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Title: The commonwealth of cells
Some popular essays on human physiology
Author: Herbert George Flaxman Spurrell
Release date: November 30, 2024 [eBook #74813]
Language: English
Original publication: London: Ballière, Tindall and Cox
Credits: Sonya Schermann and the Online Distributed Proofreading Team at https://www.pgdp.net (This file was produced from images generously made available by The Internet Archive)
*** START OF THE PROJECT GUTENBERG EBOOK THE COMMONWEALTH OF CELLS ***
THE COMMONWEALTH
OF CELLS
Some Popular Essays on Human Physiology
BY
H. G. F. SPURRELL, B.A. OXON.
[Illustration]
LONDON
BAILLIÈRE, TINDALL AND COX
8, HENRIETTA STREET, STRAND
1901
[_All rights reserved_]
To
MY ESTEEMED FRIEND AND TUTOR,
GUSTAV MANN, M.D., ETC.
PREFACE
Ever since the very beginning of my student days, when my contemporaries
took to plying me with embarrassing demands for information upon all
matters medical, I have been constantly impressed by the interest which
the unscientific public take in the workings of their bodies and the
material basis of their minds. It is this general display of interest
among my friends that has emboldened me to add yet another book to the
many already dealing with the subject. In using the word ‘unscientific’
I imply, of course, no reproach. I mean simply to denote those people
who have specialized in some branch of knowledge other than those
collectively known as Natural Science.
I usually find, when discussing physiology with such people, that they
take more interest in general principles than in details, which they
frequently find repellent, and that they frame their questions in an
appallingly comprehensive manner.
My object throughout this little work has therefore been to present the
fundamental principles of physiology in a brief, consecutive and readable
form, for those who do not care to study the text-books. There is no
lack of excellent books already, books illustrated by careful drawings
quite gruesome in their accuracy, but they are almost all intended for
“students,” and the casual reader, finding the organs divided up for
exhaustive treatment, fails to form a conception of the body as an
organic whole, and misses the very principles he is in search of, in the
heap of details under which they are buried.
It may cause some surprise that, though in my efforts to be up-to-date I
have in places outstripped the text-books, I quote no authorities. But
a moment’s consideration will show that it would defeat the very object
of a sketch like this to burden the text with an account of how my views
were formed, while, on the other hand, the pioneers of science will
forgive me. Their papers will be quoted in more durable works, and their
names honoured long after this little book has been forgotten.
H. G. F. S.
OXFORD, _March, 1901_.
CONTENTS
PAGE
INTRODUCTION ix
ESSAY I.
LIVING MATTER 1
ESSAY II.
THE CHEMISTRY OF THE BODY 8
ESSAY III.
THE MECHANICS AND PHYSICS OF THE BODY 31
ESSAY IV.
THE NERVOUS SYSTEM 58
ESSAY V.
THE BODY 94
CONCLUSION 107
INDEX 113
INTRODUCTION
The unscientific public is extremely prone, and not altogether without
reason, to take medicine as a starting-point, and arrange all biological
science around it. As it is, moreover, apt to gauge the interest and
utility of every branch of this science from a practical point of view,
and bestows most attention upon that which it imagines is of the greatest
service to the doctor, I think a series of popular essays on physiology
could not commence with more advantage, at any rate to physiology,
than by briefly discussing, not with what it deals, for that is pretty
generally known, but what is its relation to medicine. Further, as the
doctor is more easily discussed than medicine, the physiologist will
be more manageable for our immediate purpose than physiology in the
abstract, so we will devote the first few pages to the question of how
his labours benefit the patient.
Everyone knows the doctor, and everyone knows that physiology deals with
the ‘functions of different organs of the body’; but the public rarely
meet the physiologist, except in the fanciful caricatures of his enemies,
which though frequently personal are rarely accurate. These rancorous
libels, if anyone heeded them, would tend to raise doubts as to whether
the physiologist was a good companion for the doctor, and if it were not
as well for them to see as little of each other as possible.
The doctor, however, cannot move a step without the physiologist. His
business is to correct the revolt of any organ from its allotted task,
and repair the damage done by its deviation from the normal path. This
he cannot possibly do if he does not know what that organ’s normal
conditions are, and what they are it is the physiologist’s duty to tell
him. A doctor, therefore, should be an enlightened physiologist, knowing
how the body ought to work, and referring diseases to their real cause,
such as the poisons formed by an invasion of bacteria or otherwise, or
wrong feeding—that is to say, deficiency or excess of fuel for one of
the body’s many engines. Medicine is still to a large extent rule of
thumb. We don’t know to what many diseases are due, or why certain things
relieve them, if any remedy is known; and until these questions are
satisfactorily settled, it is vain to hope that disease as a whole can
be successfully combated. It is no use knowing what will stop certain
unpleasant symptoms if we do not know how to remove their real cause, and
for this end the whole body and every individual component organ is being
studied, that the process of life may be accurately understood; and the
man who is doing this for his friend the doctor is the _physiologist_.
The physiologist has many enemies, a motley array of cranks held together
by such noble bonds as general hatred of science and prejudiced ignorance
masquerading as scepticism; but he can afford to ignore them, for the
very good reason that people cannot get on without him. It is only on
account of this that they are mentioned. People say, ‘The doctor is the
person who requires a knowledge of physiology; he is the man who is most
likely to study it successfully’—presumably by his mistakes—‘and not
waste more time on it than is necessary,’ a point about which they are
most solicitous. The doctor, however, prefers to trust the physiologist.
If he did not, he would have very little time to do anything else. You
might as well expect a tailor to make his own cloth before he makes a
coat. He will doubtless be able to make better coats if the quality
of the cloth supplied him is improved; but if in order to improve the
finished article he lays down his scissors and applies his fingers to
weaving, his business will be sure to suffer.
That physiology is a thing which can take up a man’s whole energies
will, I think, be admitted by anyone who realizes how wide is its
scope. The physiologist himself must specialize, for the subject is too
vast for one man to undertake the whole. The body is composed of the
same elements as the rest of the world, and their arrangement is very
important, so he must be a proficient chemist. It is composed of solids,
liquids and gases, and diffusion, filtration, leverage, are frequent
processes, and every motion is accompanied by an electric manifestation,
so that mechanics and physics must have been part of his training. He
can scarcely study organs if he does not know their shape, so he should
know some anatomy. And, lastly, as his business is to study life and all
its attendant phenomena—and the basis of life is the cell—he must be a
histologist. To be all these things is a great deal to require of one
man; but though he may specialize for the advancement of a particular
branch of his science, he must be _au fait_ with the rest, as no vital
function is dependent upon one alone of the factors enumerated. Hard work
is required of him, though some people say he has done but little. What
he has discovered is briefly, very briefly, set forth in the ensuing
essays, with a hint or two at the knots he would like and is trying now
to unravel.
THE COMMONWEALTH OF CELLS.
Some Popular Essays on Human Physiology.
ESSAY I.
LIVING MATTER.
Physiology is the study of life, and the thing of all others which the
physiologist would like to discover is what life really is. If this were
fully known, all physiological processes could probably be deduced from
it, and disease, which is an interference with one or other of them,
could be scientifically treated. So far he has not got beyond describing
the consequences of life, and his deductions carry him no further than
this: life is a property of a substance, protoplasm, and protoplasm can
only continue to exist in the form of a cell.
This definition may seem a little cryptic to some people, and very
shocking to others. ‘Life,’ many people are accustomed to say, ‘means the
presence of a soul, and is supernatural; and as to its being a question
of chemical composition, that is absurd. My being made of cells, too,
will not account for my thinking.’ But when people dogmatize thus about
what they have not considered, they usually find themselves landed in
difficulties. They go so fast: the most spiritual of men is so dependent
upon matter and its properties that his soul will speedily quit his body
if he is prevented from breathing. And the reason of this is, that if
he cannot get air, the chemistry of the cells of which his body is made
becomes altered; he no longer consists of protoplasm, therefore he no
longer lives. Life, that it may exist in a material world, must have
a material basis, and if that is interfered with it becomes extinct
or quits the material plane; in any case, ceases to interest the
physiologist as a physiologist. I do not think anyone need be shocked at
this being recognised.
It is, of course, the ambition of the physiologist to make protoplasm,
but so far he has got no further than making some of the complicated
bodies into which protoplasm breaks up when it dies. A little while ago
this bare possibility was loudly derided, but the advance in organic
chemistry has been so great of late years, and so many complicated
substances which once seemed as unobtainable as protoplasm itself have
been made in the laboratory, that we now have hope of a precise knowledge
of the chemistry of life some day, though that day may be yet very
distant.
To give an account of life is to describe as far as we are able the
nature of the living substance, protoplasm; and as protoplasm is a
‘structure of compounds,’ a word or two about chemical compounds may
clear the ground for discussing it. If you were to take a compound,
say a lump of sugar, and start breaking it up, you could hammer for a
very long time and it would be still sugar; but if a tiny fairy with a
minute hammer and chisel were to go on breaking up the grains, he would
ultimately have molecules of sugar before him. Each molecule would
consist of exactly the same number of atoms of carbon, hydrogen and
oxygen, and if he divided it further by the removal of a single atom, it
would no longer be sugar. He could hammer away at the atoms as hard as he
liked, for they are incapable of further division.
There are seventy odd different kinds of atoms. When the molecules of a
substance are composed of only one kind, it is said to be simple; when
of several kinds, compound. Now, the difference between the various
substances we see around us consists not only in what different kinds
of atoms their molecules are composed of and their number, but in their
arrangement. This arrangement may be in chains or rings, and the relative
position which the different atoms occupy in the structure of a molecule
makes all the difference in the world.
This difference of composition gives the difference of properties to
compounds; so a compound must consist only of molecules which are all
alike. If a substance is made up of molecules of different kinds,
ununited by chemical bonds, and therefore capable of being mixed in any
proportions, it is called, not a compound, but a mixture of compounds.
But just as atoms combine to form molecules, so the smaller molecules
sometimes enter into combinations with one another to form new compounds
having larger and more complex molecules. Such a compound is said to be
composed of radicles, or groups of atoms, and on being decomposed can be
broken up, first into simpler compounds, which can afterwards be further
divided into their constituent elements.
Now, of all substances, protoplasm seems to consist of the largest
number of components, and to have them arranged in the most complicated
way known; though ‘known’ is really not the right word to use in this
connection. The reason why we do not know what life is, is that we cannot
find out in what way the constituent compounds in the protoplasmic
structure are combined. Directly we try to analyze protoplasm, it dies;
that is, it splits up into a number of simpler bodies and is altered
beyond reconstruction.
These compounds into which protoplasm breaks up when it dies are
themselves extremely complex; but though much careful study has been
bestowed upon them, we cannot as yet say how they are put together to
form the living substance. Protoplasm is too variable a body to be
considered a single compound, while, on the other hand, the chemical
relationships of its components must be too close to admit of its being
called a mixture. Its chemical position is therefore unique, and we can
only speak of it as a substance of unknown composition.
What, then, is it that makes protoplasm unmistakable and different to all
other substances? The complexity of its structure is, after all, merely
a matter of degree. The difference is not easily defined, but it roughly
amounts to this: Protoplasm is always changing, yet always remains the
same. Life is the change in the molecules.
If our definition of life seemed obscure, this sounds like a paradox;
but perhaps the following fact may help to explain it: Under certain
conditions some of the simpler compounds behave in a somewhat similar
way. For instance, there is one which is so greedy of oxygen that it
grabs it from whatever will readily give it up, and in order to do so is
obliged to relinquish that which it has already got in its molecule to
make room for that freshly acquired. Protoplasm is always behaving in
this sort of way as long as it is protected from extremes of heat and
cold, and from active chemicals which split up its molecules to form
fresh compounds. Then it dies, or ceases to be protoplasm.
But the importance of this constant change lies in the fact that by
continually breaking down its own molecules protoplasm obtains the energy
to rebuild them out of non-living compounds of high potential energy, to
modify its environment, and, in fact, to do the work of life.
It was said above that protoplasm only continued to exist in the form of
a cell; therefore, what is a cell, and why its necessity?
We have seen that protoplasm has a very complicated structure, and that
its normal condition is one of change. This being so, it obviously cannot
exist in large masses, for if it did the change would be sure to be
uneven in different parts from its very complexity; and the centre of
the lump would either be starved or poisoned by the products of its own
life. To avoid this, the mass is divided up into a vast number of small
units each complete in itself, in communication more or less direct with
its neighbours, and all equally accessible to fluids which both feed and
cleanse them.
But there is another and still more important reason for such a division.
The protoplasm is constantly discharging decomposition products, and
needs to be repairing its waste by building in fresh compounds. The raw
material around it requires dressing before it can be of use, and the
building in is a difficult business. In each cell there is, therefore,
a place set apart, where the protoplasm has peculiar capabilities, and
it is here that this elaboration is carried out. This spot is called the
nucleus. Thus it will be seen that the formation of an animal’s body by
the aggregation of cells is a necessary and ingenious way of avoiding a
difficulty.
To say, however, that an animal’s body consists of cells is to take an
entirely wrong starting-point. A cell is complete in itself, and can live
if properly fed, even though separated from its neighbours. Many whole
animals consist of only one cell. A cell is, moreover, capable of growing
and dividing, thus giving rise to two cells with two nuclei, and it is
only because cells find that it pays better not to separate, but to form
masses and specialize at different kinds of work, that we have large
animals composed of millions of cells like ourselves.
Given a cell, it is necessary to keep that cell under favourable
conditions. Otherwise the unstable protoplasm breaks up. It must have
the elements necessary to keep up its cycle of changes in the proper
form, which we may now call food, and many cells have to go and find this
requisite. It must keep away from injurious influences, and it must race
other cells for localities favourable to its growth and multiplication;
in fact, the cell must work.
That a cell can, in virtue of its chemical affinities alone, move about,
seeking favourable conditions, showing discrimination and doing work,
seems incomprehensible. In the first place, how can it move? There is
only one way: it must effect a redistribution of its substance, and
contrive that those parts of the cell whose activity is applied to this
end shall be so situated as to produce definite changes in its shape
according to the cause which evokes them. Of the way in which different
cells move we shall have a good deal more to say later.
Why protoplasm should be influenced to move still requires explanation.
Yet the gap between protoplasm and other substances is really not so
great, after all. Heat and magnetism cause movement in inanimate matter,
and the response of protoplasm as exemplified by some of the minute
unicellular animals is almost as mechanical. Some kinds which swim in
water move to the positive pole of a galvanic battery, others to the
negative, if the wires are dipped into the vessel containing them. Some
move towards light, others away from it, with unvarying regularity.
Temperature and chemical substances also cause a definite effect upon
these micro-organisms. And all these movements are wholly involuntary,
absolutely invariable, and, in fact, reactions evoked by fixed causes.
Nevertheless, it will be seen that protoplasm can only continue to exist
in the form of a cell, since, unless thus organized, it can neither
keep itself among favourable surroundings nor prepare fresh ingredients
to make good its waste. If a cell be cut up into several pieces, these
detached bits of protoplasm will live for a time; but death overtakes
them as soon as they have used up their reserve material. When this is
gone, they have to consume their own substance, a process which quickly
proves fatal. Should a fragment contain a small part of the nucleus cut
away with it, it will live a little longer; but it is only the piece
which contains the nucleus more or less intact—in other words, the cell,
damaged though it be—which can survive and recover from such mutilation.
The specialization of protoplasm to form a cell is perhaps its most
remarkable peculiarity. Not only is protoplasm differentiated to form
different structures, but it devotes the energy evolved in its ceaseless
change to different purposes. The protoplasm of the motor organs of the
cell expends itself wholly in producing the physical movements necessary
to approach and capture food. When this has been passed into the cell,
protoplasm of another variety works to refine and dissolve it, and then
passes it on to the nucleus. The protoplasm of the nucleus, again, has
different work to do. It devotes its energy to producing chemical changes
in the raw material, and converting it into new compounds which the
various parts of the cell can assimilate. Some of these it retains for
its own needs; the rest it dispenses to the motor and other organs to
repair their waste, and supply them with energy to obtain more food.
Thus do the different varieties of protoplasm which compose a cell supply
one another’s needs, and enable one another to live; and thus does a
cycle of chemical changes form the foundation upon which the whole fabric
of life rests. But into details we cannot yet go, for our investigations
of the material basis of life have not yet carried us beyond these
general conclusions.
At present we know nothing definite about the first causes of life, and,
though we have hopes, perhaps we never shall. Meanwhile we are observing,
analyzing, and classifying the phenomena in which life is manifested, in
the hope that at last light may break through upon our researches, and we
may be able, if not to synthesise protoplasm in a test-tube, at least to
demonstrate its workings in equations.
In the meantime, our actual knowledge of living matter can still be
compressed into the words in which Professor Huxley summed it up years
ago:
‘Carbon, hydrogen, oxygen, and nitrogen are all lifeless bodies. Of
these, carbon and oxygen unite, in certain proportions and under certain
conditions, to give rise to carbonic acid; hydrogen and oxygen produce
water; nitrogen and hydrogen give rise to ammonia. These new compounds,
like the elementary bodies of which they are composed, are lifeless. But
when they are brought together under certain conditions they give rise to
the still more complex body, protoplasm, and this protoplasm exhibits the
phenomena of life.’
Until we have further knowledge of the changes which constitute these
phenomena, physiology must remain descriptive rather than explanatory.
ESSAY II.
THE CHEMISTRY OF THE BODY.
I.
The cell is usually very minute—indeed, absolutely invisible without a
microscope, though in some cases it is a fair size. The whole yolk of
an egg is a single cell until its minute nucleus, a speck on one side,
starts dividing and it becomes several. By the time the chick is ready to
be hatched there are millions.
Usually, however, a cell is small—just as much protoplasm as its still
more minute nucleus can keep going; though here, again, one must be
guarded, for there may be several nuclei instead of only one. The
protoplasm on the external surface and around the nucleus is specialized
into a more or less definite membrane. To this outer envelope are
attached fine fibrils, which join up to a small body within the cell,
called the centrosome, and by the lengthening and shortening of these
its shape can be altered. The contents are fluids; so if the containing
membrane is loosened in any direction, they tend to bulge out and form an
excrescence, and in this way the cell is enabled to throw out limbs and
surround particles of food, and, by relaxing the fibrils in one direction
and contracting them in others, to crawl whither its chemical, thermal,
or physical affinities direct. (See Diagram 1.)
Not particularly inspiring is the sight of life in its simplest form, but
when a few millions of these cells group together and form one body,
dividing the labour between them, the result is something stupendous.
There are animals composed in this way, some of whose cells have
developed their digestive capabilities to such an extent that they have
almost lost all their others. These are carefully guarded in the interior
of the body. Other cells in this same beast, receiving their food in a
fluid form from these digestive specialists, secrete lime around them
till a skeleton is built up. To the levers of this skeleton are attached
bundles and strands of cells, which, if they can do nothing else, can
lengthen and shorten and make it move. Yet, again, there are cells which
have especial facilities for receiving, weighing, and transmitting
chemical and physical promptings. These cells, again, lie in a protected
corner of the interior, but they send out fine threads to one another and
every part of the body, and control the whole.
The animal in which this beautiful system of division of labour has
been carried to its greatest perfection has many and varied powers.
He can in some cases even apply to the individuals of the species the
principles of his own cellular economy, and thereby achieve not only the
making of poetry and jokes, but the building of a Westminster Abbey, the
construction of Maxim guns, and the enforcing of his economic refinements
upon his less highly specialized neighbours.
We have now traced out a general idea of life. We have seen that its
basis rests upon a chemical structure which, to maintain its identity,
must be always changing. We have seen that to do this it must keep
breaking down its substance, and giving off the products, and taking
hold of extraneous materials, and building them in, not only to repair
the loss, but in order to grow; and that to do that it has to be more or
less modified in parts, in order that the main bulk may be brought within
reach of its food, and be then able to convert it into the most useful
form. And, lastly, we have seen that just as several specialized forms of
plasm together make up a cell, so several kinds of cells, each with some
peculiarity exaggerated, aggregate, and, supplying one another’s needs,
compose a body.
Having now roughly sketched out the scheme upon which such a body works,
we can go on to a more detailed examination of the division of the
labour, and the way in which each department supplies, and is dependent
upon, the others. If we were to do this thoroughly, it would take a great
deal of time and space, for the physiology of a potato plant, though
essentially the same, presents many differences from that of a horse;
but the physiology of the great human interest is also that of the most
complicated animal, namely, man, so it is on him that we shall focus our
attention.
