on absorption in general. It has been lately shown that they are practically non-conductors of electricity in comparison with the liquid portion of the blood, or plasma, in which they float. This is due to the fact that the salts of the plasma, whose ions carry the electricity, penetrate the corpuscles with difficulty, sodium chloride, for example, scarcely passing into them at all. On the other hand, they are freely permeable to ammonium chloride, urea and other bodies. The conditions governing the passage of substances into the corpuscles are evidently very different from those which determine the permeability of an ordinary membrane. This is further shown by the fact that by certain methods of treatment the colossal molecules of the red coloring matter of the blood may be caused to escape from the corpuscles, while the much smaller molecules of the inorganic salts remain still pent within them. Such results are of great interest, for they show that cells which, as regards their main physiological office, the conveyance of oxygen to the tissues, seem to be governed strictly by the physical laws of diffusion of gases, appear to exercise a kind of 'selection' in the taking up of many substances which have nothing to do with their particular function. The suggestion is scarcely to be avoided that in this case a purely chemical or physical 'attraction' underlies the apparently selective power. And this idea is strengthened by the fact that all those characteristic reactions of the colored corpuscles can be obtained many hours after the blood has been removed from the body, and, therefore, at a time when their 'vital' activity may be supposed either to have been extinguished or to have undergone a serious diminution.
The absorption of oxygen and excretion of carbonic acid by the lungs have long been considered conspicuous examples of the passage of substances through a living animal membrane by ordinary physical diffusion. But, according to the recent observations of Bohr, oxygen may, within certain limits, be absorbed, when its partial pressure or tension in the blood is greater than that in the air contained in the lungs, and carbonic acid may be excreted when its pressure in the blood is less than that in the air of the lungs. Haldane and Smith have indeed shown that in man the pressure of the oxygen in the arterial blood is actually higher than in the outside air. These results are, of course, incompatible with a simple theory of diffusion, and show that the cells of the pulmonary membrane have the power of forcing oxygen to move in one direction and carbonic acid in the other even against the slope of pressure.
As regards the physiology of particular organs, attention has been, in recent years, attracted in a marked degree to two subjects: the so-called internal secretions of certain glands and the arrangement and actions of the nerve-cells and fibers which make up the central nervous system.
By an internal secretion we mean a substance or substances
