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.