Protoplasm is more easily studied the more specialized is the animal it
composes. When all the events of life are taking place in a speck of
matter, invisible without a microscope, it is impossible to analyze the
changes which it is working in its surroundings, or to infer those which
are going on in itself. But when large numbers of cells are examined
collectively, we can deal with what they take in and what they give out
in sufficient bulk to arrive at a fairly accurate determination. The
study is rendered still easier in an animal with extremely specialized
organs, like man, in which food is nearly all taken in by the mouth, and
thus kept quite distinct from what is eliminated; the latter, again,
being mostly given off by the kidneys is kept equally distinct. Moreover,
the intermediate changes being performed in different organs still
further simplifies investigation of the vital process; for the physical
effects are also more easily studied when exaggerated in a particular
part of the animal. The electrical changes in a single cell might long
have remained unsuspected had we not been able to observe those in a
muscle with the galvanometer.
Now, while the cells which make up the body of man differ very greatly
owing to the different tasks which they have to perform in obtaining food
and getting rid of refuse, they all require very much the same fuel to
enable them to live, and having got it, they all treat it in very much
the same way; therefore our first business is to consider what the body
wants, and what it does with it. Afterwards we can try to find out how it
gets it, and where.
The first and most indispensable requirement of protoplasm is water. The
next is probably nitrogen, compounds of which seem to form the framework
of the protoplasmic structure. The next is probably carbon, and the next
free oxygen. The two last-mentioned combine with a release of energy.
This happens in the grate when coal burns, and the result is heat. In the
tissues of a body the result may be heat, growth, or movement, all three
being present in the phenomenon of muscular activity. Finally there are
mineral salts, the most important being sodium chloride, which is placed
on the table at every civilized meal.
But though these elements are given here in order, their importance is
really equal, for all are necessary. That is about as much as it is wise
to say here. The chemistry of the living cells—their anabolism, or how
fresh material is built into their structure; their katabolism, or how
the same structure is broken down that work may be done; in fact, the
general metabolism—is so complicated, and so little understood as yet,
and requires so extensive a knowledge of chemistry to follow, that it
is best left alone by people who do not want to go into it deeply. At
best, such a discussion resolves itself into an exposition of different
observers’ theories, with the reasons why they hold them—reasons based on
laborious and technical studies. Pages might be written on the various
theories, backed by pages more of chemical formulæ, to show why this
view deserves deep consideration, while that, in spite of the obstinacy
with which it is upheld, is absurd; but though such discussions take one
nearest the secret of life, the general public is not unnaturally apt to
stigmatize this side of physiology as dry. It is a matter which interests
experts, not the casual reader.
Quite a different affair is the question of diet. That is everybody’s
business, as the number of faddist societies and blatantly advertised
‘foods’ attest. And though the preparation of the food in the body up to
the point where it merges into living matter and is lost sight of—in a
word, ‘digestion’—is again a question of chemistry, it is one which may
be approached without such an exhaustive knowledge of that science as the
previous considerations would have required. It is, moreover, to judge
from the way it is discussed, a topic of universal interest.
A casual glance at the animal kingdom will show that diet is a wide
subject. A pigeon will eat peas; a tiger would not know what to do with
the peas if he got them; while a monkey will eat almost anything he can
lay hands on. A plant takes us still further afield, for it can use the
atoms of substances with an extremely simple molecule—carbonic acid gas,
for instance.
Our task, however, is simplified by our having only man to consider; and
although most of the higher animals are so much alike that they might
be considered in general and contrasted in detail, it is a great thing
to get rid of the whole vegetable kingdom with bacteria and parasitic
animals.
One of the first requisites for the maintenance of life, as was mentioned
above, is nitrogen. Now, nitrogen is one of the commonest elements in
the world, but it is the hardest to supply to the body. Four-fifths of
the air is pure nitrogen, but pure nitrogen is useless as a food. We
draw it into our lungs at every breath, and are none the better for it,
for we breathe it out again unchanged; and if it were completely absent
from the air we should not be so very much the worse. The Ancient Mariner
exclaimed, ‘Water, water everywhere, and not a drop to drink’; a starving
man might exclaim, ‘Nitrogen, nitrogen everywhere, and not an atom to
assimilate.’
Animals have to get their nitrogen in the form of proteid, a substance
whose molecule is composed of nitrogen, oxygen, hydrogen, carbon, etc.,
and might roughly be described as dead protoplasm. Plants on which
animals feed, when they do not get their proteid by the simpler, though
less moral, method of eating one another, are able to get their nitrogen
in a simpler form; but with that we are not concerned.
The proteids are a group of substances which resemble protoplasm in the
elements of which they are composed and in the complexity with which they
are combined. The various proteids seem, however, to have a definite
chemical composition, and therefore differ from protoplasm in being true
compounds; moreover, if kept from bacteria they undergo no changes. One
of the best forms of proteid for examination is white of egg; this, as
is known, sets or coagulates when boiled, dissolves in water, from which
it may be precipitated by boiling, and displays various other chemical
properties common to all proteids. There is, however, a good deal of
difference between the several varieties of proteids, and the more
complex ones have to be converted into the simpler before they can be
absorbed. Hence the necessity for digestion.
Now, as proteid resembles dead protoplasm, it might be supposed that a
diet of proteid alone would be the most economical; but this is not so.
If it were possible to live without work, _i.e._, without movement of any
kind, it might be; but to do work, more carbon must be oxidized than the
proteid molecule contains.
Carbon, the next item on our list, is familiar to everyone in the
comparatively pure form of coal, charcoal, and the ‘lead’ of pencils. It
is commonly used to burn—_i.e._, oxidize—that heat may be obtained to
boil water and to work machinery. This is precisely what it is required
to do in the body, where it is burnt by oxygen taken in by the lungs,
that heat and energy may result. It is a commonplace that severe exercise
causes laboured breathing, and the reason of this is that the carbon in
the body is being oxidized, and the product, carbonic acid gas, has to
be got rid of. The more work is being done, the more oxygen is required
to burn carbon in the muscles. The more carbon is burnt, the more heat
is evolved, and the more necessary it is that the blood should be cooled
by drawing cool air into the lungs. Hence the more rapid breathing.
The air normally breathed out is always warmer than that taken in, and
always contains extra carbonic acid gas. After exercise the quantity is
increased, and its increase on the normal amount given off can readily be
demonstrated by analyzing samples of the air taken in and given out.
But carbon, like nitrogen, cannot be taken in in the crude form. No one
would try to make a meal of charcoal. A certain amount is contained
in the proteid molecule, enough, no doubt, to secure the basis of the
protoplasmic structure; but unless one is prepared to eat an excessive
quantity of proteid, a proceeding entailing waste and exhaustion of the
digestive apparatus, the balance must be made up by eating carbohydrate.
The forms in which people are most familiar with carbohydrate are starch
and sugar. Sugar is the better food, as it is so much more soluble than
starch; and, in fact, starch is always turned into a kind of sugar
before it is used by the body. The common cane-sugar, which everyone
knows so well, is about the most useful food we have, owing to its
purity, and therefore concentration, and its simplicity. A very small
amount of digestion is necessary to convert it into the simplest of all
carbohydrates, a substance easily stored, as glycogen, till wanted,
which is present in muscle after a meal, and is used up when the muscle
is active, being oxidized to carbonic acid gas, sarcolactic acid, and
alcohol.
The importance of carbon in the diet is therefore obvious; and people who
intend doing extra muscular work should take extra sugary food rather
than extra proteid. A locomotive which is about to make a record run
takes in more coal, not more engine-drivers, and our athletes now follow
the same principle. We shall, however, have a good deal more to say about
athletes presently.
There is yet another point to be considered in respect to carbon. Carbon
need not be taken in the form of carbohydrate, the alternative being
fats and oils. Fats and carbohydrates are both composed of the elements
carbon, hydrogen, and oxygen, but the proportions in which they are
joined are different. Fats are not such useful foods as carbohydrates,
nor to most people so pleasant—compare a spoonful of olive-oil and a lump
of sugar. But there is one important point to be urged in their favour:
they yield twice as much heat as either proteids or carbohydrates; so
their position among foods is assured.
The other chemical necessities of the body we need only mention here.
Hydrogen is one of the components of proteid, carbohydrate, fat, and
water; and if it does not enter in the last form, it—at any rate, most
of it—leaves as such, being oxidized in the tissues. Sulphur and iron
deserve honourable mention; common salt is required by the blood; lime
and phosphates go to make bone; but important as they all are, they need
not detain us further at present.
With regard to the amount of these elements which is required per day,
and which is ascertained by collecting and weighing all that is given
off, it is found that about ½ ounce of nitrogen and 10 ounces of carbon
are necessary to an average man—_i.e._, weighing about 10 stone. The ½
ounce of nitrogen and about 2 ounces of the carbon are contained in 4
ounces of dry proteid, which leaves a balance of 8 ounces of carbon to be
made up; and this is usually obtained by eating 4 ounces of fat and 18
ounces of carbohydrate.
Roughly speaking, these principles are contained in ¾ pound of ordinary
butcher’s meat and 2 pounds of bread; but it would be well to defer
considering diet for the present, until we have examined the apparatus by
which the body extracts what it wants from the raw materials, and which
of these offer it the least resistance.
II.
The way in which protoplasm gets its chemical requisites for growth is
doubtless simply by absorbing them. Some of the lower structureless forms
carry this to an absurd extreme, for when two individuals meet they fuse,
and each no doubt claims to have eaten the other. As, moreover, the first
thing which a cell does when it grows is to divide, the whole proceeding
looks rather futile. But ready-made protoplasm of an assimilable shape
is rare, and it is not often that a cell, unless it be a plant or a
parasite, finds itself in a substance which can be handed straight
to the nucleus without further elaboration. Usually the cell has to
discharge from itself a reagent, which will develop the right chemical
qualities in the matter it wants to absorb. This substance is known as
an enzyme, or ferment. Ferments, however, are an expense to the cell,
requiring a certain effort for their production; so, in order that they
may be economized, they are, in the higher forms, poured over the food
while it is in an enclosed cavity, or stomach. In the simplest animals,
consisting of a single cell, the protoplasm simply flows round the
particle of food, and it is ‘ingested’ with a drop of water. Into this
‘food vacuole’ the ferments are secreted, and when all that is useful
has been dissolved out and absorbed, the bubble moves to the surface
and bursts; or, to put it differently, the cell flows on its way, and
the vacuole, with any shell or refuse it may contain, gets left behind.
(See Diagram 1.) In other cells which are constant in shape there is an
opening leading to the interior of the cell. Round this there are little
projecting threads, which beat the water regularly. In some positions
these threads enable the cell to swim, but here their duty is to cause
a current and wash particles of food down the primitive throat into the
interior, where, as in the preceding case, they become enclosed in a
vacuole. (See Diagram 2.)
[Illustration: DIAGRAM 1.—THE AMŒBA.]
[Illustration: DIAGRAM 2.—PARAMŒCIUM.]
Moving a stage higher, we find animals consisting of several cells. Of
these it is only natural to suppose that some have greater enzyme-forming
powers than others.
[Illustration: DIAGRAM 3.—DEVELOPMENT OF AN EMBRYO: FIRST STAGE.]
[Illustration: DIAGRAM 4.—FORMATION OF A DIGESTIVE CAVITY.]
[Illustration: DIAGRAM 5.—CROSS SECTION OF A DEVELOPING EMBRYO.]
A step higher in the animal scale, or a further advance in the
development of the schematic embryo (depicted in Diagrams 3 to 6), and
we find that these special digestive cells are losing their sturdier
qualities and being placed in a position protected by cells which have
specialized in another direction. This is shown in Diagram 4, where the
hollow ball of cells which resulted from the repeated division of one
cell is represented in section. One side of the ball is pushed in, and
now the beast consists of two layers of cells, an outer protecting and
an inner digesting (Hydra and sea-anemone). Soon, however, it is found
more convenient to have a tube for digesting food, for then different
substances can be digested and absorbed in different parts; and the
refuse, of which the animal can make no use, need not be brought back to
the mouth to be got rid of.
This, however, requires a number of other changes in the structure of
the animal, which are roughly shown in Diagrams 5 and 6. It is not to
our purpose here to discuss the development of animals or an animal; but
the figures are worth glancing at, as they show not only how certain of
the cells are set apart for digesting food, but also that a large body
consists really only of a mass of protoplasm, composing kindred cells of
common origin.
[Illustration: DIAGRAM 6.—SHOWING DEVELOPMENT OF AN EMBRYO.]
Now, for obvious reasons, the longer, within certain limits, this tube is
the better. All sorts of different food-stuffs have to be acted upon in
it, and some offer considerable resistance to digestion; and the further
they have to travel in the tube, the more chance there is of their being
successfully treated. Besides, different parts have different functions,
and the longer the tube—again within necessary limits—the greater scope
is there for division of labour, and consequent economy. The comparative
length of the alimentary canal is not the same in all animals by any
means. Carnivorous animals, like the cat, whose food is soft and easily
digested, have a fairly short one. Herbivora, like the sheep, whose
food is difficult to digest and mixed with much husk, which is wholly
indigestible, have a comparatively very long one. Man, who is omnivorous,
but eats less and more judiciously chosen food than either of the above
classes, has one of medium length. But in all cases among the higher
animals there is an attempt made to obviate the necessity of increasing
the length of the animal by coiling the tube within the body. The annexed
diagram (7) illustrates this principle. It shows a schematic animal whose
digestive canal is much longer than itself.
[Illustration: DIAGRAM 7.—SHOWING HOW THE DIGESTIVE CANAL IS LENGTHENED.]
[Illustration: DIAGRAM 8.—CROSS-SECTION OF THE DIGESTIVE TUBE.]
The digestive canal has, however, another function. The cells which
compose it have not only to secrete juices, to convert the food into a
usable form; they have then to absorb it. The nearer a particle of food
is to the wall of cells, the sooner it is reached by these juices, and
the less chance there is of useful material being swept away and lost.
In view of this fact, along certain tracts the digestive canal is folded
inwards, and there are projections, which increase the number of cells to
secrete and their opportunities of absorption. (See Diagram 8.)
[Illustration: DIAGRAM 9.—SHOWING HOW GLANDS ARISE.]
Here again we have an illustration of a constantly recurring need, with
a device for meeting it—increase of surface without increase of bulk. We
met with it before in the cellular system; we shall meet with it again
in glands, lungs, and brain, at least. The importance of a device for
gaining this end is apparent when one remembers what the comparative
value of surface and bulk is to an animal, and that, while surface
increases by the square, bulk increases by the cube.
The principle is pressed to an extreme, together with the allied
principle of division of labour, in glands. The object of these is to
increase the number of secreting cells, and, as they are delicate, to
keep them protected from contact with coarse particles of food. And, in
order that nothing may interfere with their efficiency, they are absolved
from the duty of absorbing. Hence tubes grow out from the cavity of
the alimentary canal lined with the same cells, but, as no food ever
enters, the cells which line them devote themselves entirely to pouring
out digestive juices. Glands differ considerably in structure and in the
liquids which they secrete. Some are very small; some, like the liver,
very large. In some the tube is very short, in some long, coiled and
branched, and sometimes the gland is connected with the surface by more
or less of a duct. Some glands only secrete one enzyme, some several.
In each, however, the principle is that shown in Diagram 9, no matter
how its structure is masked by the bloodvessels and supporting cells or
connective tissue which envelop it.
After a meal, or, rather, when the process of digestion is over and
the animal is beginning to think about its next, the gland cells start
preparing their enzyme. There is great activity in the nucleus, and
granules stream out from it towards the lumen of the gland in much the
same way, to take a homely illustration, as bubbles in some effervescing
drink form at the bottom of the tumbler and rise till the surface is
covered with foam. At the right moment these granules are discharged,
just as the bubbles on the surface of a liquid break at a slight jog.
They are usually not the ferment or enzyme, but its precursor, a
substance which only turns into the ferment when it gets outside the
cells. The ferments, when formed, are very peculiar substances about
which we should like the chemist to tell us more, though great advances
have been made in our knowledge of them lately.
Among other peculiarities, one may mention that, though they will keep
indefinitely if bottled, they are easily destroyed by too extreme a
temperature or too acid or alkaline surroundings, that their composition
is entirely unknown, and, strangest of all, that they do not become
used up. A given amount of rennet will clot any amount of milk within
reasonable limits, and yet remain rennet. The clergyman has been quoted
as an illustration of the action of a ferment, and he makes a good one.
He can make any number of suitable men and women into married couples,
and yet his own state is unchanged.
III.
In man, the digestive process may be divided into three stages. They are
arranged progressively, so that each clears the way for the next, and
take place in the mouth, the stomach, and the upper part of the small
intestine, the rest of the canal being mainly occupied in absorption.
[Illustration: DIAGRAM 10.—GENERAL SCHEME OF THE ALIMENTARY CANAL, WITH
ITS OFFSHOOTS—LUNGS AND GLANDS.]
By far the largest proportion of the food is carbohydrate, in some form,
so one naturally expects the first stage of digestion will deal with the
constituents which represent this class. This is the case. The food is
taken into the mouth in small quantities and ground up with the teeth,
during which process it is subjected to the action of the saliva. This
fluid, which is the secretion of three pairs of glands, converts a large
proportion of the carbohydrates, starch, cane-sugar, etc., into a very
simple sugar which is absorbed directly it reaches the stomach.
One of the most sensational discoveries of the physiologist has been
that the saliva leaving the gland does not contain the ferment necessary
to effect this change until it has been subjected to the action of
putrefactive bacteria. These, fortunately for us, it is pleasant to know,
simply swarm in the mouth.
When the food is swallowed, it passes very rapidly down the first part of
the alimentary canal, which is straight, and is then kept for some time
in the stomach. The stomach differs from the rest of the canal in several
particulars, among them the following: it is a _large_ cavity, and is
closed at each end by a valve to keep the food in until it has been
thoroughly treated, and it deals with the whole mass of food taken at a
meal at one time, and yet has no contrivances for increasing its surface.
Here the food is subjected to a most important and searching examination.
Enclosed in this bag, it is thoroughly mixed with weak hydrochloric
acid, secreted by numerous glands, and kept churning round and round by
the muscular action of its walls, that the contents may be kept well
mixed. The acid is just strong enough to kill protoplasm, and hence the
putrefactive bacteria which were necessary in the mouth, but would be
a very doubtful blessing in the interior of the body, are disposed of.
Other things are also killed. Not only does the stomach execute intruding
bacteria, but it also kills a good deal of our food. Fruit and salad
consist largely of still living cells, and occasionally there is bigger
game, _e.g._, oysters. One thing, however, the acid does not kill, and
that is the cells lining the stomach, and it may as well be said here
that the parts of the body exposed to ferments have the very necessary
power of resisting them, so that a normal animal does not digest itself.
The stomach, however, is a kitchen as well as a slaughter-house. The
gastric juice, or secretion of all the glands opening into it, contains,
besides the acid, two important ferments, both of which act on proteids.
Carbohydrates are absorbed, but not digested, in the stomach, as acid
destroys saliva. One of the ferments is rennet, an article familiar
to the culinary profession, which solidifies milk. The other acts on
proteids generally, converting them ultimately into a very simple form,
peptone, which is absorbed at once. How much of the proteid in the
stomach is converted into peptone is not known, for the action of acid
alone is sufficient to enable it to be absorbed. A solution of proteid,
_e.g._, white of egg, is quite altered if made slightly acid; it no
longer coagulates when boiled, but the change of the most practical
interest is that, if injected into the veins, it seems to become part of
the blood, while ordinary proteids act as poisons.
The peptonizing ferment, however, has one very important function: it
digests the collagen of the connective tissue, the substance which
becomes gelatin when boiled. The reason why this is so important is not
only that nothing else in the body affects it, but that fat is enclosed
in it, and if it were not thus set free would pass through the body
unabsorbed.
The final stage is the digestion by the pancreatic juice. After the
food has been exposed for some time to the gastric juice, it is allowed
to escape a little at a time from the stomach, and continues its way
along the alimentary tube. Before it has gone many inches it comes to
the openings of two ducts, those of the liver and the pancreas, and
immediately the acid stimulates them, and the glands pour out their
secretion. That of the liver is largely excretion or refuse from the
blood without direct action on the food, but it enables the pancreatic
juice to do its work by making the food again alkaline, and stimulates
the muscular coats of the intestine to force its contents along. That
of the pancreas is the most important digestive fluid in the body,
containing many ferments; it acts alike on proteids, carbohydrates, and
fats—in fact, digests everything—so that the rest of the long tube is
freed from any more laborious duty than absorbing them as they pass.
