THE UNIT OF STRUCTURE
Immediately after its discovery in the seventeenth century, the
compound microscope was applied to the study of minute plants and
animals, their organs and tissues. In this connection and for this
purpose the microscope has steadily improved, until perfection has
almost been attained. Calculations based upon the physical properties
of refracting media show that the limits of the assistance which it
can give to the eye have been very nearly reached. One of the first
results of the application of the microscope to the study of parts
of plants was the discovery of their cellular structure. Robert
Brown, looking at slices of cork, saw that its tissue is divided into
compartments. It is difficult to ascertain who it was that first used
the word “cell.” The resemblance of a slice of vegetable tissue or the
surface view of a petal of a flower to honeycomb is so striking that
the same comparison probably occurred to the mind of everyone who saw
it. Further study with better instruments showed that the cells are not
empty. Each cell contains cell-juice, or cell-substance, and in the
centre of the cell-substance a miniature cell, the nucleus. Naturalists
therefore extended the connotation of the term. A cell was no longer a
space with enclosing walls; it had contents. A nucleus was invariably
a constituent of the cell. The cell was regarded as an anatomical
unit, consisting of a wall, cell-contents, and nucleus. In 1839
Theodor Schwann, using his microscope in the study of animal tissues,
recognized the similarity between animals and plants. Animals also, he
discovered, are aggregations of cells. He enunciated the =Cell Theory=.
Philosophers are always ready to generalize. It is their business.
Seeing that vast numbers of organisms are single cells, that they
feed, breathe, divide, and reproduce their kind—in fact, carry out all
the functions of life—as isolated cells, they conceived the idea that
a visible plant or animal is a community of cells, each an organism
in itself. As bees are units of a swarm, as men and women are units
of a state, cells are units which for the sake of mutual protection
remain associated in a multicellular body. The physiological or
sociological aspects of this conception we shall consider shortly; but
the anatomical basis of the cell theory was laid without a sufficient
testing of the facts upon which it rests; or, rather, one ought to
say that, although the axiom, enunciated by Virchow when he applied
the cell theory to tumours and other morbid growths, _Omnis cellula a
cellulâ_, holds good, the applications of the theory which certain of
its later exponents have made are not necessary sequents.
Every plant, every animal, commences its existence as a single cell.
An organism which is permanently unicellular divides. Each of the
separate cells into which it divides is a new individual. Higher
plants set aside certain cells as ovules, which in due course, after
conjugation with pollen grains, grow into plants. In the same way the
ova of animals, by repeated cell division, reproduce the species. The
individual commences as a single cell. Its complicated body, composed
of various organs and various tissues, is formed by the multiplication
of cells. Each of the innumerable cells of which it is composed has
the structure, and may therefore be presumed capable of performing
all the various functions, of a unicellular organism. But it does not
follow that the cells retain their individuality. Even unicellular
plants (_e.g._, the extraordinary vinegar and tan fungi, myxomycetes)
may for a time merge their individuality in a common mass formed by the
aggregation of many cells.
The cells of higher plants are not always, or even generally,
anatomically distinct. Their protoplasm, the essential part of every
cell, is united with the protoplasm of neighbouring cells by threads
which traverse the cell-walls. The cells of the connective tissues
of animals are united into a web, or syncytium. This is especially
noticeable during early stages of growth. Nerve-cells are connected
together by conducting filaments (neuro-fibrillæ). It is possible
that nerve-cells and the muscle-fibres which they innervate are from
the beginning united by nerve-filaments—that the nerve-cell and
muscle-cell grow apart without severing this thread-like connection.
Certain anatomists regard the nerve strand which connects a cell in
the central nervous system with a number of muscle-fibres, placed,
it may be, at a great distance from the nerve-cell, as the bridge
which has never been broken in the process of cell division and
displacement, which made one primitive cell into a nerve-cell and a
group of muscle-cells. Muscle-fibres are not separate cells, but cell
complexes. Each muscle-fibre contains scores, in some cases hundreds,
of nuclei (Fig. 16). It is a cylinder, perhaps 2 inches long, in which
cell division is incomplete. Tendons are bundles of exceedingly slender
fibres which lie side by side, like silk threads in a skein. The row of
cells which gives rise to a tendon undergoes incomplete cell division.
