CHAPTER III
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.