NOTE.—The digestive ferments are now prepared for examination by chopping
up the gland and placing it in glycerine; this extracts the ferment
and preserves it from the action of bacteria. The first experiments on
digestion, however, constitute one of the romances of physiology. A
Canadian named St. Martin got into trouble with Red Indians whilst in the
United States, America, and was shot through the body. The surgeon who
attended him was unable to make the wound close, and when it healed there
remained an opening in the man’s body communicating directly with his
stomach. The surgeon, Beaumont, saw possibilities in this, and, obtaining
gastric juice from his patient, made those classical experiments which
entitled him to a place among the fathers of physiology. Americans do
well to be proud of Beaumont, for it cost him many sacrifices, and his
patience and courage are above praise. Not only was he devoid of all
but the crudest appliances out in the backwoods, but his subject proved
intractable and mercenary. No sooner did he discover his value than he
crossed the border, and refused to return except upon exorbitant payment.
Even after this had been arranged, he repeated the performance whenever
he thought fresh extortion possible. In spite of these difficulties, the
investigations proved wonderfully accurate and complete.
IV.
Of the absorption of the materials thus prepared it is not necessary to
say much in a work of this compass, but the absorption of oxygen is too
important to be passed over.
Oxygen is required by the body pure, and, as it is uncombined with
anything in the air, it needs no digestion to free it. A special
organ, however, is necessary to absorb it. This is the lung. The lungs
originate, just like a gland, by a pouching of the alimentary canal
near its origin, but differ from a gland in their cells being very much
flattened, to offer a large surface to the air on one side and to the
bloodvessels on the other. Incessantly during life air is being drawn
into the lungs; that the cells to which it is there exposed may transfer
its oxygen to the blood; and then, after the cells have also transferred
the carbonic acid gas from the blood to the air, driven out again to be
replaced by fresh.
(The mechanical means by which the lungs are filled and emptied come
under another heading.)
V.
Food having been absorbed by cells set apart for the purpose, the next
problem is, How is it distributed to those specialized for other work?
The medium for this distribution is a liquid called lymph. All the spaces
in the body are filled with lymph, all the organs bathed with it, every
cell moistened with it; yet it is comparatively stagnant, and the food
has to be conveyed from the walls of the alimentary canal to the lymph in
the neighbourhood of the cell requiring nourishment by a more expeditious
agent. This is done by the blood.
[Illustration: DIAGRAM 11.—PRINCIPLE OF THE SYSTEM OF BLOODVESSELS.]
[Illustration: DIAGRAM 12.—PRINCIPLE OF DOUBLE CIRCULATORY SYSTEM.]
The blood is a fluid akin to the lymph, but confined in a system of
tubes. Through these tubes it is driven at a considerable velocity, and
in the course it takes passes within a reasonable distance of every
cell in the body. As it passes the cells of the alimentary canal, they
discharge the nutriment they have absorbed into it; as it passes through
the other organs of the body, it discharges the requisite materials
into the lymph bathing the actual cells: these are then able to help
themselves.
The lymphatic system is very simple. Lymph is practically fluid which
has exuded through the walls of the bloodvessels, and is like the plasma
of the blood, a thin solution of proteids in water containing just
enough salt to hold them in solution. From different parts of the body
a series of tubes run towards the heart, going up with increase in size
and decrease in number as they near it. Into these tubes the lymph is
forced with every movement of the body. At a slow rate, but varying with
the activity of the animal, it is forced to flow along these tubes,
regurgitation being prevented by valves at intervals, until it reaches
the place where the lymphatic vessels join a large vein, and it is poured
back into the blood-stream, thus completing its cycle.
The blood is entirely confined in a closed system of tubes, along which
it moves always in the same direction. The main principle of the system
is that of a ring. One side of the ring is split into a vast number of
fine tubes to give a large surface for absorption and discharge of food
among the cells; the other side is a single tube, with an enlargement
in which the blood from different parts is mixed (see Diagram 11). This
enlargement, which is contractile and fitted with valves, rhythmically
draws the blood in from one direction and pumps it out in another. (The
mechanics of the process we shall study later.)
As a matter of fact, this system is twofold, as in Diagram 12. In passing
through one-half of its course the blood absorbs oxygen in the lungs; in
the other it yields oxygen to the tissues, and absorbs, whilst passing
over the alimentary canal, proteid, carbohydrate, water, and salts, which
are duly distributed to the other organs. Fat is absorbed by the lymph
direct, but poured into the blood for distribution.
The blood which passes over the alimentary canal on its way back to the
heart goes through the liver. In this gland it leaves the carbohydrate
which it has taken up, and a large store is laid down there after a meal,
to be doled out as it is wanted. Blood also passes through the liver from
the spleen, where it has been, so to speak, overhauled for repairs.
[Illustration: DIAGRAM 13.—SCHEME OF THE CIRCULATORY SYSTEM.
Blood system on the right, lymph system on the left.]
As the medium for chemical communication throughout the community of
cells, the blood has another all-important and obvious function, viz.,
that of clearing away the waste products of life. Of these there is, of
course, the same quantity as of new material introduced. Carbonic acid
gas is discharged into the lungs, but all the nitrogen and most of the
other elements in the new combinations which protoplasm has made them
assume leave by the kidneys, plus a little water by the skin as sweat and
a few items discharged into the last part of the alimentary canal amongst
the unabsorbed portions of the food.
In their constituents, blood and lymph resemble one another, being
both weak solutions of salts and proteid material; but the blood is
distinguished from the lymph by the presence of innumerable extremely
minute bodies, which give it its red colour. These corpuscles, to give
them their proper name, are the vehicle by which oxygen is transported
from the lungs to the tissues. They consist of an envelope of protoplasm
filled with a red fluid (hæmoglobin), which combines loosely and easily
with oxygen. In shape they are discoid, with a thickened rim and
biconcave sides, another device for increasing surface and reducing bulk.
(See Diagram 14.)
[Illustration: DIAGRAM 14.—A RED BLOOD CORPUSCLE.]
With one more fact we may now conclude the chemical survey of the body.
The blood has to pass through certain glands, or it becomes poisoned, and
this quite apart from whether the gland secretes healthily or not.
Disease of the thyroid (a ductless gland in the Adam’s apple) causes
goitre; of the suprarenal, Addison’s disease; of the pancreas, diabetes.
Whether these organs secrete some substance into the blood which
counteracts poisons formed in it, or whether they remove injurious
elements from it, is not certain, but they are necessary to keep the
great means of chemical communication in order.
NOTE.—The thyroid gland no longer secretes anything into the alimentary
canal, and its duct disappears at an early age. If, however, it become
diseased or is surgically removed, the distressing symptoms of goitre
supervene. Such a patient may be completely cured by grafting a thyroid,
excised from another animal, anywhere in his body. Doctors usually,
however, give the patient extract of sheep’s thyroid either in pills or
injections.
ESSAY III.
THE MECHANICS AND PHYSICS OF THE BODY.
I.
In the preceding essay we regarded protoplasm as a chemical factor in the
universe.
We have seen how it is always changing, always taking in food, always
giving off waste materials. We have seen, too, that it grows and that it
does work, and that in a large mass the cells which compose it share the
labour instead of each component cell performing all the vital functions.
We have now to consider the work which protoplasm does—in a word, the
mechanical effect of the chemical actions just described.
The simplest movement of protoplasm is to be seen by the aid of the
microscope in certain vegetable cells, where granules seem always
streaming about in different directions. A step higher, and we find
this streaming movement converted into movements of the whole cell.
In the simplest unicellular animals the fluid protoplasm is contained
in a membrane, or denser bounding layer, to which are attached fine
filaments springing from a minute body known as the centrosome. These
centrosomes—for there are sometimes several in a cell—seem to control the
mechanical department, just as the nucleus does the chemical. Along the
fibrils at intervals are minute globules, and by watching the distance
between them it is seen that the fibrils undergo changes in length,
pulling in the membrane when they shorten, and letting the cell flow out
in any direction when they relax. By adjusting these two movements to
balance one another, the cell can move in any direction, surround and
engulf particles of food, and assume a strange variety of shapes. (See
Diagram 1.)
[Illustration: DIAGRAM 15.—CELL DIVISION.]
In some cells, probably in all, the centrosome presides over division.
Cells, however, do not always divide in the same way. Some simply
lengthen, the nucleus also lengthening inside, become constricted in the
middle like a dumb-bell, and separate. (See Diagram 15.)
[Illustration: DIAGRAM 16.—CELL DIVISION.]
Others manage differently. In them the nucleus simply bursts, and turns
its essential elements, a number—always a constant number—of coarse
threads, adrift. Meanwhile, two centrosomes have moved to opposite ends
of the cell, and there anchored themselves by fibrils; other fibrils
springing from them become attached to the nuclear threads, and when all
is ready pull them apart, equally divided, to their respective ends,
where they re-form into two fresh nuclei. (See Diagram 16.)
Unicellular animals, which are constant in shape and swim instead of
flowing when they want to get anywhere, have at first sight nothing
in common with those which do the latter. From their surfaces spring
fringes of free protoplasmic threads, called cilia, from their fancied
resemblance to eyelashes, which serve as motor organs, and beat the water
like oars. (See Diagram 2.) Waves of movement, as they lash one after
another, all in the same direction, seem to pass over the cell, and it
is propelled through the water; while others, which are situated in the
neighbourhood of the cell’s mouth, stir the water into eddies, and drive
food particles into it.
[Illustration: DIAGRAM 17.—CILIA OF AN EPITHELIAL CELL.]
These cilia are important, as they are adapted for many purposes in
large animals. The cells which line the cavity at the back of the nose,
the tubes of the lungs, and other parts of the body, have a few cilia
on their free surface, and it is in them that the structure of these
organs can best be made out. At the foot of each cilium is a minute
globule, from which a fine fibril passes into the cell, and the fibrils,
collectively forming a leash, are attached to its opposite end. (See
Diagram 17.) It seems highly probable that the globule is a centrosome
giving rise to two fibrils, one attached as described, the other passing
up one side of the cilium, and fast to its apex. The result of this
arrangement is that when the fibrils contract the cilium is bent over
with a jerk to the side up which the fibril runs, and when they relax it
slowly straightens itself. There is, therefore, no fundamental difference
between this and the other mode of progression; both are dependent upon
the centrosome.
Finally we have muscle cells. These are only found in a fairly
complicated animal, since they are a product of the division of labour
principle, and their sole business is movement. There are two varieties
of muscle, but the principle is the same in both—a long thin cell, with
fibrils traversing its length whose contraction causes the cell to
shorten and thicken, thus reducing the distance between its two ends. At
present the development of muscle and the way in which it ‘contracts,’
to use the word accepted in this case for describing a redistribution of
bulk, are little understood, and there are accordingly many opinions; but
I think careful study will eventually show that some modification of the
centrosome, with its contractile fibrils, is responsible for the movement.
[Illustration: DIAGRAM 18.—MUSCLE.]
The two varieties are: the smooth, or involuntary, and the striped, or
voluntary, muscle. Smooth muscle consists of spindle-shaped cells with
one elongated nucleus. (See Diagram 18, Fig. 1.) It only contracts very
slowly, and is not under control of the will; but it is very abundant
in the body, since it effects practically all the movements of the
alimentary canal and bloodvessels. Voluntary or striped muscle, so called
from its appearance under the low power of a microscope, consists of
long fibres, each containing many nuclei. (See Diagram 18, Fig. 2.) Its
protoplasm is rich in hæmoglobin, and in it, under powerful microscopes,
can be made out two kinds of fibrils: Rutherford’s fibrils, the
complicated structure of which gives muscle its striped appearance; and
Marshall’s fibrils, which are much finer and more difficult to see. The
muscle of the heart, though not under control of the will, is striped;
but it differs from ordinary striped muscle in being made up of small
branched cells with only one nucleus.
The way in which the three elements of striped muscle contribute to a
contraction is practically unknown, and the subject of much dispute. In
fact, one could hardly wish for a better soil for theories, and some
which grow in it are very wonderful indeed. We have reason for supposing
that there are two contractile substances—one which gives a sharp twitch,
the other a slow, hard pull; and on the whole there seems good reason
to believe that Rutherford’s fibrils give the sudden movements, while
Marshall’s give the more forcible ones; and that the ordinary protoplasm
of the cell is restricted to the duty of nourishing the fibrils.
[Illustration: DIAGRAM 19.—STRIPED MUSCLE FIBRE, MORE HIGHLY MAGNIFIED
THAN IN DIAGRAM 18.]
The muscle cells are modified from among those of the bud forming the
middle layers of the embryo. (See Diagram 5.) Other cells of this bud
form connective tissue, by, so to speak, spinning long fibres of the
substance called collagen, which turns to gelatin when boiled. (See
Diagram 20.) This connective tissue permeates the whole body, affording a
firm foundation for the many layers of cells which form the skin and the
single layer of digestive cells; supporting the other organs throughout,
and keeping the different parts of the body in their places, in doing
which, however, it is assisted by other fibres which are not collagenous,
but elastic. It also forms tracts which become lymph and blood vessels.
In parts of the animal which require special support it forms solid
rods, the collagen combining with calcium salts to form a clear, hard
substance—cartilage. At one period in the development of an animal or
animals we find the only solid support is cartilage, but cartilage is not
sufficiently rigid for a very large beast, especially on land, so is only
used for outlying parts, the main framework being bone.
[Illustration: DIAGRAM 20.—A CONNECTIVE-TISSUE CELL GIVING RISE TO LONG
COLLAGENOUS FIBRES.]
Bone is formed very much as if Nature were rectifying a mistake. When a
rod of cartilage is unequal to its work it is eaten hollow, and fresh
connective-tissue cells immigrate and fill up the cavity, eventually
laying down a fine network of cells in its place, the meshes of which
are filled with inorganic calcium salts, chiefly phosphate of lime.
Nature then benefits by experience, and the last bones to be formed are
not preceded by any makeshift cartilage, but built up straight away in
ordinary connective tissue.
This brings us back again to muscle, for the object of nearly all the
voluntary muscle is to cause movement among the bones. For this purpose
the muscle cells or fibres are arranged parallel to one another, and
bound up together by connective tissue, the whole bundle being known
as ‘a muscle.’ The two ends of a muscle are attached to two bones by
connective tissue, which sometimes forms a short cord, or tendon. Then,
when the muscle contracts, the two places of its attachment are pulled
towards one another, and something has to move. But before saying more
about the way in which the bones are jointed and muscles attached—in
fact, what movements are possible in the human body—it would be as well
here to describe the chief properties of muscle and the way in which they
are studied.
II.
[Illustration: DIAGRAM 21.—APPARATUS FOR RECORDING A MUSCULAR
CONTRACTION.]
The way in which voluntary muscle is studied is very simple. A frog is
killed by thrusting a probe into the brain and down the spinal cord, and
a muscle is then dissected out and attached to a piece of apparatus (see
Diagram 21) in such a way that on its contracting it raises a lever,
and draws a line on a moving surface. The rate at which the surface
is moving is ascertained, so that the nature of the curve, which is a
graphic record of the contraction, can be analyzed. (See Diagram 22.) For
instance, when an electric shock is used to make the muscle contract, we
find that a slight shock causes a small contraction, as shown by a low
curve, while a stronger one, up to a certain point, causes an increase.
[Illustration: DIAGRAM 22.—GRAPHIC RECORD OF A RESPONSE TO A SINGLE
STIMULUS APPLIED AT A.
Lower line = tuning-fork records of ⅟₁₀₀″.]
But having described how muscle is studied, it is only necessary to
state a few facts concerning it; to discuss muscle, fully describing the
experiments by which its more obscure properties have been elucidated,
and the devices by which causes of error have been eliminated, would fill
volumes.
[Illustration: DIAGRAM 23.—CONTRACTIONS WITH TWO STIMULI AT DIFFERENT
INTERVALS OF TIME.]
Muscle is thrown into a state of contraction by an impulse reaching it
from a nerve, but it contracts quite as readily if excited directly
by a mechanical or electrical shock. A second shock causes a second
contraction, or, if the muscle is still in a state of contraction owing
to the first, causes it to contract still more. (See Diagram 23.) If a
number of stimuli are applied to a muscle in such rapid succession that
the effect of the preceding one has not passed off by the time the next
arrives, it will contract as far as possible, and remain contracted—a
state known as tetanus. (See Diagram 24.) A muscle is therefore kept in
a state of contraction by a continuous nervous effort, not arranged and
then left contracted.
[Illustration: DIAGRAM 24.—TETANUS.]
[Illustration: DIAGRAM 25.—FATIGUE CURVES.
Fast drum: a, point of stimulation. Every tenth contraction recorded.]
[Illustration: DIAGRAM 26.—EFFECT OF FATIGUE ON MUSCULAR CONTRACTION.
Slow drum. Every contraction recorded.]
Various conditions alter the character of a muscular response. With
repeated stimuli at short intervals a muscle fatigues, and each
contraction becomes smaller in extent and longer in duration. (See
Diagrams 25 and 26.) If the muscle has to lift a load it has a certain
check on its contraction, and its relaxation time is shortened.
Temperature also affects muscular contraction, moderate increase causing
a sharper, and moderate cooling a slower, rise and fall of the lever
on stimulation. (See Diagram 27.) Lastly, we have drugs which exert an
influence, but the only one of these which it is necessary to mention
here is veratria, which makes the slowly contracting fibrils continue
their activity after the quick ones have subsided. (See Diagram 28.)
[Illustration: DIAGRAM 27.—EFFECT OF TEMPERATURE.]
[Illustration: DIAGRAM 28.—VERATRIA CURVE.]
Finally, there are the electrical changes in muscle. These, again, may be
passed over briefly, since they are not easily understood or described.
To put the facts in a nutshell, the part of a muscle which is in activity
is negative to all other parts. Thus, if a muscle be dissected out and
cut across, the activity at the seat of the injury, while it lasts,
causes a current to pass through a galvanometer from uninjured parts
to the wounded. (See Diagram 29.) Again, if a muscle be dissected out
without injury, connected at two points with a galvanometer, and then
stimulated at one end, as the wave of contraction passes along it, first
one, then the other, contact becomes negative. (See Diagram 30.) S,
Stimulating electrodes; N, contraction which marks the wave of excitation
passing along the muscle; G, galvanometer which shows that the seat of
activity (N) is negative to the rest of the muscle.
[Illustration: DIAGRAM 29.—INJURY CURRENT: CROSS-SECTION OF MUSCLE
NEGATIVE TO REST.]
[Illustration: DIAGRAM 30.—ACTION CURRENT.]
In passing, it may be mentioned that, as the heart is a muscle slung
obliquely across the body, and waves of contraction are continually
passing down its long axis, the whole body is affected by continual
electrical changes. By very delicate instruments it can be demonstrated
that with each beat the two hands alternately become electrically
positive and negative to each other.
Whilst dealing with the electrical phenomena of muscle, it may be as well
to state that nerve fibres, which are studied with very much the same
apparatus, show the same electrical changes, the point of injury or of
the greatest activity being negative to all the rest. Single cells are
less easily investigated, but in glands it is possible to show that the
same rule holds.
Undoubtedly the most curious fact about the generation of electricity by
protoplasm is that, by a modification of muscle and nerve, which causes
them to lose their ordinary properties, they are converted into a special
organ for giving electric shocks. Armed with powerful batteries of this
description, an otherwise rather helpless class of fish are enabled to
defend themselves from their enemies, and deal unexpected death to their
more agile prey.
* * * * *
Having now run over a few of the physical properties of protoplasm, we
may pass on to a brief investigation of the movements we find in the body
of man.
III.
In describing the movements of the body, we shall have to treat them as
several and distinct, as indeed they are; but the fact should not be
lost sight of that they cannot really be isolated: one idea embraces the
whole. Two kinds of movement may, however, be distinguished in the vital
functions: movement of the actual cells, such as muscles; and movement of
non-protoplasmic elements acted upon by the cells—_e.g._, lymph.
There is a parallel to this in the chemical side of life, where we
find some phenomena peculiar to the living elements, and others, like
digestion, going on in the living body, but outside the cells.
Taking the movements in the natural order—that is, proceeding from the
simpler to the more complex—the first to be considered is undoubtedly
that of the leucocytes, or general scavengers of the tissues. The body
consists, so far as we have defined its anatomy, of three layers of
cells, and its shape is that of a tube with hollow walls. (See Diagram
6.) Within the cavity of the body are various organs, such as the
muscles, which are formed from the middle layer; and its space is largely
reduced by glands, lungs, and other ramifications of the inner layer
which forms the alimentary canal.
These organs hang more or less freely in the body cavity, slung to its
walls by enveloping sheets of connective tissue, the whole being bathed
in lymph. Now, in such an arrangement the products of wear and tear must
accumulate. Cells here and there die for various reasons, and pieces
of cells become detached even in adult animals. The interior of a bone
is always being eaten away to decrease its weight, or in order that it
may be replaced by fresh bone of a closer texture, and in young animals
and embryos there are many structures which, useful for a time, have
eventually to be removed; as an instance, we may quote the tadpole’s
tail. In fact, if the tissues were left to themselves, the body would
soon be choked with débris, and to avoid this it is supplied with an army
of scavengers, the leucocytes.