Their nuclei divide, and a small quantity of soft body-substance is
set apart for each nucleus. The rest of the mass consists of fused
cells. It constitutes a continuous rod, which becomes fibrillated as
it grows. Vegetable cells are separated by cell-walls. Animal cells
tend to develop intermediate partitions; but the partitions are so
thick that they can no longer be described as walls. In cartilage the
cell-bodies are embedded in a great mass of intercellular substance,
or matrix. In this intercellular substance elaborate developments may
take place. Elastic fibres may make their appearance in it to form
elastic cartilage, as in the case of the epiglottis. In these various
instances, although it is perfectly true that tissues are formed by
cell division, the cells are not, strictly speaking, separate units.
They are not completely divided one from another. It is impossible to
recognize their anatomical boundaries.
But there is a much more serious difficulty in applying the cell
theory—the difficulty of deciding what are the essential parts of
a cell. Long ago it was recognized that many animal cells—white
blood-corpuscles, for example—have no cell-wall. It was therefore
decided that cell-body and nucleus are the only essential parts. But
what is to be said of the red blood-corpuscles of mammals? (Fig. 4).
Are they cells? They have neither cell-walls nor nucleus; nor does
their substance present the structure which is usually associated with
the “body-substance” of cells. They are not produced, if the view held
by many histologists be sound, by cell division, in the ordinary sense
of the term, but appear as spots, gradually growing into discs inside
the body of a blood-forming cell. The discs are extruded when they
reach their full dimensions. Yet the tissue, blood, is composed of
these blood-discs and the intermediate substance blood-plasm. Mammalian
blood might be dismissed as a non-cellular fluid secretion containing
formed elements, if it were not for its history. In all animals below
mammals the red corpuscles are cells with nuclei and cell-bodies. The
absence of nuclei in mammals is due to the recognition by Nature of the
fact that, as the blood-cells will never be called upon to divide, it
is a waste of material to provide each of them with a nucleus. Not only
would the nucleus be useless, but it would take up space, diminishing
the capacity of the corpuscle for carrying hæmoglobin. The process of
cell division is in consequence curtailed. There are, it is true, other
ways of looking at this problem. The cells which line the bloodvessels
stand in some sort of nutritive relation with the blood. When the
lining cells of the bloodvessels are injured or inflamed, the blood
clots. But here again it is somewhat straining a point to say that
these lining cells are the cells of the blood, and the blood a kind of
intercellular substance; especially as a distinction would have to be
made between mammals with non-nucleated blood-corpuscles and birds with
complete blood-cells.
The physiologist, if he is to feel sure of his ground, needs to know
the minute anatomy as well as the naked eye anatomy of the body. But
what is there that he does not need to know? He must be chemist,
physicist, biologist, pathologist, and expert in various other branches
of science. Microscopic anatomy, or histology, as it is commonly
termed, will be called upon in this book only when it has evidence to
give which bears directly on physiological problems. We have dwelt
at some length upon the cell theory because the physiologist needs
starting-points. He needs to have in his mind a conception of the
fundamental structure of the body. Protoplasm is the material which
lives. We begin with protoplasm albeit our conception of protoplasm is
so difficult to formulate that we are obliged to admit that in using
the term we are almost guilty of playing with words. Protoplasm is
the most living substance. The substance which is most alive always
presents itself to us as an imperfectly transparent, viscous material,
which proves on analysis to contain a large quantity of certain
proteins mixed with various organic and inorganic compounds. Protoplasm
is organized into, or distributed amongst, cells, which in any given
tissue present a fairly uniform size. What determines the size of
cells? Speaking generally, cells are small—say about 0·01 millimetre
in diameter. In early stages of growth, cell division occurs as soon
as the cell attains to something like this size. It would seem that
when nutriment is abundant cells add to their protoplasm more than they
lose. Having attained certain dimensions at which the conditions most
satisfactory for cell life reach their limit, cell division occurs. The
big drop falls into two smaller drops, each of which grows more rapidly
than the big one was growing at the time when it began to divide.
