650 THE MECHANISM OF ABSORPTION 



anaemia, or poisoned with osmic acid, &c. It seems clear, there- 

 fore, that in the absence of a normal living epithelium, consider- 

 able absorption of serum can still take place, possibly by imbibition, 

 but at any rate by some physical process. 



Further, it does not seem by any means certain that osmosis 

 and diffusion are really excluded in these experiments. Reid 

 seems to assume that the serum proteids are taken up by the 

 epithelial cells without digestion. The possibility of such an 

 occurrence has long been maintained, but recent work has thrown 

 very grave doubts upon it. For instance, Mendel and Rockwood 

 compared the absorption of casein and caseose from the small 

 intestine. The loops of gut were washed out very thoroughly 

 beforehand with salt solution. At the end of five hours they 

 recovered 91 per cent, of the casein unabsorbed and only 3 per 

 cent, of the caseose. The slight absorption of casein may well have 

 been due to digestion, for casein can be split up by erepsin as 

 well as by trypsin. Reid in his experiments only washed out 

 the loops of gut with salt solution sufficiently to remove debris 

 of food and worms, and does not mention that he continued the 

 washings in order to remove digestive ferments. Unless it is certain 

 that in these experiments the proteid was not absorbed by diges- 

 tion, the absorption of the salts and water could be accounted for 

 by osmosis and diffusion. And there is strong presumptive evidence 

 that the proteid must have been absorbed by digestion. 



At first sight, however, there appears to be a difference in 

 kind as well as in degree between the absorption of serum from 

 a damaged and a normal loop. Reid found that the anasmia 

 produced by excitation of the mesenteric nerves reduces the rate 

 of serum absorption to half the normal, but does not otherwise 

 alter the relative proportion in which the water, salts, and organic 

 solids are taken up. On the other hand, when the epithelium 

 is further damaged by clamping the mesenteric arteries or by 

 poisons, not only is the rate of absorption further reduced but 

 the quality is altered ; for the organic solids are taken up 

 relatively to the salts and water more freely than normal. 

 Nevertheless he looks upon this result as physical and directly 

 connected with the removal of the epithelium. We can conclude 

 only that however likely it may be a priori that the living 

 epithelium does actually transport material, these experiments 

 bring forward no evidence in favour of such a process. 



