FROM THE SMALL INTESTINE 653 



(3) Reid's third series of experiments was devised with the 

 object of testing how far diffusion and osmosis play a part in 

 absorption, under conditions removed as little as possible from 

 the normal. The practical difficulty is how to devise experiments 

 which shall give definite results and not at the same time injure 

 the epithelium. As has been pointed out, the only criterion of 

 epithelial injury at present available is the retention by the 

 epithelium of its one-sided permeability to NaCl. But this at 

 best is a very rough guide, for Cohnheim showed that it was 

 possible to poison the cells up to a certain point without depriv- 

 ing them of this power. 



The first experiments dealt with the absorption of water, and 

 were based on the following considerations. If water is taken up 

 from a solution in the gut by osmosis, then the amount absorbed 

 must bear relation to the sustained excess of osmotic pressure on 

 the blood side of the membrane, due to substances to which the 

 membrane is impermeable existing in solution in the blood. If, 

 then, the osmotic pressure of the blood were raised by an addition 

 of NaCl, which is a normal constituent of the blood and cannot 

 diffuse into the gut, water should be absorbed from a solution in 

 the intestine more rapidly after than before the addition of NaCl 

 to the blood. Reid injected slowly into the circulation enough of 

 a 5 per cent, solution of NaCl to raise the percentage of NaCl in 

 the blood by about O'l per cent. He injected actually 0*3 grm. 

 NaCl per kilo of body weight, which was about 100 c.c. of the 

 solution. He placed in the intestine a 4 per cent, solution of 

 dextrose. Comparing the absorption before and after the injec- 

 tion of the NaCl, he found after the injection that the absorption 

 of water was unaltered. The absorption of dextrose was generally 

 decreased, but this point is only incidental to the other. As the 

 percentage of NaCl in the intestinal contents did not rise after 

 the injection, Reid considered that the epithelium was uninjured, 

 and concluded from his results that the uptake of water is not a 

 simple function of the osmotic pressure of substances to which 

 the normal epithelium is impermeable from the blood to the gut 

 side. In these experiments it is clear that the epithelium had 

 escaped gross injury, but it is not so obvious that it would be 

 uninfluenced by the NaCl injection. It is only necessary to 

 remember MacCallum's experiments to see that the epithelium is 

 just as susceptible from the blood as from the gut side ; and it 