The leucocytes are detached cells which owe their origin to the middle
layer. In size they are, of course, very small, quite invisible to the
naked eye. In appearance they resemble unicellular organisms of the
amœba type, which we have had occasion to mention several times already
(Essay II., Section II.; Essay III., Section I., Diagram 1). They are
of several different varieties, some being larger and more active than
others; but they all wander about in the lymph and blood like independent
animals, creeping in and out between the cells of the organs, and
devouring any foreign matter they come across. They sometimes multiply,
like independent animals, by division, especially in the presence of
inflammation, or when they have much work to do, and a rapid increase in
their numbers is needed; and they have been induced to live, and feed,
and multiply, outside the body (in which case they must be considered to
have become independent organisms), thanks to the careful attentions of
the experimenter.
Apart from their duties of devouring the inside layers of bones and
clearing away dead tissue, they are supposed by some to assist in the
absorption of food by creeping between the cells lining the alimentary
canal, and, after throwing out arms to engulf particles of food,
returning with their spoils into the body. Perhaps, however, the most
interesting, or at any rate most romantic, of their many and important
functions are what may be called their emergency duties. Frequently
people, especially those who live in smoky towns, draw into their lungs
particles of dust and soot, which if left adhering to the walls of the
air cavities would cause dangerous irritation. As if by magic a leucocyte
will discover the presence of such a nuisance, and, crawling between
the cells forming the wall of the lung, in which, by the way, it is
outside the body proper, will engulf it and carry it away with him. This
exploit, however, pales beside the warfare which goes on in the body
between leucocytes and invading bacteria. A bacterium thrives in the
blood or lymph, since it finds itself in a warm alkaline fluid containing
complex organic substances, by breaking down which it can easily obtain
energy. Unfortunately, the products of such a process are frequently
virulent poisons, the effect of which upon neighbouring cells produces
the distressing symptoms which we associate with disease. No sooner,
however, has the bacterium begun to generate poisons, than leucocytes,
influenced by chemical attraction (Essay I.), swarm upon it. First come
leucocytes of a small kind, full of zymogen granules, which crowd round
the bacterium till they have covered it. After a time they creep away,
leaving it dead. They are now in an exhausted condition, and no longer
contain granules, having doubtless discharged them as a destructive
ferment upon their enemy. Then a leucocyte of another kind moves to
the attack, or, rather, to clear up the remains, for he is a large,
non-granular, active fellow, and eats up the dead bacterium by the simple
process of engulfing him whole. (See Diagram 31.)
A natural question arising out of the study of leucocytes is, What
becomes of them? Particles of soot and similar refuse can hardly be
considered nutritious, or even digestible, food, and one is rather
drawn to the conclusion that the leucocyte performs its functions for
the good of the body at large, not of itself, and that when its work is
done it must die. Many leucocytes, probably, loaded with unconsidered and
undesirable trifles, cast themselves into the alimentary canal, and are
got rid of with the useless portions of the food; but they do not always
have the luck or energy to get to a natural outlet. An unpleasantly
familiar phenomenon is the boil. Here we have some irritating substance
under the skin setting up inflammation, and leucocytes swarm up to remove
the cause of the trouble. Before, however, this is done, many have
perished in the fray, and they have collected in numbers to the formation
of what is commonly known as pus, or matter. Their dispersal into the
body is now neither easy nor desirable, and the surgeon usually lets them
escape from the surface by a touch of the lancet.
[Illustration: DIAGRAM 31.
A, Eosinophile leucocyte; B, bacterium; C, leucocytes killing bacterium
with their enzyme; D, leucocytes leaving bacterium dead; E, hyaline
leucocyte devouring dead bacterium.]
Such, then, is very briefly the story of the leucocyte, neglecting such
problems as the differences between those found in the blood, called
white corpuscles to distinguish them from the red corpuscles, with
which they have no sort of connection; those found in the lymph, called
lymphocytes to distinguish them from those found in the blood; those
caught in the act of devouring bone, called osteoclasts; and those found
with bacteria inside them, therefore known as phagocytes; and without
speculating on how long an individual lives, and whether the different
varieties differ in origin or are merely at progressive stages of
development. The study of leucocytes is one of the most fascinating in
physiology, but we have many other things calling for our attention, and
we have said enough about the part they play in the life of the body to
justify our passing on to consider another essential movement.
IV.
Next in natural order for consideration come the movements of the
alimentary canal.
So far we have considered this structure as a chemical laboratory, a tube
consisting of a single layer of cells which secrete ferments into the
lumen, where digestion takes place, and then absorb the products, and we
have not yet accounted for the food travelling along the tube, without
which its functions, as described in the earlier part of the book, could
not be performed. That the passage of the food is not due to gravitation
is obvious from the many directions of the tube’s coils—not to quote the
old instance of a horse drinking, in which case the liquid first travels
upwards. One must therefore conclude in favour of some muscular method of
propulsion.
We have so far described the alimentary canal as a single layer of cells,
but it must be obvious that these soft secreting portions of the tube are
not capable of vigorous movement. The canal proper is surrounded by a
tough sheath of connective tissue which prevents its being overdistended
or ruptured, and, by means of a layer—or, rather, two layers—of
non-striped muscle which it contains, produces the movements which result
in the passage of its contents along the tube. These two layers lie well
to the outside of the connective-tissue sheath. The fibres of the inner
layer are arranged circularly, so as to form rings round the tube; those
of the outer have a longitudinal direction, running, therefore, parallel
with its long axis. When the former contract, the diameter of the tube is
reduced, while contraction of the latter has the effect of enlarging it.
(See Diagram 32.)
The movements of the intestine are what is known as peristaltic.
Contraction of the muscle fibres is not simultaneous in all parts, but
passes in waves along it. Just in front of the food the longitudinal
fibres contract, and thus offer less resistance, while just behind the
circular fibres reduce the size of the tube, and so get up a pressure.
The result of a number of successive waves of contraction passing down
the alimentary canal is that the food is propelled along it.
[Illustration: DIAGRAM 32.—TO ILLUSTRATE THE PASSAGE OF FOOD ALONG THE
INTESTINE.]
The arrangement of the muscle varies in places to suit special needs.
Where the tube suddenly enlarges to form the stomach, and where the
stomach suddenly narrows to the intestine, there are two strong rings
of muscle, whose constricting influence converts the enlargement into a
closed chamber during gastric digestion; while the coats which actually
clothe it here run obliquely, and their activity causes the contents to
be slowly churned about inside.
Thus it will be seen that it is not only the voluntary muscles which
give the alimentary system its opportunities; without these unobtrusive
non-striped cells we should toil for our bread and swallow it in vain.
V.
Our next step, after having surveyed the principle of movement by which
the chemical necessities of the body are exposed to its absorbing
surface, must be to see how the fluid which transports them is made to
pass along the tubes containing it. We have already had occasion to
describe how these blood and lymph vessels ramify through all the organs,
when we were dealing with the chemical influence of the blood and lymph.
The tubes through which the lymph is brought back to the blood-stream
have thin walls, and no muscle of their own. They are subjected, however,
to a constantly varying pressure by the movements of the limbs and
trunk, and as, owing to valves inside them, the lymph can only escape in
one direction, there is a constant flow towards the junction with the
bloodvessels.
The bloodvessels are quite different. A far more certain and expeditious
current is necessary—hence the steady circulation through a system of
closed tubes.
In order to understand this passage of the blood, it is necessary to keep
in mind the great principle with which hydrostatics supplies us, viz.,
that a liquid always flows from a region of high to a region of lower
pressure. The problem of the vascular system is, therefore: How can the
pressure within a ring of tube be so arranged as to maintain a regular
flow always in the same direction?
Let us begin with the structure of the system. The tube through which
the blood first passes on leaving the heart is composed of four distinct
and essential elements: A lining of endothelial cells, which we need not
discuss at length; a main substance of tough white fibrous connective
tissue; elastic fibres and muscle fibres, the two last arranged in the
substance of the connective tissue. All these parts are present in
the main arteries which leave the heart, but in the fine meshwork of
capillaries to which the arteries give rise by repeated branchings there
is nothing left of the outer coats, only the lining of endothelial cells
separating the blood from the organ traversed. In the veins which these
capillaries unite to form, the connective-tissue sheath reappears, and
also some muscle; but the elastic coat is quite absent. The heart is
really a double coil of the tube (see Diagram 12), in which the muscular
coat is predominant, and is divided into four chambers by the valves,
which insure the blood flowing in the right direction when it contracts.
(See Diagram 33.)
[Illustration: DIAGRAM 33.—SCHEME OF CIRCULATION]
The way in which these structures work is as follows: Two of the chambers
of the heart (the auricles) receive blood from the veins, and when full
suddenly contract, driving their contents into the other two chambers
(the ventricles). The blood does not run back into the veins, although
the pressure in them is very low and there are no valves to prevent
it, because there is still less pressure in the ventricles, and also
because the veins enter the auricles obliquely, and the tendency of the
increasing pressure is to close their orifices. Having discharged the
blood into the ventricles, the auricles relax, and the pressure within
being a minus quantity, they are speedily filled with blood from the
veins, blood not being able to return after entering the ventricles, as
valves close automatically to prevent it.
Stimulated by the blood distending them, the ventricles then contract
simultaneously like the auricles, only with much greater force: for the
right ventricle has to drive the blood all through the vessels pervading
the lungs back to the left auricle; whilst the left ventricle, which is
proportionately stronger than the right, has to send its contents to
the furthest extremities of the body. They then relax, in order that
conditions of their internal pressure may favour another inflow from the
auricles, return of blood from the arteries being, as in the preceding
case, prevented by valves.
The pressure in the arteries during life is always fairly high; indeed,
the ventricles have to get up a considerable force before the valves
leading from them will open. The result of this is not only that the
blood is driven along them with a rush, but also that they are slightly
distended at each beat; and so, owing to the elasticity of their walls,
the blood continues to flow forwards even between the beats of the heart.
The rest of the journey is quite simple; the pressure in the capillaries
is lower than in the arteries, and the pressure in the veins lower than
in the capillaries, and lower in the veins, too, as they approach the
heart, till, where they join the auricle, it is actually minus, and the
blood has no other course open to it but to return to the auricle. It
looks as though accidents might happen in the veins owing to there being
so low a pressure there to direct the current, but this is prevented by
the presence of valves at intervals, to stop any return.
The rate at which the blood travels is another point which has an
important bearing on the nutrition. It does its work—_i.e._, gives out
nutriment and picks up refuse—whilst flowing through the capillaries; so
here one finds that it moves slowly. On the other hand, the sooner it
reaches them the better, so it races fast through the arteries. Finally,
its return to the heart need not be delayed, so it is quickened up again
through the veins. The principle by which this variation in the rate
of flow is obtained is simple and inevitable. If a tube through which
liquid is flowing is not the same size all the way along, the liquid
will be found to flow faster in the narrow parts than in the wider ones.
Now, in branching, the arteries do not keep becoming smaller in regular
proportion, and the result is that the capillaries have collectively a
diameter five hundred times larger than the aorta; hence the blood flows
through them only one-five-hundredth of the pace at which it leaves the
heart. But in uniting again to form the veins their cross-section is
reduced once more, so that that of the large veins near the heart is only
two and a half times larger than that of the aorta, and hence a flow only
two and a half times slower results.
The pace of the blood-stream must depend, obviously, on the pressure of
the blood in the arteries. This pressure is altered either by changing
the rapidity of the heart-beat or the diameter of the arteries, which
are capable of considerable variation owing to their muscular coat. The
regulation of the blood-pressure is managed by the nervous system, so
does not belong here, and we may leave it after mentioning one or two
facts. High pressure is due to a large quantity of blood being in the
arteries, and this may be due either to the rapidity with which it is
injected by the heart or to the reduced capacity of the bloodvessels
themselves. High pressure, due to the latter cause, throws a great strain
upon the heart, owing to the hard work it has in pumping blood into
the arteries; with a low pressure the heart beats feebly, having less
resistance to overcome.
Blood-pressure can be raised by stimulating the muscular coat and
reducing the capacity of the bloodvessels, and lowered by causing the
heart to beat more slowly or by removing blood from the body. This
latter operation was a favourite way with doctors of the old school; but
as our knowledge of physiology, and with it our control over the vital
functions, increases, such crude and heroic remedies are able to be
replaced by others which are less dangerous.
VI.
Comparisons are rightly regarded as objectionable, so it would hardly be
safe to say that the group of movements whose primary object is filling
the lungs, and which we must study next, is the most important in the
body, especially when we have just been speaking of the circulation,
which, however, would be of but little use if the blood could not
be oxidized; but we can at least say that its importance cannot be
overrated, so far-reaching are its effects.
The lungs are, as we have described them above, a pair of delicate
membranous sacs connected by a tube, the trachea, with the alimentary
canal, from which they originally budded out. They are subdivided,
though how we need not describe in detail, into a vast number of small
compartments, so as to give the maximum surface in the space accorded
them, and the whole somewhat resembles a cluster of grapes, the stalks
being the branches of the trachea. The membranous parts are pervaded by
an elastic network, enveloping the compartments in such a way that it
would reduce them permanently to the resemblance of a bunch of raisins
rather than grapes, were it not that they are enclosed in an airtight
box—the thorax—from the walls of which they cannot shrink without causing
a vacuum. Owing, however, to the latter arrangement and the trachea being
open to the external world, they are always more or less distended with
air.
The thorax, which they thus must always exactly fill, is a conical-shaped
box, its walls being the ribs, and its floor a sheet of muscle known as
the diaphragm. It contains, besides the lungs, only the heart and large
bloodvessels. The problem, therefore, of drawing air into the lungs and
(after the gaseous interchange described in Essay II., Section IV., has
taken place) of expelling it again, becomes solely a matter of increasing
and decreasing the capacity of the thorax. (See Diagram 34.) This can
be done in two ways: the diameter through the ribs can be increased, or
the diaphragm can be pulled down, increasing its depth. Actually, both
these methods come into play together. Diagram 35 will probably give a
better idea of how this is done than could easily be conveyed by a verbal
description. An attempt is here made to show the action of the ribs and
the diaphragm—first, of each separately, then of the two combined. The
elasticity of the lungs themselves is sufficient to drive out the tidal
air if the diaphragm and the muscles of the ribs are relaxed, though in
hard breathing a muscular movement may depress the ribs and a contraction
of the abdominal muscles force up the diaphragm.
[Illustration: DIAGRAM 34.—MODEL (ADAPTED FROM RUTHERFORD) FOR SHOWING
HOW THE LUNGS ARE FILLED WITH AIR BY ALTERING THE SIZE OF THE THORAX.]
But though the primary object of raising the ribs and depressing the
diaphragm may be to fill the lungs, its secondary influence upon
the trunk as a whole is hardly less important. The effect upon the
circulation is profound. The compartments of the lungs are enveloped in
innumerable capillary bloodvessels, and, as these lie around and between
them in the cavity of the thorax, they must, when breath is drawn in, be
subjected to a negative pressure before the lung itself, and be the first
to experience a positive pressure when the air is expelled. Here, again,
a diagram is the best explanation. (See Diagram 36.)
The pulmonary vessels, moreover, are not the only ones influenced. The
reader who attentively examined Diagram 13 must have been struck by the
peculiarities of the circulation through the spleen, intestine and liver,
and the obstacles which this repeated breaking up into fine vessels must
offer to the flow of blood, as described in Section V. of this essay.
[Illustration: DIAGRAM 35.—SHOWING HOW THE CAPACITY OF THE THORAX IS
INCREASED BY RAISING THE RIBS AND DEPRESSING THE DIAPHRAGM.]
[Illustration: DIAGRAM 36.—MODEL FOR SHOWING EFFECT OF MOVEMENTS OF THE
THORAX ON THE PULMONARY CIRCULATION.]
The liver forms the crux of the situation. (See Diagram 37.) A vein
carrying blood from the intestine and spleen is broken up into fine
capillaries to pass through that organ, and the pressure in this vein
is extremely low. How is a sufficiently rapid flow of blood to be
maintained? The answer to this riddle is best given by Diagram 38, which
shows how, by the contraction of the diaphragm at each breath, the large
veins entering the heart are subjected to a negative pressure which draws
blood out of the liver, while, simultaneously, that organ is squeezed and
the blood it contains forced out. Obviously this natural pump influences
not only the flow of blood, but also that of the lymph, and what was
said about the hepatic vessels also holds good for the thoracic duct, up
which the lymph, rich with fat absorbed from the intestine, passes to be
emptied into the large veins near the heart. So, though vigour in the
action of the diaphragm is more favourable to health than necessary to
life, deep breathing is an essential factor in the well-being of the body.
[Illustration: DIAGRAM 37.—A DIAGRAMMATIC VIEW OF THE CIRCULATION THROUGH
THE ORGANS UPON WHICH THE DIAPHRAGM PRESSES WHEN IT DESCENDS.]
[Illustration: DIAGRAM 38.—ILLUSTRATING THE INFLUENCE OF THE DIAPHRAGM
UPON THE CIRCULATION THROUGH THE VISCERA.]
VII.
All the movements as yet described are absolutely necessary to the
continuation of life; they are, moreover, independent of the efforts
of the will. But there remains yet another kind of movement without
which the body, left to itself, would die. This is the movement of the
limbs—organs by which the body is able to move from one place to another,
to capture its food and convey it, viâ the mouth, to its stomach—in a
word, to satisfy its chemical and physical needs.
To understand how the limbs work requires a knowledge of their anatomy,
for which we have not time or space here; but the principle throughout is
that of a system of levers, the bones, worked by the voluntary muscles.
Here, as before, a diagram will probably be found to convey more than
could ever be expressed in words. (See Diagram 39.)
The diagram represents very roughly, but it is hoped very plainly, the
main principle of the elbow-joint; but for an exact knowledge of the
mechanism of the joint, and the comparative strain upon, and therefore
strength of, each muscle, the reader must consult some work on anatomy.
He will there find, if he goes on to read the description of the hand,
what a wonderful precision, complexity, and amount of movement can be
obtained by variations of this simple device.
[Illustration: DIAGRAM 39.—DIAGRAM OF THE ARM.
1, Lifting; 2, pressing.]
Here, however, we must leave the study of the manner and object of the
bodily movements, and proceed to investigate the far more intricate
question of how they originate and are controlled.
ESSAY IV.
THE NERVOUS SYSTEM.
I.
Now comes the final problem. Protoplasm forms a structure always
changing, always making good its waste by chemical action upon raw
material, always capturing raw material or in search of it, always,
when it exists in large quantities, and the labour is therefore divided
between many cells, economically apportioning the work and the spoils.
How is it that all the actions, chemical as well as physical, of a vast
number of cells composing a large body are, no matter how complicated,
always harmonious, and always with purpose directed to the advantage of
the whole animal?
In the first essay in this book we discussed the phenomenon of life, and
described briefly the chemical and physical peculiarities of protoplasm.
These in the two succeeding essays we have gone into more fully; but
there is one characteristic of that interesting substance which yet
remains for us to examine in specialized cells, viz., its extreme
readiness to respond to changes in its environment.
In Essay I. we saw that chemical agents, light, heat, electricity,
etc.—had a definite effect upon protoplasm, and that, though they might
influence different kinds in different ways, the effect was nevertheless
invariable; in a word, the response of protoplasm to circumstances is
automatic. But the most remarkable thing about this is that the response
is not confined to the protoplasm actually affected, but is transmitted
to that nearest to the part stimulated, and again passed on to that
beyond, so that a wave of excitation passes through the whole mass, not
stopping till it has reached the extreme confines of the cell. It may
even pass beyond these and set up activity in neighbouring cells. The
power of conductivity once grasped, it may easily be seen that certain
cells, by specializing in this direction and adapting their shape to the
needs of the body, might by throwing out long threads to reach distant
parts set up an organic system of telegraphy.
The organs developed for the control of the body owe their origin to
the outer layer. (See Diagram 5.) This was only to be expected. In the
second essay, in which we treated of the chemistry of the body, we, of
course, touched upon all three layers from which the body is built up;
but the one which chiefly occupied our attention was the innermost layer,
which is so admirably arranged as a chemical laboratory. In the third
essay we dealt chiefly with the middle layer, which both by its position
and its bulk might have been guessed to be the foundation of most of
the motor organs. Now that we have come to the organs of perception and
transmission of impressions, it is only natural to expect that they
should be specialized from the cells already in contact with the external
world, and which, since they form the envelope of the animal, must allow
all such stimuli as reach the subjacent motor layer to pass through them.
Hitherto we have not dealt at great length with the development of the
organs whose functions we have been describing, either from the point
of view of the embryologist or the evolutionist. Nor have we spent much
time upon their gross anatomy. With the nervous system we must proceed
rather differently; for to understand how its higher functions can be
performed they must be traced from their origin step by step, while their
complexity is largely vested in the structure of special organs.
