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.