But if there be an optimum size for nutritive purposes, this limit
is suspended in many cases, and for various reasons. Take the ovum
itself as an example. It is vastly bigger than the cells into which
it divides. The yolk of a hen’s egg is, when first formed, a single
cell. By the time the egg is laid cell division has already set in. In
the embryo there are cells which surpass the average dimensions—the
unexplained “giant cells” which appear in the liver as soon as it can
be recognized as such (_cf._ p. 65). These disappear from the liver,
but are for a time evident in the spleen. The large cells found in the
marrow of bone, some with a great single nucleus, others containing a
bunch of separate nuclei, also show that there is no fixed limit of
size. It is generally considered that the giant cells of marrow—or,
at any rate, those which are multinucleated—are leucocytes which are
engaged in scooping out the bone; consuming the hard tissue on the
inner surface of the hollow cylinder in order that, by deposition of
new material on the outside of the cylinder, the size of the whole
bone may be increased—leucocytes battening on bone which, owing to
interference with its blood-supply, is breaking down. They have not
time to divide. Nourishment is superabundant. Although much too large
for a vigorous standard of cell life, they continue to grow, putting
off the duty of cell division until the supply of nutritious food
begins to run short.
The most remarkable variations in size are to be found amongst the
cells of the nervous system. It may be given as one of the most
distinctive characters of nervous tissue that its cells have no fixed
or standard dimensions. A nerve-cell enters into connection with
other nerve-cells and with muscle-fibres by means of branches, or
cell-processes, as they are termed. The cells may be globular, as in
the sympathetic system, or star-shaped. Each cell gives off a certain
number of processes, which divide like the branches of a tree, and one
process which may run for a very long distance without dividing. This
latter thread-like process places it in communication either with a
distant part of the central nervous system or with the muscle-fibres
which it controls. By means of such a thread a cell in the spinal cord
may be connected with muscle-fibres of the hand or of the foot. The
thread is really a bundle of filaments (neuro-fibrillæ) which separate
to supply a number of muscle-fibres. It is, in its whole length, a
part of the cell in which it originates. The size of the cell varies
as the number of filaments in this bundle (termed the “axon”), and
possibly also as their length. Hence it comes about that nerve-cells
may be amongst the smallest, or they may be the very largest, in the
body. The so-called “granules” of the cortex of the cerebellum and
of the cerebrum are almost as small as red blood-corpuscles (Fig.
23). Each of them has five or six minute branched processes and an
exceedingly delicate axon. The large cells of the cerebral cortex,
which send their axons far down the spinal cord, and the large cells of
the spinal cord which supply the muscles of the body, have a diameter
ten or twelve times as great as that of a granule. But larger still
are the nerve-cells which supply the electric organs of the torpedo
and other electric fishes (p. 295); and largest of all are the cells
which innervate the curious “fishing-rods” of the strange angler fish
(_Lophius piscatorius_). It is difficult, owing to their irregular
shape, to say how large these cells are; but they are visible to the
naked eye.
The anatomical unit of structure is the cell. Cells are the bricks of
which the body is built. Some are large, others small, as befits the
part which they take in the construction of the body. If the tissue be
merely a supporting tissue, connective tissue, cartilage, bone, its
cells are uniform in size and small. If it have functions to perform
which in some cases are carried out best by small cells, in other cases
by large ones, the cells are adapted in size to the work that they have
to do. Of the various kinds of wandering cells, some—the bone-forming
cells (osteoblasts), for example—are small; others—the bone-eating
cells (osteoclasts)—relatively large. Nerve-cells, like telephone
exchanges, are large or small according to the size of the area which
each supplies.