The way in which the nervous system was evolved is shown in Diagram 5.
Originally, no doubt, the cells of the outer layer, when the latter was
in its simplest form—that is to say, only one cell thick, not several,
as it is in our skin—would, when influenced in any way directly call
forth the activity of the motor cells lying beneath them. (See Diagram
40, Fig. 1.) In Fig. 2, however, we see one cell of the outer layer
becoming specialized. It has thrown out a process above the surface of
the skin the more readily to catch impressions, and has sent another down
into the body the better to distribute them. Diagram 41, Fig. 1 shows the
nerve cell at a further stage. The principle is the same, but the cell
is removed to a safer place. In Fig. 2 it is not exposed to the outside
world at all, but by receiving its impulses second-hand from several
cells the same work is done with greater economy and uniformity. Some of
the special sense organs are still developed in this way.
[Illustration: DIAGRAM 40.—SHOWING ORIGIN OF A NERVE CELL.]
[Illustration: DIAGRAM 41.—SHOWING THE DEVELOPMENT OF A NERVE CELL.]
Once the nerve cell is developed and safely shifted into the interior of
the body, it is clothed with a protecting feltwork of connective tissue,
and the nerve fibres are also surrounded by connective-tissue cells which
secrete around them the fatty substance which makes nerves look white.
Such is the nerve cell or intermediary between the world and the muscles;
but thence to harmonious movement in a body with complex organs capable
of varied actions is a long step. To obtain precision and uniformity
throughout the body, all the impressions received must be collected and
balanced, and stimuli, the correct outcome of this balancing, must be
transmitted to the muscles, glands, etc., whose activity circumstances
require. The way in which cells of the outer layer become enclosed to
form a central nervous system is shown in Diagram 5; but its development
will be better seen in the figures of Diagram 42.
[Illustration: DIAGRAM 42.—TO ILLUSTRATE THE DEVELOPMENT OF THE NERVOUS
SYSTEM.]
[Illustration: DIAGRAM 43.—CROSS-SECTION OF THE SPINAL CORD, SHOWING HOW
IT GIVES OFF NERVES.]
[Illustration: DIAGRAM 44.]
This diagram shows how certain cells of the outer layer are budded off
and transferred to a safe place within the body. In this position the
cells are further developed, throwing out one long fibre, which goes to
some distant organ of the body, and short fibres, which, though they do
not join those of other cells and become continuous, closely interlace
and put them into communication. They are also separated from one another
by connective tissue, which supports them, holding them suspended with
only their fibres approaching one another (Diagram 43). Diagram 44 shows
how the bone which replaces the supporting rod (see Diagram 6) throws an
arch round the feltwork of connective tissue in which the nerve cells are
suspended, giving them still further protection.
It will be noticed in the figures of Diagram 42, which is fuller than
Diagram 5, that there are three of these buds—one central and two
lateral. The central one becomes a tube running the whole length of
the animal, while the lateral buds form solid clusters or ganglia,
arranged in pairs at intervals beside it (see Diagram 45). Fibres from
these ganglia go to the skin, and bring to the nerve cells information
from the outside world, which they duly pass on to the cells of the
central column. The cells of the central column, when set in motion by
the ganglion cells, send out impulses to the muscles, whose contraction
is necessary to perform the movement which circumstances indicate. A
movement brought about in this way is called reflex.
[Illustration: DIAGRAM 45.—CENTRAL NERVE TUBE AND GANGLIA.]
The reflex movements are, however, not quite the simplest. For instance,
the food is moved along the alimentary canal by the contraction of two
sets of muscle fibres—an outer longitudinal coat and an inner circular
one. Between these two coats are some nerve cells, which are thrown into
activity by the presence of food and the iron compounds of the bile
secreted by the liver in the tube. These sympathetic cells do not send
their impulses to any centre for examination, but at once stimulate the
muscle fibres between which they lie, thereby producing the peristaltic
movements we have already described. Yet it should be remembered that,
though these cells act independently of the central nervous system, they
are under its control, and can, if need be, have their action modified
for the benefit of the body as a whole.
For convenience’ sake, we had better here specify the chief kinds of
nervous action. First there is what we may call the immediate nerve
action, such as that we have just been describing; secondly there is
reflex action, the centres for which are in the spinal cord and the base
of the brain; and thirdly there is voluntary movement, which arises out
of the interaction of centres in the hemisphere of the brain, where the
most complex machinery of all is kept.
II.
Of the first kind we need say no more. The instance of peristaltic
movement illustrates it sufficiently; so we can at once begin a more
careful examination of reflex action.
The simplest instance of reflex action may be taken from the schoolroom.
If a boy suddenly sticks a pin into an unsuspecting schoolfellow, the
latter invariably starts, and frequently lets fall an exclamation also.
In this case the presence of an injurious agency is reported to the
nearest motor centre, which is in the spinal cord, and this automatically
convulses the body, jerking the limb out of danger.
This is reflex movement; the nerve fibre, which conveys an intimation of
the injurious influence, is a prolongation, or really two prolongations,
of a spinal ganglion cell. (See Diagram 46.) The near end of this fibre,
which enters the cord, has several branches. Some run a little way up the
cord, and some a little way down, so as to communicate with several motor
cells; but one branch runs right up the cord, and sends the message on to
the brain. Our outraged schoolboy starts a fraction of a second before
he is conscious of the pain of being pricked, and this first response
is involuntary and unvarying; the sensation, however, is reported to
his brain, and the workings of that wonderful organ are less easy to
predict. It leads to his taking stock of the aggressor, on the strength
of which he decides whether it is safe to attempt a reprisal, and, if so,
in what form it will be most effective and least likely to attract the
master’s attention. This knotty point settled, the motor cells of the
brain send down messages to the motor cells of different parts of the
spinal cord, and these in turn set the necessary muscles in motion for
delivering a surreptitious kick or aiming a splash of ink, as the case
may be. This is voluntary movement.
[Illustration: DIAGRAM 46.—EVOLUTION OF A SPINAL GANGLION CELL.]
[Illustration: DIAGRAM 47.—SCHEME OF THE CENTRAL NERVOUS SYSTEM.
→ shows the path taken by an impulse in reflex action.
↣ shows the path for a voluntary action.]
The difference between reflex and voluntary movement is, as may be seen
from the above instances, very much a matter of degree; but we had better
leave a comparison between them, and any discussion as to the extent to
which the manifestations of consciousness are automatic, until we have
finished describing reflex movement, and set forth the little we know
about voluntary movement.
Time and space forbid a complete list of reflex movements. The following
are, however, a few typical examples of how the body is automatically
made to perform such acts as are necessary, and of how such as do not
require deliberation are brought about without taxing the intellect.
A reflex action which is unpleasantly familiar is the cough, also the
somewhat similar phenomenon of the sneeze. In this case, a foreign body
which obstructs the windpipe, or causes irritation to the membrane lining
the nose, is, on being reported at the spinal cord, incontinently blown
out by an explosive blast of air from the lungs.
An organ which is very important, and at the same time very
sensitive—viz., the eye—has many protective reflexes. The external
surface of the eye is covered by a very delicate membrane, which must be
kept moist and scrupulously clean. Whenever this membrane gets in the
least dry, or any dust falls on it, the eyelids are closed for a moment,
thereby bathing it with the secretion of the tear glands. Few people are
aware, I think, that they blink their eyes on an average twice every
minute. The eyes are also closed quite involuntarily by a reflex when any
danger threatens them—for instance, a sudden dazzling light, a strong
wind, or a blow aimed at the face; and if any foreign substance—say
a fly—does get into one of them, the secretion of the tear glands is
enormously increased to wash it out.
The size of the pupil, again, is quite involuntarily, _i.e._, reflexly,
altered in proportion to the strength of the light.
Reflex actions are, however, by no means only protective. The act of
swallowing is reflex. So is the secretion of the digestive glands when
the lining membranes of the stomach are stimulated by the presence of
food. The very act of standing depends on the reflex principle, the
tendency of the body to collapse and fall being unconsciously perceived
and corrected by the spinal cord. Walking is also a reflex action. It may
be objected that we think about walking, and do so with intention; but
it is of common experience that we can walk along ‘thinking of something
else,’ and the way in which an intellectual though absent-minded man will
run into people, charge lamp-posts, trip over steps, and tread upon dogs,
is sufficient to absolve the organ of thought and intention from any
share in the performance.
The blood-pressure is also automatically regulated, both the diameter
of the bloodvessels and the frequency of the heart-beat being under
reflex control; and we may, as a final instance of reflex action,
describe one of Nature’s most perfect and merciful contrivances—fainting.
Suppose a man receives a severe wound—say, has his hand struck off by
a sword—the shock to his system causes an immediate dilatation of the
large bloodvessels of the abdomen; this results in a great fall of
blood-pressure, and the heart, finding that it has much less resistance
to overcome, slackens its beats so that soon the flow of blood is very
slow indeed. Hence, it has time to clot over the wound, and the man
does not bleed to death. Incidentally, the feeble current of blood is
insufficient to keep the most delicate organ of the body, the brain, in
its normal state of activity, and the man is relieved from his pain by
unconsciousness, which passes off when the heart again quickens its beat.
It is perhaps needless to remark that fainting fits are not always and
only caused by flesh wounds; they may be due to weakness or other causes.
Now, if we consider the instances quoted above, we are able to deduce a
few general principles from them. In the first place, it may be noticed
that reflex action compels us to perform the movements necessary to our
existence whether we like it or no. It is not for us to decide whether we
will breathe or not. We must. The strongest-willed man who ever lived,
no matter how much a philosopher, could not commit suicide by holding
his breath, as Cato boasted he could. Directly he lost consciousness,
supposing he managed to hold out till then, the tainted blood bathing
the respiratory centre would awake it to activity, and he would start
breathing afresh. Again, it is noticeable that many of these actions
could not possibly be performed by a voluntary effort. We can, to a
certain extent, regulate the depth and frequency of our breathing, and
we can blink our eyes voluntarily; but an average man would be quite
at a loss what to do if asked to make the pupil of his eye dilate and
contract, the glands of his stomach secrete, or his heart alter its
rhythm.
It is a familiar fact that some reflex actions can be altered by an
effort of the will; in other words, an impulse from a brain cell will
prevent a nerve cell in the spinal cord from discharging. But it is an
equally familiar fact that with continuous stimulation the impulses
accumulate and ultimately overcome this resistance. Most people have at
some time or other striven to resist the inclination to cough, consequent
upon a tickling sensation in the throat, and know that there comes a
time when they can restrain themselves no longer. This is because the
accumulated stimuli from the throat, having reached a greater strength
than the prohibitive impulse from the brain, succeed in compelling the
cells in the cord to discharge.
Lastly, reflexes can be learnt. When a young child first endeavours to
stand upright, the sensation of falling is doubtless conveyed to the
brain, and thought taken of how the erect position can be maintained. But
it is not until after many experiments and failures that the brain-cells
can send messages to the right cells in the cord, and these set the
necessary muscles in motion. Experience teaches what must be done, and
constant practice eventually enables the spinal cord to act for itself
without referring for orders to the brain. It is on the same principle
that we learn to ride the bicycle. At first we have to devote our whole
attention to keeping our balance, but in a short time we find we are
doing it with our mind free to contemplate the scenery.
What can be done by reflex action can only be appreciated by observing
an animal from which the brain has been removed. A frog which has been
treated in this way—the operation, it should be said, if performed under
an anæsthetic can cause no pain, either at the moment or afterwards—will
live for weeks—in fact, almost indefinitely—if proper precautions be
taken. But it is an automaton pure and simple. Unless touched it sits
absolutely still. If touched it hops once or twice straight ahead
regardless of obstacles. If placed in water it swims, equally regardless
of obstacles. If turned on its back it immediately resumes its normal
position. If small chips of wood are placed on its back it kicks them
off. If the table on which it is sitting be tilted it will crawl up
the incline until it reaches a level. But it will starve in the midst
of plenty, having lost all power of thought, memory and perception. If
diligently fed by hand a frog, a fish, or a bird will live for a long
time without any brain, since their repertoire of movements is small and
mostly reflex, and their occasions for deliberated action comparatively
few. But the higher we get in the scale of life the more the brain takes
over the duties of the cord, the less automatic become the greater number
of the actions, and hence the more open does the animal’s conduct lie to
moral criticism.
III.
We have now seen how protoplasm exists in a large body, sharing the work
of living amongst specialized cells, and how it responds as a whole to
the influences exerted upon it by its surroundings. The next thing to
consider is how it is situated with regard to matter which does not form
part of its own body; how protected from, and how put into communication
with, the rest of the universe.
With regard to the former, we have seen that in the single cells,
constituting unicellular organisms, there is always a bounding membrane
of denser texture than the rest of the protoplasm. As the cell develops
its capabilities, we have a shell or case of non-living matter secreted
around it, with apertures for communication with the outside world, and
increasingly effective protection is provided as protoplasm, whether
in the single cell or the body, leaves the water, and has to face the
inclemencies of terrestrial life.
[Illustration: DIAGRAM 48.—SHOWING THE FORMATION OF THE SKIN.]
[Illustration: DIAGRAM 49.—STRUCTURE OF SKIN.]
In the schematic embryo (Diagram 6) and other diagrams contained in
this volume, the skin has so far been represented as consisting of a
single layer of living cells; but we must now admit that the skin of man
is quite different. Such a covering would be no protection from heat,
cold, or irritating chemicals, while, in order to prevent its drying
up, it would have to be kept moist with slime, and we should look very
like frogs. In order that an adequate defence may be provided for the
body, this layer of cells divides tangentially, forming two layers. The
inner of these two then divides tangentially again, and a second layer
is interposed between the innermost and that first formed. The skin
now consists of three layers, and so the process is repeated until
it is several layers thick. (See Diagram 48.) It is the innermost and
best-nourished layer which keeps dividing; the other layers, as they
get pushed outwards, are only reached by a little lymph which filters
between the cells, and are eventually starved even of that. As they
get pushed away from the dividing layer, however, they set to work to
surround themselves with a horny wall, which thickens and thickens, until
eventually there is hardly any cell left. (See Diagram 49.) Finally
the cells die and the horny envelopes form a dead cuticle, protecting
the living layers beneath, and are ultimately sloughed off when their
successors are ready to replace them.
[Illustration: DIAGRAM 50.—SHOWING THE DEVELOPMENT OF HAIR.]
Not even a horny layer of dead cells is, however, always sufficient
protection, and the growing layer has sometimes to supplement it by
hair or feathers. How hair is developed is shown in the accompanying
diagram (50). The growing layer sends a strand straight downwards into
the connective tissue, which forms the basement of the skin. The cells
in the middle of this strand, which behaves like ordinary skin, are the
least well nourished, and accordingly die and leave a tube. This tube,
if no further development took place, might become a sweat gland; but if
it is to give rise to a hair it becomes cup-shaped at the base, enclosing
a small loop of bloodvessel. The cells just above the capillary, being
better nourished than the rest, grow more rapidly than their neighbours,
and the result is that a column of cells which we know as a hair pushes
its way up through the tube. (See Diagram 50.)
This outer layer comes everywhere between the main bulk of the body and
the outer world. Hair and sweat glands do not by any means represent its
only modifications. Teeth are formed from it in somewhat the same manner
as hair, while we have already seen that it gives rise to the whole
nervous system.
The next thing which we have to consider is how knowledge of the
external world reaches the central nervous system. Sensations of touch,
temperature, and pain are fairly easy to understand, since the nerves
which convey such impressions have numerous endings in the skin. End
organs of nerves in the joints and muscles doubtless enable the animal to
perceive and estimate strain and resistance in moving or lifting things.
But the power of perceiving the chemical peculiarities of things; light,
involving the formation of visual images, which we call seeing; sound;
and position and equilibrium, it is not possible for the whole surface
of the body to possess. The principle of division of labour is extended
to the task of perception as well as to that of motion; and cells, with
their property of responding to light, vibration, chemical stimulation,
etc., are grouped together to form special organs, connected with the
central nervous system by special nerves.
Perhaps the most important factor which can influence protoplasm is the
chemical nature of its surroundings; and in the first essay, on the
general nature of protoplasm, we touched upon the way in which it is
drawn towards some substances, and repelled by others.
In the body there are two sets of cells deputed to act for the rest in
this particular. One set is situated in the membrane lining the nose,
over which the air we breathe passes; and these cells examine our
gaseous surroundings, and warn us, by what we term ‘smell,’ whether the
atmosphere is fit for us or we had better seek a purer. The other set
is for the examination of liquids. Against these we are protected by
our skin, and, as we do not absorb anything through it, it is devoid of
the power of examining the things it touches. But with our food it is
different; we must have the power of testing that. Accordingly, there
are Customs officers in our mouth in the form of little groups of cells,
which report upon the liquids and solids moistened by saliva, and enable
the animal to reject pernicious imports. Thus, the stimulation of a small
portion of the protoplasm composing a body is transmitted over the whole,
and is able to awake in it the necessary response.
[Illustration: DIAGRAM 51.]
[Illustration: DIAGRAM 52.]
[Illustration: DIAGRAM 53.]
[Illustration: DIAGRAM 54.]
[Illustration: DIAGRAM 55.]
[Illustration: DIAGRAM 56.]
So much for the chemical sense organs; they are comparatively simple.
But between a single cell, which always makes towards or always hurries
out of a ray of light passing through the water in which it swims, and
an animal with eyes capable of recognising the colour, shape, size, and
distance of objects in space, there really does seem to be a wide gulf.
It is not, however, too wide to be bridged.
After the single-cell stage has been passed, and we have beasts
consisting of an inner layer of cells which is digestive in function,
and an outer layer which is protective, motor, and sensory, the power
of perceiving light is doubtless vested in the outer layer. When we get
beasts consisting of three layers progressing along the straight path
of development which leads to man, we find the outer layer becoming
too opaque for this purpose, and the torch is handed on to the sensory
tube derived from it. (See Diagram 5.) As more and more protection is
required, the skin thickens, and the neural tube comes to lie deeper,
as in Diagram 51. In order not to lose the light altogether, it has to
throw out buds, which concentrate in themselves the peculiar faculty of
perceiving it, and at the same time little pits are formed in the skin
just over them to help the light to reach them. (See Diagram 52.) In
Diagram 53 both the nervous elements and the integumentary are developing
their possibilities; and in Diagram 54 a large surface has been prepared
for the reception of light, and a lens formed to focus the rays upon it.
Diagrams 55 and 56 give the concluding stages in the development of the
eye: the formation of the cornea and its protecting eyelids. The two
cavities are filled with clear liquids, and the whole eyeball supported
by connective tissue.
So fascinating is everything connected with the eye that the temptation
to describe it in detail is great; but in a book of rough outlines,
and in consideration of the many important matters yet awaiting their
turn, we must confine ourselves to briefly mentioning a few of the more
important points concerning it. The light is focussed by the lens upon
the nervous curtain at the back, and produces there a picture, as in
the photographic camera. Thus we perceive the shape of objects. The
different rays of the spectrum affect different elements in this curtain
or retina, whereby we get sensations of colour. Finally, the clearness
of the picture, its size, the degree of convergence of the two eyes, and
the effort of focussing—for the curvature of the surface of the lens
can be altered—enable us to estimate the size and distance of an object.
And now, though it would take volumes to do justice to the physiology of
vision, we must pass on to deal equally briefly with the functions of
that no less important organ, the ear.
The essential part of the ear is a membranous bag, formed by the pouching
in of the outer layer of cells—as shown in Figs. 1, 2, and 3 of Diagram
57—which comes to lie in a bony chamber beneath the skull, and assumes
the somewhat complicated shape depicted in Fig. 4. We have not time,
nor is it for our purpose necessary, to trace all the steps in the
development of the ear, either external or internal, nor need we spend
much time upon its structure, beyond indicating its position. But its
position, which is shown in Diagram 58, must be grasped in order to
understand how it is influenced by sound.
[Illustration: DIAGRAM 57.—SHOWING DEVELOPMENT OF THE MEMBRANOUS
LABYRINTH OF THE EAR.
U, Utricle; C, cochlea; S, saccule; S.C., semicircular canals.]
It will be seen that the membranous bag, which is fitly termed the
labyrinth, is situated in a bony cavity which fits so closely as to be
termed the bony labyrinth (C). The membranous labyrinth is filled with
a liquid, called endolymph, and the bony labyrinth (C) is also filled
with a liquid, called perilymph, in which the membranous bag swims. All
this is called the inner ear. The inner ear communicates with a second
cavity—the middle ear (B)—by two apertures in the bony wall, which are
closed by membranes. The middle ear is full, not of liquid, but of air,
and is separated from the external ear, the cavity marked A, which is
open to the external world, by another membrane called the tympanum, or
drum, of the ear. The middle ear is connected by a tube with the throat,
so that the pressure of the air on both sides of the drum may be the same.
