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

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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.