The fact that the cell suffers also is a striking illustration of the essential unity of the nerve-cell and all its branches, whether they are long or short—three feet in length, as the axones that run from the lower part of the spinal cord to the foot may be, or a thousandth of an inch, like some which arise and terminate within the gray matter of the cord and brain.
Simpler, at first sight, in their action and organization than nerve-cells or muscular fibers are the gland-cells which secrete the digestive juices. The cells of the kidney which separate from the blood the constituents of the urine, and the cells which line the intestine and are engaged in the absorption of the food appear to be simpler still. And simplest of all are the flat, scale-like cells that line the lungs and have to do with the taking in of oxygen and the elimination of carbonic acid and the similar cells which form the walls of the capillaries and are concerned in the production of lymph. Accordingly we have seen of late years, in connection with researches on the functions of such cells, a revival of formal discussion of the general problem of physiology: whether the vital processes can be completely explained in terms of the laws of unorganized matter. This is a question which has had a singular fate. Answered at certain epochs by an almost unanimous negative, it has emerged again with every fresh advance in mechanical, physical or chemical knowledge, and for a time has seemed about to be settled in the affirmative. It was so in the seventeenth century when the discoveries of the new geometry and the new mechanics were hailed by Descartes and the iatro-mathematical school who were his lineal descendants, although they denied their parentage, as the key which was to unlock all the secrets of that cunningly devised automaton, the animal body, and particularly to explain its movements. At a later date, the determination of the laws of the diffusion of gases appeared to solve the problem of the passage of gases through the lungs, and the determination of the laws of diffusion of dissolved substances and of endosmosis, the problem of absorption from the intestines. With Ludwig's researches on the formation of urine, secretion seemed about to pass out of the group of mysterious 'vital' phenomena, and to become a mere process of filtration. But always as renewed investigation has brought into clearer light the peculiarities, the wizard tricks, one might almost say, of those rare mechanisms that ply so deftly even in the common business of the bodily machine, the gulf that separates the inorganic from the organized world has opened wide as ever, and physiology has still had to wait for a new Curtius to close it.
Quite recently the experiments of de Vries, Van't Hoff and others on osmosis have supplied further physical data for the solution of this perennial problem, and have, therefore, become the starting point of numerous physiological researches. Among these may be mentioned
