284 DIFFUSION, OSMOSIS, AND FILTRATION. 



either side of a living membrane, and a current is found to pass from 

 one side to the other, when the possibilities of nitration and electro- 

 osmose are excluded, we have no physical explanation. Thus Heiden- 

 hain x has demonstrated that serum is absorbed by the intestine. The 

 pressure in the gut in relation to that in the capillaries, it is true, was 

 not measured, and the serum was not the animal's own serum, yet these 

 objections are not of great force, especially the former, since an excess 

 of pressure in the intestine would probably cause collapse of the capil- 

 laries or venules. 2 It is absurd to maintain that the motion of the 

 blood in the capillaries aspirates the serum through the epithelium, 

 because the rate of the blood stream is too slow to have any appreciable 

 effect in this direction, and weak salt solution is moved across exsected 

 and still living gut with equality of pressure on the two sides and no 

 stream. 3 



This class of absorption experiment appears to be the only one in 

 which it is justifiable to speak of " vital action," for differences in the 

 ratios of " diffusion " of two substances into serum outside the body, 

 and in the cavities thereof, are, per se, no proof of such action, since, as 

 has been already indicated, the physical permeability of membranes 

 differs much to one and the same substance ; and again, the fact that a 

 drug affects the rate of absorption of a substance, after exclusion of the 

 action of that drug (if any) on the circulation, is as well (and as little) 

 explained by stating that the permeability of the membrane is altered 

 by its combination with the drug, as by stating that the activity of the 

 cells is affected. 



In spite of the magnificent labours of Dutrochet, Graham, Pfeffer, 

 van 't Hoff, and Arrhenius, the enigma of the physical chemistry of 

 protoplasm in many cases still puts a limit to the physiologist's concep- 

 tion of the mode of motion of fluids through the membranes and cells 

 of the body. 



1 Arch. f. d. yes. PhysioL, Bonn, 1894, Bd. Ivi. S. 570. 



2 The author has repeated Heidenhain's experiment, using the animal's own serum, and 

 measuring the pressure in the gut, and in a mesenteric vein throughout. Active absorption 

 occurs, of the water, of the organic, and of the inorganic solids of the serum, when the 

 pressure in the gut is far below that in a mesenteric vein, and when all the lacteals leaving 

 the loop have been ligatured. 



3 Reid, Brit. Med. Journ., London, May 28, 1892. 