[Illustration: DIAGRAM 58.—SHOWING THE POSITION OF THE EAR.
A., Outer ear; B., middle ear; C., inner ear.]
Now, the object of this arrangement is that the ear may be able to fulfil
one of its principal duties, namely, the perception of sound. Sound, as
the reader is doubtless aware, is transmitted through the air as waves of
condensation and rarefaction, due to the swinging backwards and forwards
of its particles; it resembles the passing on of a bump along a line
of trucks on the railway when the engine runs up against the end one
preparatory to coupling. The magnitude of this oscillation we perceive as
the loudness, the frequency as the pitch of a note. Now, when the waves
of sound strike against the drum of the ear, they cause it to vibrate
backwards and forwards also. Supposing there was no middle ear, and the
sound waves beat directly upon the membranous windows of the inner ear,
these could not be made to vibrate, as there is liquid behind them, and
liquids are incompressible; so, in order that the movements of the drum
may be transmitted to the liquids of the inner ear, they are carried
across the middle ear by a chain of small bones, by which their extent
is curtailed, but their force increased, and brought to bear upon one
only of the two openings. The consequence of this is that the membrane
closing it is able to vibrate and pass on the vibrations to the liquid
within, since when it is pushed in, the membrane covering the other hole
is pushed out.
[Illustration: DIAGRAM 59.—THE SEMICIRCULAR CANALS.]
Exactly how the different parts of the membranous labyrinth contribute
to our perception of sound we do not quite know. It appears as though
the difference of pressure in saccule and utricle originally conveyed to
the brain a sensation of noise without any idea of quality, while the
cochlea was developed later to analyze sounds and give information as to
pitch and tone. Whether the rest of the labyrinth has any longer a part
to play in the perception of sound, we cannot say with certainty; but it
seems pretty certain that the cochlea is the organ for receiving musical
impressions. Here, again, though, we are at a loss, for we do not know
with certainty how the cochlea acts. In shape it is a long tube, and in
the head is coiled spirally—like a snail’s shell to look at. Along its
whole length is a ridge of cells with short hairs projecting from their
inner surface into the liquid it contains; and to the cells along this
ridge a branch of the auditory nerve is distributed. But as to whether
one of the cells along this keyboard responds to each of the notes we
can distinguish, or whether they are affected as a whole, physiologists
are not yet agreed.
At least one other important duty the ear performs; it tells us in what
position we are, and how our whole head moves or is moved. On the top of
the saccule, in Diagram 57, Fig. 4, there are shown three little loops
which are called the semicircular canals. They are shown again more
clearly by themselves in Diagram 59.
Fig. 1 shows their position with regard to each other. It will be seen
that two of them are vertical, with their loops forming a right angle
with one another, and that the other is horizontal—in fact, that they lie
in the three planes of space. Fig. 2 shows the structure of one of them;
it has a swelling at one end (_a_), and a knob projecting into it where
the nerve joins it (_b_). In Fig. 3 is shown a section through this knob,
which gives the key to the use of these structures. A little head of
cells projects from the wall of the canal into its lumen, and from these
cells hairs bristle out into a dome-like covering of jelly, weighted, to
prevent its moving too easily, with small particles of lime. Now, if you
take up a round vessel full of liquid—say a bowl of gold-fish—and give
it a twist round, you will notice that, though the bowl turns, the water
inside does not; the fish remain in their old position. If there were
a rod projecting from the side of the bowl, it would, of course, move
with it, and if a fish came in its way would strike against it. This is
the principle of the semicircular canal. For if we turn our head, the
tube of the canal turns, passing over the liquid in it, which of course
does not move, though it appears to flow in the opposite direction. The
consequence is that the hairs on the side of the knob in the direction
in which the head is being moved are pressed upon by the dome of jelly,
which, as it floats in the liquid, tends to remain where it is. The
nerves, stimulated in this way, inform the animal generally of the
movement.
These little organs are very important to us, though we have our eyes to
correct our ideas of position, and they are still more so to the fish,
which dart and turn in the wide expanse of the ocean, and the birds and
bats, which wheel about in the air. There are, however, some occasions
when we do not feel inclined to bless them; for, inasmuch as they
faithfully report every roll and plunge of a ship to a person on board,
it is they which are mainly responsible for sea-sickness.
* * * * *
And now that we have seen how the body lies with regard to the external
world; how it is efficiently protected from its surroundings; how it
is placed in communication with them; and have briefly examined the
organs by which it makes its chemical and physical investigations,
looks out into space, and is kept aware of what is going on therein, we
may return to the means whereby it responds as a whole to the stimuli
thus reported—the central nervous system—and try to learn how the right
response is brought about.
IV.
There is but one thing more to describe in the mechanism of the body—the
connecting link between the last two sections. In the last we saw how the
body receives stimuli from the external world; in the one before, that
when these stimuli reach the central nervous canal it in turn stimulates
the organs to perform such movements as circumstances require. What,
therefore, remains to be described is the working of that canal by which
these necessary movements are ordered and controlled.
Now, in speaking of reflex action a few pages back, we said that the
nerves which bring in stimuli from the periphery distribute them about
the neural canal to those cells whose activity, by sending out fresh
stimuli to the muscles, produces the requisite movements. These motor
cells, however, are not scattered about the spinal cord anyhow. They are
collected into clusters, or nuclei, as they are sometimes called, and
each cluster has special duties—_i.e._, a special organ to control. Thus,
we say that there are in the central nervous system centres—a nervous
centre to control the leg; another to work the diaphragm; another
for the muscles of the ribs; more for the arm, hand, etc. And these
centres are in communication with one another, so that they may not pull
different ways.
In the first example of reflex action given in Section II. of this
essay, the sensation of a pin-prick was first conveyed to the centres
controlling the limb injured, by whose activity it was drawn away from
the danger. But the nerve which gave the warning which produced this
elementary movement distributed the impression that something was wrong
to the higher centres, so that the whole body was involved in protecting,
doctoring, and avenging the outraged member; from which it would appear
that the lower centres are under control of higher ones. And this is the
case. If we may be allowed the metaphor, there are captains of tens,
who are under the direction of captains of fifties, and the captains of
fifties receive their orders from captains of hundreds. The nerve canal,
the manner of whose formation as a simple tube is shown in Diagrams 5 and
42, has therefore different functions in different parts, and this to
such an extent that considerable differentiation in bulk and structure is
produced.
The neural canal may be roughly divided into two parts—a comparatively
simple tube, running the greater part of the animal’s length, containing
many centres from which nerves run to the organs they control; and a
complicated bulbous enlargement at one end, with thickened walls, in
which are the centres controlling those in the cord, and thereby managing
not so much organs as the whole animal. The former is called the spinal
cord, the latter the brain.
This division, accustomed as we all are to take it for granted, offers
plenty of food for reflection. Why should an animal have such a brain
placed in its head? Why, indeed, should it have a head, regarding that
member as a group composed of eyes, nose, mouth, ears and brain? The
mouth gives us the key to the riddle; the mouth is the essential organ,
and all the rest are its accessories.
In the first essay we saw that the basis of life was chemical, and in the
second that the materials necessary for the chemical action, or food,
must, in the higher animals, be taken into the digestive tube through the
opening which we call the mouth. Therefore, as it is highly important
that only the most beneficial substances shall be received into it, and
that all which are actively injurious shall be excluded, it is plain that
the organs of chemical perception must be placed in its neighbourhood—the
organs of smell to enable the mouth to find its food, and the organs
of taste to aid in selecting it. As, moreover, our humble ancestors,
the fishes, move literally mouth foremost, it is not surprising to
find the organs of space perception, the eyes, also situated in its
neighbourhood, especially when one considers that their food is often
of a lively character, and requires precision of movement to secure it.
The inevitable consequence of thus grouping the more important organs of
perception under the fore-end of the neural canal is that it grows and
develops more highly here than elsewhere along its length, and soon is
in a position to dictate to the rest of the body. Another reason why it
must develop is that it must contain centres for turning its impressions
to practical account, not only by producing complicated movements in the
jaws, eyes and gills, but also by ruling the centres in the cord, and
instructing the body to carry the mouth whither it needs to go.
[Illustration: DIAGRAM 60.—SHOWING PRIMARY DIVISION OF NERVOUS TUBE.]
In the preceding diagram (60) the origin of the brain is shown as a
dilatation of the end of the neural canal into a bulb with thickened
walls, which has already become constricted in places, so that it is
subdivided into three. The next diagram (61) is intended to give, in
no matter how crude and schematic a way, some idea of the lines on
which the development continues. We do not show all, or even half, the
structures which go to make up the brain. To do so would be out of place
in a book like this. Further, we shall endeavour as far as possible to
speak of the brain in general terms, avoiding the five-syllable bastard
Græco-Latin names with which the early anatomists have endowed almost
every square inch of its substance, and confine ourselves to summing up
its functions as briefly as can be done with justice.
[Illustration: DIAGRAM 61.—GIVING A ROUGH IDEA OF HOW THE BRAIN IS
DEVELOPED.]
In pursuance of this method, attention must be drawn to the fact that
only the foremost of the three original bulbs (marked A in the diagram)
and the hindermost (C) continue to grow. The middle one (B) remains
comparatively simple. From the foremost lobe buds grow out to form the
eyes in the manner which we have already described, and other buds push
forwards to meet the nerves from the nose. The latter have, even in the
early stages shown in Diagram 61, reached an extraordinary size; and
when we come to trace them further, we shall find that they become very
complex, and acquire remarkable and unexpected powers, considering their
humble origin. Strange changes also take place in the hindermost bulb.
It splits along the top, so that the cavity it contains is open like a
saucer, though bridged over by a three-lobed body called the cerebellum.
Following the spinal cord up into the brain, we are conscious of no
sudden line of demarcation separating the one from the other, only of an
increasing size and complexity. The lower parts of the brain send out and
receive nerves much as the cord does; three pairs go to the muscles which
turn the eyes; other pairs bring in sensations from the face and throat;
others control the muscles of the face, tongue and throat. But the brain
differs from the cord in being directly connected by nerves, not only
with adjacent parts, but also with the distant and more important organs
in the interior of the body—heart, lungs, etc.; in containing groups of
cells which have stimuli sent on to them from all over the body viâ the
cord; and in possessing centres which control those lower down in the
nervous system. It therefore not only receives and balances stimuli from
all over the body, but, by governing the centres which preside over the
bodily movements, is able to wield and direct the body as a whole.
The hinder divisions of the brain, which we shall consider first, have
no connection with consciousness or volition. They only produce reflex
movements, which, however, owing to the wealth of material they have to
work upon, are wonderfully complex and far-reaching.
Let us take a few examples. In the hindermost division of the brain (C
in the diagrams) there is the centre which presides over the oxygen
supply, the importance of which we saw in the essay on vital chemistry.
This centre perceives when the lungs have been filled with a gas, and
causes them to be emptied; it perceives when they are empty, and again
does not allow them to remain too long in that state, before ordering an
inspiration; it notes the quality of the air which is passing through the
nose, and it notes the quality of the blood which bathes its own cells.
The condition of the blood, indeed, is closely watched. An excessive
quantity of carbonic acid gas, poverty of oxygen, even temperature, all
produce through it an effect upon the rhythm of the breathing.
Close by the respiratory centre is the centre which controls the
circulation. But enough has been said in the section on reflex action,
wherein the process of fainting was described, to give an idea of the
part it plays in the body; so it need not detain us here.
We cannot, however, pass over its neighbour, the centre of temperature,
so briefly. Its methods not only afford one of the most striking and
interesting examples of harmonious regulations by reflex action, but the
subject of temperature itself is so important that we must describe in
some detail how that of the body is kept level.
As we said when discussing protoplasm generally, life—that is, the
change always going on in the protoplasmic substance—is influenced by
temperature: the single cell becomes less active at a low temperature,
and dies at a high one; so obviously there is a temperature at which its
functions are most easily carried on. Inside the body the cells are all
kept at the temperature best for them by the circulation of the blood;
but the absolute temperature of the whole body depends upon the heat
which is generated within it by chemical action, and the heat which it
loses to, or receives from, its surroundings. Under normal conditions
this temperature in man is 98·4° F., when the production of heat from
its own metabolism is balanced by the loss of heat by radiation. If,
however, the atmosphere be very hot, less heat is developed in the body,
the general metabolism being slower; and more heat is lost, since by
reflex action the skin is bathed in sweat and cooled by its evaporation,
and the small bloodvessels under the skin are dilated, so that more
blood being brought to the surface, its chance of being cooled by
radiation is thereby increased. If, on the other hand, the atmosphere
is cool, the loss at the surface is minimized by constriction of the
cutaneous bloodvessels, and a checking of the perspiration and consequent
evaporation; while internally more heat is generated by increased
metabolism. The cells which are mainly responsible for the production
of heat are those of the muscles; and when much heat is required they
increase in activity, not only in their general tone, but even by a
visible movement, which we describe as shivering. So, within reasonable
limits, whatever the temperature of its surroundings may be, that of the
body remains the same, and though we may raise or lower our temperature
by lying in a hot or cold bath, reflex adjustment of the sweat glands,
bloodvessels and muscles brings it quickly back to normal when we emerge.
With a passing mention of the cerebellum, the three-lobed organ shown in
Diagram 61, and seen again in a more advanced stage in Diagram 63, we may
dismiss the two hinder divisions of the brain.
The cerebellum lies on the upward path of fibres from the cord to the
higher centres in the fore-brain. It is a somewhat complicated organ,
and its functions are not yet fully known. The older physiologists took
a very extreme view of its importance, assigning to it, among other
romantic duties, that of providing a habitation for the soul. This
opinion on the strength of later research we can hardly endorse. The
cerebellum really seems mainly concerned in co-ordinating the action
of the muscles, especially in maintaining equilibrium in standing and
walking.
Our knowledge of the whole brain is very far from complete. We should
like to know the peculiar function of each little group of cells that
can be made out under the microscope, and the paths of all the fibres
connecting the different parts of the nervous system. As it is, we have
to wait with the best patience we can while they are being investigated,
and hope. In few departments, however, have the labours of the
physiologist proved more fruitful and interesting than in the study of
the fore-brain (A in the diagrams).
In the simpler form, as shown in Diagram 61, A, and Diagram 62, Fig. 1,
A, the fore-brain is remarkable in that it throws out buds for the two
most important sense organs—those of sight and smell. So important are
these senses, especially in our humble ancestors, as we have already
pointed out, that it is not surprising to find the impressions of the
other senses brought on up from the hinder parts of the brain to be
compared with them. The fore-brain is, in fact, a sort of terminus
whither the whole of the afferent or incoming stimuli are brought, and
whence, since information is only received in order to be acted upon, the
supreme orders to the body issue.
In the fore-brain there are centres for specially governing all the
motor organs; but by a strange arrangement the main root of the brain
is overwhelmed by its own offshoot, the hemisphere, or lobe which gives
rise to the olfactory bud. In fact, so great is the importance of the
sense of smell to an animal whose one object in life is to find food,
that, instead of the hemisphere being subordinate to its parent, it
seems to take over most of the latter’s business, receiving a report
of the sensations collected by it, and sending out orders upon its own
initiative. Yet, unimposing though the history of this division of the
brain may be, it ultimately becomes the seat of consciousness, whereby
the mental processes are carried on, and whence all voluntary movements
spring.
[Illustration: DIAGRAM 62.—SHOWING HOW THE CEREBRAL HEMISPHERES ARE
DEVELOPED FROM A, THE FOREMOST BULB OF THE BRAIN.]
Of course, in order to do this, the hemispheres have to grow
considerably, and thus we find them enveloping the rest of the fore-brain
and swamping it in structure as well as in function. Diagram 62 indicates
how this is done, while Diagram 63 shows roughly the proportion and
position the different parts of the brain ultimately attain. Finally,
Diagram 64, which is rather more realistic, but still much simplified,
presents a view of the organ in the head.
The size of the cerebral hemispheres, compared with the rest of the
brain, is especially remarkable. So, too, is their endeavour to increase
their surface still more by throwing it into deep folds. (See Diagram
64.) These two features vary with the position of the animal in the
scale of development; in man, who stands highest in intelligence and
dexterity, the hemispheres are very large indeed compared with the other
organs, and seamed all over with a maze of winding furrows. Another
remarkable feature is the extreme degree to which specialization is
carried out. Different parts of the body are represented, each by a
small area of the cortex, or surface layer, and we know at what spot on
the cortex such sensations as sight and hearing are perceived, and from
exactly what little patch the impulse to move each limb emanates. In
the accompanying diagrams (65 and 66) these areas are mapped out, their
locality being fixed by the principal folds which act as landmarks on the
surface of the hemisphere.
[Illustration: DIAGRAM 63.—RELATION OF DIFFERENT PARTS OF THE BRAIN.]
There is another important fact which we must not omit to mention in
speaking of this localization: each hemisphere presides over the opposite
side of the body. Early in development the nerve fibres from the eye
cross over to the opposite side of the brain, and the afferent fibres
from the lower parts of the body have accordingly to follow suit. Then,
as the efferent fibres—_i.e._, those which set the muscles in motion—have
to bring about the movements in response to information received, they
must also cross to get back to the side from which it came. So, if a
tumour grows inside the head on the right side, it is the left eye which
becomes sightless, or the left hand which grows numb and powerless,
according to the part of the cortex which is pressed upon.
Perhaps the most interesting part of the whole body is that little band
of the cortex running upwards from behind the temple to the crown of
the head, in which (_cf._ Diagrams 64, 65 and 66) the motor areas of
the limbs, and the perception of those sensations which we have grouped
together and called ‘touch,’ are situated. The minute structure of this
region is roughly shown in section in Diagram 67, as it has been made
out with the microscope; but only a few of the nerve cells are shown,
the connective tissue of feltwork in which they are suspended, and
the bloodvessels by which they are nourished, being left out. All the
structures represented are of course very, very small; the large black
patches which represent cells would really be invisible, and the whole
field of the diagram only a mere speck, to the naked eye.
[Illustration: DIAGRAM 64.—POSITION OF THE BRAIN IN THE HEAD.]
A represents the nerve by which impulses are brought in. It runs straight
up to the surface of the cortex, and there its branches end, interlaced
with those of a many-branched distributing cell (B). The two cells (C
and D) shaped like pyramids, which send up branched processes from their
apexes, receive an impulse from the distributing cell, and transmit it
along the fibre which runs downwards from the middle of their base.
Where the fibre from the smaller one goes to we are not sure—probably
to another part of the brain to insure harmonious working—but the large
pyramidal cell sends its fibre right away through the lower parts of the
brain, passing the cell-stations they contain, on into the spinal cord,
till it reaches the centre there, which immediately works some particular
limb.
[Illustration: DIAGRAM 65.—MAP OF THE CEREBRAL HEMISPHERE, SHOWING THE
AREAS IN WHICH VARIOUS FUNCTIONS ARE LOCALIZED.]
Supposing we anæsthetized somebody, throwing him into deep
unconsciousness, and then opened his skull, laying bare the brain as is
done in Diagram 64, only not quite in such a wholesale manner. If we then
stimulated the part of the brain we are now considering at different
places with electric needles, using a weak induction current, we should
see him moving different members according to the different regions
touched—now an arm, now a leg, now the whole head. If we were to place
the electrodes in the centre for the hand, and then gradually increase
the strength of the current, the activity of the hand centre would throw
other centres into activity. The arm would move next, raising the hand
towards the face. Then the eyes would turn, and the whole head to meet
the hand. Lastly the mouth would open. The movements are those of putting
something into the mouth—the ruling passion strong in unconsciousness.
[Illustration: DIAGRAM 66.—MAP OF THE CEREBRAL HEMISPHERE, SHOWING THE
AREAS IN WHICH VARIOUS FUNCTIONS ARE LOCALIZED.]
Such experiments were, of course, first made upon animals, but they have
been fully verified on the human subject. The story of how this was done
is not, however, a romance with a martyr or a criminal for the central
figure. The _corpus vile_ was not provided by a volunteer, or kidnapped
and bound in a dark cellar, but treated as a patient in the airy wards
of a hospital. With increasing knowledge of the brain, it was found that
epilepsy was cortical in origin. A little piece of the cortex becomes
diseased and hyperexcitable. The sufferer suddenly becomes acutely
conscious of one of his members—a hand or a foot, say—not because there
is anything the matter with it, but because the corresponding area in the
brain is morbidly active, and he refers the sensation to the part from
which it receives its nerves. The next moment the limb begins to twitch,
and the excitement spreading, as in the experiment we described above,
to other centres which are not diseased, they, too, become morbidly
active, and the whole body is thrown into convulsions. This is a disease
which must be checked as soon as possible. The surgeon accordingly lays
bare the part of the brain affected, knowing now where to look; finds the
exact spot which is diseased by reproducing the first twitchings of a fit
by electric stimulation, and removes the source of the trouble.