All animals of complex organization, from starfishes and sea-urchins
to Man, are inhabited by =motile cells=. In addition to the bricks
which enter into the construction of its fabric, each fixed in its
place and definitely united to its neighbours, the animal contains
leucocytes which wander through its tissue-spaces or float down
the streams of lymph or blood. We are disposed to speak of these
wanderers as inhabitants of the body, to distinguish them from the
elements which enter into the construction of their habitation. It
is difficult to avoid the temptation of describing the body as a
habitation. Allegorical as Aristotle’s distinction between body and
soul—between the habitation and that which inhabits—may seem, when
contrasted with the exact language of modern science, it would save
many a periphrasis if we might still use the monosyllable “soul.” The
fixed tissues constitute a unity, bound together by nerves. The work
done by glands and muscles is done in response to directions conveyed
by nerves. It is impossible to say where the control of the nerves
ceases—to point out any fixed tissue which is not co-ordinated with
other tissues, nor susceptible to the influence of the environment
as impressed upon the central nervous system, through the medium of
sense-organs. The fixed tissues constitute a habitation for the “soul.”
They share in a common life. The wandering cells are as independent
of control as the parasites which occasionally find entrance into the
body. Each must have a soul of its own in Aristotle’s sense. Like
parasites, they carry on all the business of nutrition, respiration,
cell division, without reference to the needs of the fixed tissues.
They take what they require from the lymph as it leaves the intestines
loaded with the products of digestion; they take it from the lymph
in the tissue-spaces; they take it from the blood. When nutriment
or oxygen runs short, they do not share the privations of the fixed
tissues. Only indirectly is their well-being affected by that of
the body as a whole; only accidentally is the death of the body the
occasion of their death. The same might be said of such parasites as
the “blood-worms” of Egypt, or the trypanosomes (the cause of “sleeping
sickness”) of Equatorial Africa. Occasionally, in the rare disease
lymphocythæmia leucocytes multiply exceedingly, not, apparently,
in response to a call for their presence in large numbers, but in
defiance of the needs of the economy, and with baneful results. To the
indispensable services which wandering cells render, frequent reference
will be made. In the present connection, and while we are searching
for the principles of construction of the animal body, it would be
desirable, if we could do so, to define the status of wandering cells.
If they entered the body from without, they would be parasites of
commensal type, intruders who share in the food and shelter of the body
in return for service. But they do not enter from without. They are
cells of the growing body which, detaching themselves from the cells
which are forming tissues, assume a wandering life. They are not to be
recognized in the embryo until development is considerably advanced.
Their origin is far from clear, but histologists believe that, although
they are not recognizable as wandering cells in the earliest stages
of growth, they, or rather their parent cells, are set apart at a
very early date. Probably they are not formed in the embryo proper,
but in the “extra-embryonic area,” from which they emigrate into the
embryo. In this sense they come in from outside. But, after all, the
extra-embryonic area equally with the embryo is a product of the ovum.
Looking at the body as a whole, we recognize a common life, a soul
in Aristotle’s sense, which inhabits the framework of fixed tissues;
and at the same time we see a multitude of independent cells, each an
organism in itself, produced, like amœbæ, from similar independent
cells by cell division, absorbing the body-fluids, consuming invading
germs and fragments of decaying tissues, dying, disintegrating, in
their turn absorbed. Wandering cells are autonomous in the largest
sense.
