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 filtration and electro- 
osmose are excluded, we have no physical explanation. Thus Heiden- 
liain 1 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.- 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 stillliving 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 bod}-, 
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. ges. Physio?., Bonn, 1894, Bd. lvi. 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. 