[Illustration: DIAGRAM 67.—SHOWING THE SYSTEM OF NERVE CELLS IN THE
CORTEX.
A, Afferent fibre; B, distributing cell; C, small pyramidal cell; D,
large pyramidal cell.]
Returning to general considerations, an important point is the way in
which the different centres are connected by fibres, which put them
into relation. The brain may consist of many centres, just as the body
consists of many organs, but both body and brain must live as a whole.
If the heart and lungs get out of harmony there is trouble, and if the
bridge which connects hearing with motion in the brain breaks down, as
occasionally happens for a time in an overworked man, he is mentally at a
discount. He can hear and understand, but he cannot write or talk sense:
he is sane, but quite helpless, and generally very frightened.
Still more important are the intermediate stations and sidings on these
lines of communication, for it is here that the most exhaustive weighing
and comparing of incoming stimuli is carried on—the final balancing
before a voluntary action; in a word, thought.
These courts of inquiry are called association centres. It used to be
believed that they were all in the fore-part of the brain, under the
forehead; but this is evidently not the case. Several men in war or
by accident have had the frontal lobes of their brain damaged beyond
repair; and when they have been discharged from hospital, where, thanks
to the advance made by surgery since anæsthetics and antiseptics were
discovered, they have been successfully treated, they have gone back
to their work seemingly in no way different from men whose brains were
whole. In some cases they have even been reported as having become
quicker and sharper than before, probably owing to there being fewer
association centres, and thought being accelerated by simpler machinery:
facts are thenceforward shaken through a larger-meshed sieve.
A few general considerations, and we have done. There is no centre
for memory in the brain. The facts which we remember are not stored
as in a box, nor can one imagine how they could be, considering that
the physical basis of an idea is molecular change. The whole nervous
system is probably concerned in memory, a particular change, which has
momentarily occurred in its tissues, being more likely to occur again
under certain circumstances than a fresh one, and certain tracks becoming
well beaten and more permeable than others. Pleasure and pain are other
general phenomena: they are not to be localized in the brain like vision
or hearing. Pleasure is the consciousness that the whole body is under
favourable conditions; and pain, the knowledge that the protoplasm of
certain cells of the body is being acted upon by injurious agents,
chemical or physical. There seems to be good evidence that separate
nerves convey impressions of injury, distinct from those of touch and
temperature; but it is the revolt of the whole body against conditions
affecting a part which constitutes pain.
Amidst the maze of perplexities which lies between physiology and
psychology, there is, however, one fact which stands out clear and bold:
the brain can create nothing. We have seen how matter is taken into the
body and matter is cast off from the body. We have seen how energy is
released in the body from chemical compounds, and made use of by the
body. So now, after a moment’s thought, it must be plain that every
stimulus which goes to the brain must have its effect there, and that a
man’s thoughts and conduct are entirely dependent on what has, at some
time or other, come in from the external world. The association centres
can evolve wonderful thoughts, but they are structurally derived from the
grosser sense organs, and must get all the material they work upon from
them.
The nervous system puts the body into relation with the external world
as a whole, but for convenience it is subdivided into the afferent
system, by which impressions come in, and the efferent system, by which
the muscles are set in motion. Of the two halves, the afferent system
has a just right to priority, for the efferent system is merely its
consequence. Sights, sounds, smells, etc., reach the brain by afferent
paths from the external world, and are there moulded into thoughts. Their
effects we see in poetry, architecture, sculpture, or laundry work,
according to the method of the brain in treating the raw material it
receives, and of a quality corresponding to the fineness with which the
brain examines them, and can control the motor organs of the body.
Whatever goes in at the afferent door, and some people’s sensory
apparatus is much more easily affected than others, produces its effect
within. Sometimes the energy is expended in thought, sometimes in action;
sometimes it trickles away as laughter. But all these phenomena have
a material basis: matter producing changes in matter. ‘Those delicate
tissues wherein the soul transacts its earthly business,’ as Stevenson
so picturesquely describes the brain, stick to their earthly business.
There is no astral department opened yet. A man may evolve a great idea
from the data he receives, but he must give it a material coefficient
if he does not wish it to be lost to his earth-bound brothers. He may
write it in a book, or he may sculpture it in marble; but the most
convenient means of communicating with his fellows is by sound, which he
can command by expelling the air in his lungs over vibrating cords in his
throat. These cords are adjusted at the position and tension to give a
desired note; and the cavities of the chest, throat and mouth acting as
resonators, a noise is produced, which is shaped by the tongue, lips and
teeth into words.
By means of language the human body is enabled to co-operate with others
of its kind for the development of the resources of the earth, the
shaping of society, and the forming of individual character. But here
physiology ends and other sciences begin.
ESSAY V.
THE BODY.
I.
Such, as far as it can be compressed into four short essays, is the
nature of protoplasm. We have sketched out its powers; described how
it exists only in the form of a cell; and shown how cells, by forming
a community and sharing the work, simplify the business of living, and
secure great advantages for themselves individually. But now we have
something fresh—the body.
The body is an organic whole, like the cell. It is composed of cells,
but these cells only develop one of their many powers to its utmost that
they may justify their existence in the community; they do not acquire
fresh properties. So really the body is merely a mass of protoplasm in
which—though in a greater quantity—the same changes are going on that we
find in a single cell. Yet how utterly different is the body from the
cell! what a wide gulf yawns between a man and an amœba!
The body has powers of its own, distinct from those of protoplasm;
and the protoplasm may still be alive when the body it helped to
compose is dead. A few thousand pounds of protoplasm could not have
built Westminster Abbey, but a few hundred men did; for just as out of
protoplasm arises the body, so the body gives rise to the mind, a thing
as much above it as it is above the cell.
So far we have taken the cellular units as our starting-point in
discussing life; but a bird’s-eye view of physiology would be incomplete
which contained no mention of the major tactics of protoplasm—the life of
the body. We must consider the needs of a man as such.
A man requires, as we have described before, air and food. The air must
contain the proper amount of oxygen, and the food must consist of liquids
and solids.
The important liquid is water. More than half by weight of the whole body
is water, and we are always losing it: through the skin, through the
lungs, and through the kidneys. People may drink alcohol with water, but
not instead of it. Indeed, the more alcohol they take, the more water
they require, for if they take their spirits in an undiluted form, they
take water out of the body. If you dip a piece of wet cloth into alcohol
for a minute, it dries with remarkable rapidity, because the water has
been absorbed out of it. In the same way, if you drink neat spirits, they
pass into the blood and stimulate the nervous system, ultimately being
excreted by the kidneys; but on their way through the tissues they absorb
a great deal of water, which must be replaced.
Milk is often spoken of as the ideal food, and so it may be for very
young animals; but it lacks one important constituent—iron. A young
animal is born with enough iron in its body to do without any in its food
until it can take something better than milk. If it is weaned late it
becomes anæmic. An adult animal requires iron and also a certain amount
of solid food. Its alimentary canal is provided with a good deal of
muscle; and this muscle, to be kept healthy, must have something to work
upon—in fact, be exercised.
A man’s food should contain a certain amount of coarse
material—cellulose, for instance, which forms the envelope of vegetable
cells. And here we see a distinctive feature of the body. Protoplasm can
make no use of cellulose, there is no digestive juice which will act upon
it; but its presence in the alimentary canal in the form of husk and
small seeds stimulates the walls by contact, and produces peristaltic
movements.
In Essay II., which dealt with the chemistry of the body, we said that
if we were only to live upon meat we should tax our digestive apparatus
severely by having to eat more proteid than we required in order to
get enough carbon. The converse, however, is equally true: if we fed
only upon vegetables, we should again have to over-eat. Plants most of
them contain large stores of carbohydrate; potatoes and rice are rich
in starch, onions in sugar; but an exclusively vegetarian diet would
necessitate our consuming huge quantities, as its proteid-supply is
bad. It is defective not only in amount—for sometimes, as in beans, the
proportion is fairly large—but in being so indigestible that a good deal
of it passes through the body unabsorbed.
Both science and experience teach that we live most economically upon the
carbohydrates of vegetables and the proteid of animals, and that our food
is the better for being cooked. In cooking, parasitic animals which are
strong enough to survive the ordeal by acid in the stomach are killed,
and the food itself is made more accessible, the indigestible cellulose
envelopes of the vegetable cells being burst open, and the collagen of
the connective tissues converted into gelatin.
No less important than the quality of our food is the quantity; and here,
again, we get a good illustration of the necessity for regarding the body
as a whole. A healthy man’s appetite is his best guide, and if he follows
it he cannot go far wrong. People who arrogate to themselves a wisdom
superior to that of Nature little know the harm they do when they force
food down an unwilling throat. The cells of the alimentary canal digest
and absorb what is sent to them, minding their own business, which is not
to criticise the appetite.
‘Theirs not to question why;
Theirs but to do and die.’
So the digestive and excretory systems embark on hard and profitless
labour, and the whole body suffers.
Passing on to another subject, we find that the body eats that it may
work, and works that it may eat. This cycle comes naturally enough to
animals which have to go and find their food, but men with a sedentary
occupation have, in view of the artificial conditions under which they
live, to take constitutional exercise.
There are many reasons why the numerous and bulky muscles with which
the body is endowed must be constantly used. In the third essay in this
volume we saw that the flow of the lymph and the blood in the veins is
largely dependent upon the movements of the limbs. Muscular exercise,
therefore, must be taken to prevent the circulating streams from growing
sluggish. Obviously, many evils must arise if they do so. Not only would
the muscles be starved by the slowness with which they received their
food, but they would also be poisoned by the slowness with which the
products of their own metabolism were removed. The blood-stream would
become tainted, and the brain, which requires pure blood, would suffer.
In other ways, however, than by their action on the vessels themselves
do the muscles help the circulation. Exercise has secondary effects
upon the circulatory and respiratory centres in the base of the brain,
and makes the heart beat more strongly and the diaphragm contract more
forcibly. The influence of the latter upon the circulation we have
already described; but its vigorous action is required not only to aid
the circulation through the liver and viscera, but to inflate the lungs
to their fullest extent. In the breathing of a man who takes no exercise
only a small part of the air which the lungs hold is pumped out at
each breath, the greater part remaining stagnant in chambers which are
practically unused. Thus, not only is the revenue of oxygen diminished,
but there are numerous little crannies in the body filled with still,
warm air which are ideal nurseries for bacteria. The devil of consumption
does not allow such dwellings to long remain swept and garnished.
Without exercise the sweat glands of the skin will not act, and its pores
get closed up. The muscular coats of the alimentary canal, too, reflect
in their state of health the condition of the voluntary muscles; laziness
is followed by constipation, for if the voluntary muscles rust, so do the
involuntary.
The muscles, moreover, must be well developed and kept in a healthy
condition for their own sake. They form a large part of the whole bulk,
and no healthy man can have unhealthy muscles. Ignorant people sneeringly
say that they have no ambition to lift weights or bend pokers; but
they should remember that they are dependent upon their muscles for
their bodily warmth, and also to save their internal organs from being
oppressed by their own weight.
The bones of the skeleton do not rest one upon another; they are jointed,
and kept slung in position by springing bands of muscle attached to their
levers. If these muscles are not properly developed, they tire under
the strain of holding the frame up, and a disastrous rearrangement of
the organs is made to save labour. The chest is drawn in, the hips and
knees thrust forward, and the man stands with cramped chest, compressed
viscera, and his diaphragm under dire constraint. The result of the
redistribution of weight is that his bones tend to rest upon one another
like a column of bricks, and his whole weight is upon his heels. Such an
individual cannot walk; he stumps along, jarring his whole body at every
step.
A pleasant contrast is the athlete. The athlete is a man who endeavours
to develop the latent powers of his body to the utmost; and in the
achievement of this desirable object the physiologist takes great
interest. Physiology has revolutionized our ideas of training, as well
as many other things, during the last half-century. We now recognise
two kinds of training—the preparation which a healthy man makes for an
occasion on which unusual exertions will be expected of him, and the
slower and permanent strengthening of the whole body, now usually called
physical culture. The former is a comparatively short process now that
athletes no longer think it _de rigueur_ to live in bestial intemperance
when they have no contest in immediate prospect. A little extra exercise,
to stimulate excretion and clear away any waste products that may have
accumulated in the tissues; a little extra proteid in the diet, since
there is at first a slight inclination to growth; a good deal of extra
carbohydrate, since the muscles want extra fuel; rest—and the man is
ready. Many athletes live continually in training, and are ready at any
moment to ‘fight for their lives.’
The second kind of training is for those who are weakly or those who
wish to excel. Its object is not only to improve the health, but to
increase the absolute strength and size of the body, and its effects are
permanent. It necessitates careful diet and constant exercise for a long
period, and proves equally beneficial to both sexes. This book has been
written in vain if the reader has not grasped by now that the activity
of the muscles implies the activity of all the organs in the body.
Accordingly, the whole muscular system is by appropriate methods given
frequent exercise: gentle at first; never exhausting; but constantly
increasing as the strength grows. The result is a general development
of the organs throughout the body, which will in time work a complete
metamorphosis in the individual’s physique.
Yet by strength alone no athlete can excel; success depends upon skill.
He must have the strength to work with, but he must have the knowledge
to apply it without wasting energy, and the ability to do this with
precision. He must practise well the sport he intends to adopt—in other
words, train his central nervous system.
Here, again, we must hark back to the main idea—the unity of the body.
We have already dealt in this essay with the bodily needs in the way of
food and exercise, and we must now consider the needs of its nervous
components. The chief of these is education; but education of the nervous
system means, of course, education of the whole body. There are still
people who cling to the old fallacy that the mind can be developed at the
expense of the body, but a visit to a hospital or a lunatic asylum will
afford many opportunities of seeing how Nature avenges ill-treatment of
‘those delicate tissues wherein the soul transacts its earthly business.’
Physical culture must come before mental: _Mens sana in corpore
sano_—hackneyed, but true.
I do not, of course, say that a man must develop equally both his body
and mind; only, that the former must be functionally competent. The
absurdity of supposing that the brain can benefit by forming part
of an unhealthy body is, surely, obvious to all. Determined invalids
may produce splendid work, as Darwin did, in spite of ill-health, but
not because of it, and men of great mental energy will sometimes wear
themselves out prematurely by their restlessness; but starvation and
maltreatment of the body will not create intellect, however morbidly it
may stimulate the imagination.
Granting the tenement of a healthy body, the education of the central
nervous system must proceed along four distinct lines. A child must learn
useful reflex actions, such as walking; have its association centres
trained, that it may reason quickly and correctly; be endued, if it is
not to live upon a desert island, with a sense of moral responsibility
and ethical principles; and have its head stored with useful facts, from
the meaning of words and the A B C to the value of the coinage.
Few people seem to realize how much a child has to learn before it gets
to the A B C. It begins life with very little beyond a capacity for
learning, and even its sense organs tell it little until it has had
practice in using them. If baby is so unfortunate as to get a scratch
from a pin, he wriggles, and makes the whole house aware of it; but he
does not seem to have a clear idea at all as to where he is hurt. He has
to learn the way about his own body. He passes his hand over his face,
and learns that he has features with a definite position and magnitude;
he then waves his arms in the air, and learns that there is such a thing
as empty space; finally, he knocks his knuckles against the edge of his
cradle, and learns that there are other things in existence besides
himself. Of course, his eyes help him considerably to form his ideas of
things, but his eyes tell him nothing until he has learnt how far to
believe them by correcting their impressions by touch. He learns the
properties of matter by experiment, not intuition.
Very interesting experiments have been made upon people who have
been born blind, and to whom sight has been given late in life by an
operation. They generally take some time to appreciate their good
fortune. Things, they say, are all pressed up against their eyes, and
they are afraid to move. Objects with which they have carefully been
made familiar before the operation—wooden spheres, cubes, cones and
prisms—they have been absolutely unable to recognise by sight until they
have handled them. They have mistaken sparrows for tea-cups, and it
is sometimes only after weeks that they have suddenly discovered that
pictures are something more than a mixture of irregular splashes of
colour on a flat surface.
Babies have to learn to interpret what they see in much the same way,
and take longer about it. The child who cries for the moon is probably
not so unreasonable as people think. He focuses his eyes upon the
little, bright, sharply-defined disc, and it appears to him, if not
actually within arm’s length, at any rate near enough to be caught with a
butterfly-net. It is only after he has seen it sink behind a large tree
on the distant horizon that he gathers a vague idea of its real size and
remoteness.
The term ‘physical culture,’ as usually applied, is supposed only to
mean the development of the muscles and viscera; really it only begins
there. After the organs of nutrition have been got into a healthy state
the motor organs are developed. Finally the nervous system should be
trained. Mere muscle will not make even a good runner. He must practise
carefully till he can take his full stride, and do so without wasting
energy by any needless movements. Then he must make the action which his
brain has decided is the most effective for his build the property of
his spinal cord, that in a race he may use his strength economically,
with his thoughts free to deal with the tactics of his opponents and the
peculiarities of the course, or he will not make his supreme effort at
the most advantageous moment. This is only one instance. Many men having
a normal body develop organs of perception, not motion: the musician and
the wine-taster, as well as the juggler and the athlete, are products of
physical culture. Even the philosopher must foster the physical basis of
intellect.
The education of the nervous system goes on all through life; and just as
oft-repeated actions become automatic, so are habits of thought formed
which are almost as regular; in fact, we might almost call them cerebral
reflexes. Without constant exercise, men lose their flexibility of mind
as well as of body.
But we have already passed the boundaries of our subject, and it is time
for us to pull up, lest we trench further upon another science; for the
study of the mind is the province, not of the physiologist, but of the
psychologist.
II.
Notwithstanding the strange powers of protoplasm, and notwithstanding
that these are accumulated and intensified in the body, as we saw in the
last chapter, there are immovable limitations to vital activity.
This is a fact familiar to all. We can trace diminishing vitality through
a series of stages, from slight fatigue right up to death itself. Sleep
is perhaps one of the most interesting, though it is little understood.
During sleep and the hypnotic trance, we know that the cells of the
hemispheres pause in their work and chemically recruit themselves; that
there is an interruption of consciousness; and that changes occur in the
respiratory and circulatory, and, in fact, in most of the functions.
But exactly how these states are induced we do not know. It has been
suggested that during sleep less blood passes through the brain; but this
is unlikely, and still less probable is it that the nerve cells draw in
their processes and shut up like sea-anemones, as another daring theorist
supposed. We can only draw parallels between the cells of the central
nervous system and any others; all need rest.
The simplest unicellular animals, which we have mentioned so often
already, spend their lives in alternate spells of activity and rest. In
the third essay we mentioned briefly the weakening of each successive
response when a muscle, in which tissue fatigue has chiefly been studied,
is stimulated. Before the muscle contracted it contained a form of sugar;
when it is tired the sugar is gone, and has been replaced by the products
of the chemical action by which the energy was evolved. A period of rest
must then follow, for the muscle to be cleansed and replenished. The case
of glands, described in Essay II., is somewhat similar. After the gland
cell has discharged its ferment, it must spend some time secreting a
fresh stock before it is ready to discharge again. In fact, a cell seems
to load itself up with supplies, like a locomotive with coal, and, after
working till the fuel is nearly exhausted, it has to stop to take in more.
All the cells in the body rest at times; even the cells of the heart,
carefully as they are nourished and incessant as their work seems, rest
between each beat, and the cells of the nervous system form no exception.
The brain no less than the body requires periodic rests to renew its
chemical stores, and these rests have to be all the longer, as during
the waking hours the brain works harder and less intermittently than any
other organ. It is only because the brain is the seat of consciousness
and the source of voluntary movements that these phenomena are suspended
during sleep.
Death may seem at first sight a very simple affair, the breaking up of
protoplasm into simpler non-living compounds; but the death of the body
is anything but simple—in fact, it is not always easy to say when the
body is dead. Usually, however, it is considered dead when the central
nervous system has succumbed, though the muscles may continue to live for
several hours.
Death may begin in many ways. The loss of some organs will bring death
only after a considerable time, while the failure of others disturbs its
economy fatally, and causes an almost immediate cessation of the vital
functions. Any interference with the normal conditions of the brain,
heart, or lungs is very dangerous, and it is injury or disease of one of
the three which puts an end to most men’s troubles. If the brain weakens
so that it no longer keeps the heart beating or the muscles, which fill
and empty the lungs, to their duties, the body, for obvious reasons, can
no longer keep up the cycle of changes we call life. On the other hand,
if the lungs cannot oxidize the blood, or the heart drive fresh pabulum
to the brain, that organ collapses immediately, and, if a stream of pure
blood is not quickly restored to it, dies. No return of the circulation
can then restore it; the death of the rest of the animal must follow.