All multicellular plants and animals are formed by division of a
primitively single cell, the segments remaining in contact. As the
scale of life is ascended, the cells which are massed together in
the body, whether of a plant or of an animal—we are still unable to
find any word other than body for the thing as a whole—tend more and
more to differ in appearance. Some are large, others small. Some have
cell-walls; others have none. Some remain “protoplasmic”; others are
largely composed of “metaplasm.” Better terms are wanted to connote
“most living substance” and “less living substance” respectively. It
would be easy to coin suitable words, but, alas! the nomenclature
of physiology is already hopelessly encumbered, and there is little
prospect that a bad word will die when a good one is available in its
stead. Differences in structure indicate differences in function. A
division of labour has set in. The cell starts with capacities for
every function. Its particular situation renders it desirable that
it should cultivate one capacity at the expense of the rest. It
specializes in a particular direction. If it happens to be placed
in the centre of the body on the course of the bloodvessels which
bring to the embryo food and oxygen from its mother, it develops a
great capacity for taking up food. It accumulates in its substance
a vast quantity of nutriment which it cannot consume, holds it, and
passes it on into the blood-stream as it is required. Thus the liver
is formed. In the embryo it attains to a great size, equal to about
one-half the whole body-weight; but whether storing food be its chief
function at this stage, or whether the other special functions for
which it is responsible are equally important, remains a question
for further research. In subsequent life its main work is to store
food. After birth, when the child prepares its own food by processes
of digestion in its stomach and intestines, the blood-supply of the
liver is so modified that the blood from the digestive organs is
passed through it. Now and for the rest of life the liver is the
storehouse of food, the larder of the body. It is a reservoir from
which supplies are distributed as required. A liver-cell retains many
primitive characters. It is soft and destitute of envelope. But under
the microscope it appears, unless it be taken from a starving animal,
unlike any other cell (Fig. 7). It is always loaded with masses of
glycogen. Sometimes it contains fat globules also. This is perhaps the
simplest of all instances of specialization of function. An amœba can
take up food. Presumably it always absorbs as much as it can get, the
simple law of growth with cell division making it impossible for it
ever to get too much. The cells which in the liver are so fortunate as
to be placed on the route along which food is carried into the body
retain the appetite of an amœba, but lose its capacity for growth and
cell division. They return to the blood-stream, when it is deficient in
food, the stores which they took up when food was in excess.
The specialization of a gland-cell is opposite in kind to that of a
liver-cell. It takes up no more food than it requires, but it has
developed a great capacity of producing from the food a substance which
would no doubt be needed for its own purposes were it an isolated cell,
but which the gland-cell places at the service of the body as a whole.
An amœba can digest proteid substances. A cell of the pancreas produces
the ferment necessary for the digestion of proteins, and secretes it
into the alimentary canal.
To take another instance of specialization. An amœba responds to
stimulation by changing its shape. It contracts in one direction,
expands in another. A muscle-fibre has developed the capacity of
contraction at the expense of all other functions. During the course
of its growth it changes from a round cell into one that is elongated.
The elongation is in the direction in which it acts with greatest
efficiency. Its cell-substance is very highly specialized in order that
it may have the maximum capacity of contraction in this direction.
Sensory cells develop to a maximum the capacity of responding to
external force; nerve-cells, the capacity of conducting the impulses
generated in sensory cells. The body is a republic in which every
citizen develops to the highest degree the capacity of doing the thing
which his situation makes it desirable for him to do.
The possibility of isolated cell life, and the necessity within
certain limits of cell division, have led biologists to dwell too
much upon the independence of the separate cells of which the body is
composed. Protoplasm organizes itself into cells, but cells are not
necessarily anatomically distinct. They may be the partially separate
elements of a syncytium, or there may be but the faintest traces of
cell separation. The objection to looking upon cells as isolated,
self-complete units does not hold good to the same extent when they are
viewed from a physiological standpoint. A cell is an administrative
area. For purposes of nutrition, respiration, and cell division it is
autonomous. It is responsible for its own local affairs. If a part is
cut off from it, this part loses its vitality; this, at least, is the
conclusion drawn from the atrophy of the axons of nerves when they are
cut off from the cells of which they are outgrowths. Apparently we must
understand by “the cell,” when speaking of the cutting off of a part,
the portion of the cell which retains the nucleus; although we must
be careful not to lay too much stress upon the nucleus as the centre
of cell life. Red blood-corpuscles, as already pointed out, have no
nuclei, and yet they live. Cell growth, estimated by mere increase in
size, does not depend upon the nucleus. Many cells of the skin and its
appendages increase considerably after the nucleus shows changes which
clearly indicate that it is far advanced towards decay. But increase
in protoplasm, cell growth in a legitimate sense, and especially cell
division, are dependent upon the presence of an active nucleus. While,
therefore, histologists no longer formulate the cell theory in the
restricted terms in which it was enunciated some years ago, they still
regard the cell as the unit of structure and unit of function. The body
is built of cells, and whatever is done by the body as a whole is done
by its individual cells.