RECENT PHYSIOLOGY. 87 



on absorption in creneral. It has been lately shown that they are prac- 

 tically 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 tlie other hand, they are freely 

 permeable to ammonium chloride, urea and other bodies. The condi- 

 tions governing the passage of substances into the corpuscles are evi- 

 d(>ntly very different from those which determine the permeability of 

 an ordinary membrane. This is further shown by the fact that by cer- 

 tain methods of treatment the colossal molecules of the red coloring mat- 

 ter 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 diffu- 

 sion of gases, appear to exercise a kind of 'selection' in the taking up 

 of many substances which have nothing to do with their particular func- 

 tion. 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 charac- 

 teristic 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 ox3^gen 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 