Being the most delicate, the cells of the central nervous system usually
die first, and we then say that the man is dead. So the body may be,
but much of the protoplasm of which it is composed—whole organs, in
fact—remains alive; the muscles will respond to electrical stimulation,
and in case some people may dispute that this is a sign of life, if
pieces of his skin be removed and grafted into another person, they
will grow there, produce hair, and become, in fact, a part of the new
body. This they could not possibly do if they were dead; we cannot endow
inanimate matter with life.
As death creeps on over the tissues, the leucocytes die, and in doing
so form a ferment which solidifies one of the proteids dissolved in
the blood, so that the familiar clotting takes place. By a similar
process certain constituents of the muscles are also clotted, the
muscles stiffening and passing into what is technically, but also pretty
generally, known as rigor mortis. Rigor is said to set in soon after
death if the body is kept in a warm place, or if death has been preceded
by violent exercise; but death in this instance means only the death of
the body. It is at the precise moment that a muscle fibre dies that it
passes into rigor. By keeping it cool, so that the processes of life may
go on slowly, especially if it be in a healthy condition, its death may
be deferred for hours; while, on the other hand, at the end of a severe
and protracted battle, exhausted soldiers sometimes die instantaneously
on being shot, and are found fixed in the position in which the fatal
bullet found them—on their knees, with gun to shoulder, in the act of
firing.
But if the manner of death is not to be lightly dealt with, its causes
are still more obscure. It seems natural enough that people should be
killed by violence or by diseases with an external or septic origin, or
even by one particular organ wearing out and involving the whole body in
the fate of its part. But why should people die of old age? Why should
their vitality ebb till they quietly go out? Life is a mechanical cycle
of changes. For a time even after it has stopped growing, the body
replaces what it wastes, and keeps itself in a condition of equilibrium.
Why, then, without any apparent external cause, does it, after a more or
less circumscribed period, enter into a decline? And, finally, could we,
by taking the proper precautions, delay or prevent old age and death?
In the first place, regarding protoplasm as a chemical structure, why,
if kept under favourable conditions, should it ever break down? We have
no reason to suppose that it need. It is hard to see how the minute
animals, consisting of only one cell, can die of old age, provided that
no injurious influence be brought to bear upon them. When an individual
has grown to a certain size, it divides in two, and each enters upon life
afresh. Why, therefore, should not all the cells of the body continue to
renew their youth?
The reason why the body can only last a certain time, in spite of the
many quacks with recipes for immortality—recipes including such items
as the avoidance of all trouble, worry, or work—must remain a secret
until we know the chemical basis of life. It seems to lie in the cell.
If a unicellular organism, as described above, be placed in a vessel
of sterilized water and left to live alone under otherwise ideal
conditions, it will start dividing and multiplying, as though it meant to
reproduce itself indefinitely. After a time, however, the shoal begins
to deteriorate; each successive generation is feebler than the last, and
eventually all die. If, however, before this happens one of the effete
cells be placed in another vessel with a similar individual derived from
a different ancestor, the two will fuse and form a single fresh animal
with entirely restored vigour, ready to multiply to the same extent as
either of its original ancestors. A few individuals of a different stock
will in this way revivify the whole brood.
There is, therefore, evidently something in the cell which wears out
after it has divided a certain number of times—something which must be
restored by blending with cells of another strain. What this is we do not
know, and perhaps never shall. The most we know is that it seems to be
something inherent in the nucleus, not the main body of protoplasm of the
cell, for some unicellular animals do not fuse, under the circumstances
related above, but exchange only pieces of their nuclei, and yet derive
the advantages of mutually increased vitality. But if we apply the fact
to our conception of the body as a vast colony of cells with a common
origin, we find that it has an important bearing upon the duration of its
life.
The single eggcell which gave rise to our schematic embryo in Diagram
3, Fig. 1, was formed by the fusion of two cells shed by two distinct
animals. How this one cell grows and multiplies by division is roughly
shown in the diagram and those immediately following it; but though the
cells do not separate, but hang together and form a body, it is obvious
that the colony only amounts to a shoal of unicellular organisms, like
that described above as growing weaker with each successive division
unless blended with individuals of a different stock. This cannot be done
in the body; what would become of our individuality even if such a thing
were possible? The body can help to give rise to new bodies, but its own
tissues must wear out, and when the colony of cells is exhausted it must
die. Careful diet and regular habits, the minimum of wear and tear, may
enable the body to run its full term; but they cannot lengthen the lease
of life.
So far only can the physiologist take us. Physiology may teach us how to
develop our powers and economize our strength; it is already beginning to
convert medicine from an art into a science; it will, it is to be hoped,
shortly work a revolution in our at present barbarous ideas of how to
rear and educate children; it may, in short, teach us how to make the
very most of life and die easily; but not until, if ever, it understands
the physical basis of life, and perhaps not even then, will it be likely
to succeed in prolonging a man’s days much beyond the traditional
fourscore years.
CONCLUSION.
While the physiologist is quietly working, making slow but sure
progress, his critics, friendly and otherwise, buzz about him like bees.
There are some who are in a chronic state of excitement, expecting a
revolutionizing discovery from hour to hour; there are others who assure
him that he has reached the limit of human powers of comprehension, and
can never know much more than he does to-day; and, lastly, there are
those who declare that he has done next to nothing, and that his utmost
endeavours have failed to effect any real result, and leave all the
important secrets of life untouched.
No one knows better than the physiologist how mistaken is the
oversanguine class first mentioned. In no department of science,
certainly not in physiology, is it possible to reach the top of the
ladder by a bound; each rung must be mastered in order. In invading the
unknown land the scientist must thoroughly explore and effectively occupy
as he advances. He must annex as he goes along; the flying columns which
try to reach the enemy’s capital by a dash are never heard of again. In
physiology the publication of the various steps is sometimes withheld
until the objective has been reached, but our knowledge of life is like
Solomon’s temple: a David collects the material, and his successor raises
the edifice. The world watches it grow. It is not like those bewildering
and unstable palaces of the ‘Arabian Nights,’ built by genii in a single
night, and often vanishing as mysteriously.
In every age there have existed people who declare that men can never
know more than they do at the moment. There were plenty of them when the
science of physiology was unborn, and there will be plenty more of them a
hundred years hence; only then they will refer with tolerant amusement to
the crude and elementary ideas of their predecessors at the beginning of
the twentieth century.
The third class, who take such delight in minimizing the achievements
of the physiologist, usually are found, if anyone takes them seriously,
to know very little either about the science of physiology itself or
the history of its growth. I leave the reader to form his own verdict
upon the value of the results obtained from their exceedingly brief and
sketchy description in this little volume, with the remark that the
science is barely more than three-quarters of a century old, and the
most important additions to our knowledge have been made within the last
twenty years.
There were, paradoxical as it may sound, great physiologists before then;
the work of Harvey, who three centuries ago discovered the circulation
of the blood, is above all praise; but how nebulous must have been
their ideas may be seen from the following facts: It was only at the
beginning of the nineteenth century that the atomic theory of matter
was formulated; it was not until twenty years later that the world was
startled by a daring chemist who showed that organic compounds obeyed
the same natural laws as inorganic; and not until ten years later was
the cellular structure of animals, the groundwork in all study of life,
recognised.
Even when the science was set upon firm foundations, progress was
at first necessarily slow: the organic chemist took some time in
examining and classifying the compounds met with in the body—he has not
finished yet; and even when the cell theory was grasped, it required
much ingenuity and long patience to devise ways of examining organs
under the microscope, so that their structure could be made out. The
microscope itself was a poor toy fifty years ago, magnifying a diameter
ten times where now it magnifies a hundred, and giving only a dim and
distorted image. The perfecting of the microscope, and the introduction
of anæsthetics and antiseptics, have led to enormous strides being made
within the last two decades.
The result of the advance in chemical knowledge, and the introduction of
fresh aids to investigation, led to the discarding of vital force as a
working hypothesis. Vital force was the bane of the earlier biologists.
They made it accountable for all they could not understand, and with
this restatement of their difficulties—a restatement which they called
an explanation—refrained from further research. But when it was found
that many of these inexplicable phenomena, though refractory, yielded
to careful study, and could be explained by chemical and physical laws,
the physiologist ceased to say of them, ‘They are problems connected
with Life, and therefore explained by Vital Force, which is past man’s
understanding,’ and frankly admitted that there were many things which he
did not as yet fathom. Recently a vitalistic school has cropped up again,
declaring that all that it cannot understand must infallibly be due to
some occult agency. It shows remarkable vitality in surviving the shocks
of successive discoveries.
Turning once more to the present day, we will conclude with a brief
glance round a physiological laboratory, and see by what methods the
physiologist is preparing future surprises. The chemical department first
claims our attention. The imports and exports of animals are carefully
balanced, and the changes produced in the food examined. The animal is
enclosed in an airtight chamber, air of known composition being pumped
into it, and the air which escapes analyzed. The animal most used for
this experiment is man himself, since he will take rest and exercise
to order, the latter usually on the treadmill, by which it can be also
measured, and can be relied upon not to while away the tedium of his
imprisonment by gnawing holes in the walls or upsetting his food.
All the substances used as food, found in or excreted by the body, are
being thoroughly studied; but it should be remembered that this is
chemistry, not physiology. Physiology is only concerned with protoplasm,
and the physiologist who goes deeply into the chemistry of non-living
matter has to discipline his mind against forgetting its ceaseless
change, and trying to regard it as though it were constant. The actual
chemistry of protoplasm will be a very hard nut to crack, and may
defy us until we can depict molecules as well actually as we now can
symbolically. Some idea of the difficulty may be formed if we consider
that it is impossible to imagine a pure sample. From the restless
activity which is the condition of its existence, it is always working
changes in its surrounding, always mixed with raw material, and always
masked by the products of its own metabolism. Even if we withhold the
former, it consumes its own substance until the moment of death. It does
not even look homogeneous under the microscope.
Before, however, we can pursue the chemical methods further, it
will be necessary to describe the histological. The reader may have
already wondered how we managed to find out so much about the cellular
structure of the body. It is no easy matter to cut up soft tissue,
of the consistency of an unboiled egg, into thin slices which can be
examined under the microscope. It is done in the following manner: The
bloodvessels of the freshly killed body are injected with a fluid which
instantaneously kills and fixes the cells in much the same way as an
egg is fixed by being hard-boiled. The natural shape of the cells is
thus preserved, and the loss of any of their chemical constituents by
putrefaction prevented. The piece of organ is then impregnated with
and cast in the middle of a solid block of paraffin wax, which is put
into a machine and shaved up into thin slices, about 40,000 to the inch
sometimes. One of these shavings is then stuck upon a glass slide, and
on the wax being dissolved away with some such substance as benzine, a
section of the tissue, about one cell thick, is left on the glass ready
for microscopic survey.
To do anything like justice to the histological methods would require
a volume in itself. When the sections are fixed upon slides, they are
treated with a number of reagents to show their chemical and structural
peculiarities. One section is stained specially to show the nucleus;
another to show the centrosome; another zymogen granules, etc. And, as
all these cannot be shown at their best in one cell, the differently
treated sections have to be separately drawn or photographed, and the
typical structure compiled from several. By careful staining, the
chemical composition of the different parts of the cell is being worked
out, and the effects of rest, activity, feeding, and other influences,
studied.
Take as an example the effects of a meal. A number of animals of the
same litter are fed together out of the same trough. One has been killed
before the meal, and the rest are killed at intervals dividing the
time which must intervene before their next feeding-time comes round.
Series of sections from their organs are prepared, one from each animal
being mounted in order upon the same piece of glass, dipped in the
same reagents, and examined under the same microscope. From a number
of these sections the progressive effects of a meal upon each of the
several constituents of the cell are traced out, and some of the chemical
processes deduced.
Turning to the physical side of physiology, it is unnecessary here to say
more about the means employed for studying the properties of muscle and
nerve than that many of the phenomena occur with such extreme rapidity
that they can only be perceived by the photographic plate. In the study
of the large organs, the physiologist finds a fascinating employment in
devising models in which, so far as possible, all the physical conditions
are reproduced, and this not only for the benefit of his pupils, but to
help himself in perceiving their meaning. Too much reliance must not be
placed on these models, of course, but they have added considerably to
our knowledge of the eye and throat.
It requires no great imagination to perceive the difficulties which lie
in the way of studying the nervous system. Tracing nerve fibres under
the microscope through interminable series of sections is a labour which
can neither be hurried nor scamped. It is greatly aided by pathological
specimens. An animal which has been through life with only one eye will
obviously have central organs of vision showing wide contrasts. Those
connected with the blind eye will be undeveloped, because never used,
while the corresponding lobes of the brain connected with the other eye
will show the effects of doing extra work.
Many of the problems which meet the physiologist can only be solved
by experimenting upon a live animal, and these experiments form by
no means the easiest part of his work. The animal must be kept, so
far as possible, under physiological conditions—that is to say, free
from pain and fright and unpoisoned by drugs. Thanks, however, to an
extensive knowledge and skilful use of anæsthetics, the obstacles to
this method of investigation have been overcome, and its results have
proved very profitable. The absence of pain is a very important factor
in an experiment, and even if the physiologist took the wanton delight
in inflicting suffering which the imagination of his enemies attributes
to him, he would have to restrain its indulgence in his laboratory, or
forego the hope of even moderate success. In this country, moreover,
the Government will not allow such experiments without its express
permission, and the license is very rightly only granted to men whose
researches promise an adequate return, and who are likely to conduct them
humanely and successfully.
Physiological research is not a hobby to be lightly taken up. It is not
one merry round of exciting tussles with tortured and infuriated cats
and dogs; on the contrary, it entails arduous labour and needs infinite
patience. The experiments, often tedious in themselves, have to be
repeated again and again in as many different ways as possible, until
every slight difference in result can be accounted for; and the certainty
that both the methods used and the interpretation given will, when
published, receive the closest, and not in every case the friendliest,
scrutiny by other members of the profession serves as an admirable
corrective to jumping at conclusions. It is, however, an occupation of
absorbing interest, and the physiologist feels amply repaid if he can
think that his labours have added, no matter how little, to that control
over Nature which the severe conditions of modern life make every day
more pressingly necessary.
INDEX
A.
Abdominal circulation, 54
Absorption, 25
Acid, hydrochloric, 23
Afferent system of nerves, 92
Alcohol, 95
Alimentary canal, length of, 19
movements of, 46
origin of, 17
structure of, 46
Amœba, 16
Amœboid movement, 31
Amount of food, 15, 96
Animalcula (unicellular micro-organisms), 5, 8, 16, 31, 102, 105
Aorta, 51
Arm, 57
Artery, 48
Association centres, 91
Athletes, 98
Atom, 2
Auditory mechanism, 74
Auricles, 49
B.
Bacteria, 23, 44
Beaumont, 25
Bile, 24, 62
Blind, experiments with the, 100
Blood, 26
circulation of the, 48
clotting after death, 104
corpuscles, 29, 45
course of the, 27, 55
pressure, 51, 66
Body, the, 9, 94
Boils, 45
Bone, 36, 56, 98
Brain, 63, 79, 102
Breathing, 25, 52, 82, 97
C.
Canals, semicircular, 77
Capillaries, 48
Carbohydrate, 14, 96
digestion of, 22
Carbon, 11, 13
Carbonic acid gas, 13, 25
Cartilage, 36
Cavity of the body, 43
Cell, 4
a chemical laboratory, 6
division of, 32, 105
exhaustion of, 102
movements of, 5, 31
nerve, 60, 87
spinal ganglion, 63
Centres, nervous, 63, 78, 88, 91
Centrosome, 31
Cerebellum, 84
Cerebral cortex, 86
hemispheres, 85
Chemical compounds, 2
needs of body, 11
Chemistry of the body, 8
Chords, vocal, 93
Cilia, 33
Circulation, 27, 97
course of, 28, 55
mechanism of, 48
Circulating fluids, 26
Clotting of blood, 104
Cochlea, 76
Cold, 83
Collagen, 24, 35, 96
Compounds, chemical, 2
Conductivity of protoplasm, 59
Connective tissue, 24, 35, 96
Conservation of energy, 92
Contraction of heart, 49
of muscle, 34
Cooking, 96
Cord, spinal, 63, 78
Corpuscles, red blood, 29
white blood (leucocytes), 45
Cortex, 86
Cough, 65, 67
D.
Death, 103
Diaphragm, 52, 97
Diet, 11, 95
Digestion, 11
by body, 22
by cell, 16
Doctor, ix
Drum of ear, 75
E.
Ear, 74
Education, 99
Efferent system of nerves, 92
Egg, white of, 13
Electric manifestations in tissues, 40
organs of fish, 42
stimulation of tissues, 37, 88
Embryo, development of, 17
Endolymph, 74
Enzyme, 15
Epilepsy, 89
Equilibrium, 66, 84
Exercise, 97
Eye, 73
education of, 100
protection of, 65
F.
Fainting, 66
Fat, 14, 24, 27
Fatigue, 40, 102
Ferments, 15
Fibres, connective tissue, 35
muscle, 34
nerve, 42, 60
Fibrils, muscle, 35
Fish, electric, 42
Food, 10, 95
amount of, 15, 96
vacuole, 16
Forehead, 91
Frog experiments on muscle, 37
reflexes, 68
G.
Ganglion, spinal, 62
Gastric juice, 23
Gelatin, 24, 35, 96
Gland cells, 21
Glands, origin of, 20
salivary, 22
sweat, 71
tear, 65
H.
Hæmoglobin, 29
Hair, 70
Harvey, 108
Head, 79
Hearing, 74
Heart, 48, 66
Heat, production of, 13
regulation of, 83
Histological methods, 110
Huxley, 7
Hydrochloric acid, 23
I.
Internal secretion, 29
Intestine, movements of, 47
Involuntary movements, 65
muscle, 34
Iron, 95
K.
Kidneys, 29, 95
L.
Labyrinths of ear, 74
Leucocytes, 42
Limbs, 56
Liver, 24
circulation through, 27, 54
Living matter, 1
Localization of function in the brain, 86, 88
Lungs, 52, 93
absorption by, 25
Lymph, 26
circulation of, 27, 48
M.
Marshall’s fibrils, 35
Meat in diet, 15, 95
Medicine, ix
Medulla (hind-brain), 82
Memory, 91
Micro-organisms, 5, 8, 16, 31, 102, 105
Microscope (histological methods), 110
Milk, 95
Mixture, 3
Molecule, 2
Mouth, 79
Muscle, 34
how studied, 37
of alimentary canal, 46, 95
Muscles, 97
of limbs, 56
stiffening at death of, 104
N.
Nerve cells, 60
centres, 63, 78
fibres, 42, 60
Nervous system, 58
Nitrogen, 12
Nucleus, 4, 21, 32, 106
O.
Olfactory lobe of brain, 85
sense organs, 72
Oxygen, 11, 13, 25, 82, 95
P.
Pain, 91
Pancreas, 24
Paramœcium, 16
Peptone, 24
Perilymph, 74
Peristaltic movements, 24, 63, 95
Physical culture, 98, 101
Physiological methods, 37, 88, 109
Physiology, ix, 107
Pleasure, 91
Pressure, blood, 50, 66
Proteids, 12, 23, 96
Protoplasm (living matter), 1
Pulmonary circulation, 53
R.
Reflex action, 62, 65
centres, 82
Removal of refuse, 27
Rennet, 21, 24
Respiration, 25, 52, 82
Ribs, 52
Rigor mortis, 104
Rutherford’s fibrils, 35
S.
St. Martin, 25
Saliva, 22
Salt, 11, 14, 29
Scavengers of body, 43
Sea-sickness, 78
Secretion, 21
internal, 29
reflex control of, 66
Semicircular canals, 77
Shivering, 83
Sight, 73, 100
Skin, 69
Sleep, 102
Smell, 72, 84
Sneeze, 65
Sound, 76
Speech, 93
Spinal cord, 63, 78
Spirits, 95
Spleen, 27, 54
Standing, 66, 84
Starch, 14, 96
Stomach, 23, 47
Sugar, 14, 96, 102
Sympathetic nervous system, 62
T.
Taste, 72, 84
Tear glands, 65
Temperature, 83
Tendon, 37
Tetanus, 39
Thorax, 52
Thought, 91
Thyroid gland, 29
Trachea, 52
Training, 98
U.
Unicellular animals, 5, 8, 16, 31, 102, 105
V.
Valves of heart, 48, 50
Varieties of protoplasm, 6
Vegetable food, 96
Veins, 48
Ventricles, 49
Veratria, 40
Vision, 73
Voluntary movement, 56
muscle, 34, 56
W.
Walking, 66, 84
Warmth, 83
Water, 95
_Ballière, Tindall and Cox, 8, Henrietta Street, Strand_
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