Читать книгу The Body at Work: A Treatise on the Principles of Physiology онлайн

28 страница из 88

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.

motile cells

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 (ssss1). 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.