THE ABSORPTION OF THE FOOD-STUFFS 831 



The most cogent argument perhaps in favour of an active inter- 

 vention of the cells of the gut in the process of absorption is furnished 

 by the study of the absorption of blood serum. It has been shown 

 that if an animal's own serum be introduced into a loop of its intestine 

 the serum undergoes absorption. This absorption affects the water 

 and salts more than the protein, so that the percentage of the proteins 

 in the fluid remaining in the intestine is increased. Finally, how- 

 ever, the whole of the serum is absorbed. In this case the fluid 

 within the gut is identical with the fluid within the blood-vessels. 

 There are no differences in concentration, quality of salts, or osmotic 

 pressure of proteins. Nevertheless water passes through the cells 

 of the gut from their inner to their outer sides, entraining with it 

 the salts of the serum and a certain proportion of the indiffusible 

 proteins. It is impossible to explain this result as due to the 

 digestion of the proteins and their conversion into diffusible pro- 

 ducts, since the intestinal loops were washed free of any trypsin 

 that they contained, and serum has itself a strong antitryptic action 

 whicli would prevent its being attacked by even a strong solution of 

 trypsin. 



The active intervention of the cells in the absorption of salt 

 solutions, as well as of serum, can be abolished by any means which 

 diminishes or destroys their vitality, such as the addition of sodium 

 fluoride to the fluid to be absorbed, or destruction of the epithelium 

 by previous temporary occlusion of the blood-vessels supplying the 

 loop of intestine. 



We must conclude that, when a fluid is introduced into the intes- 

 tine, an active transference of water from the lumen into the blood 

 stream is effected by the intermediation of forces having their origin 

 in the metabolism of the cells themselves. This work of absorption 

 of the cells may be aided or hindered according to the physical 

 conditions present. If these act against the cells, e.g. if the fluid 

 be hypertonic, the absorption is effected more slowly, while with 

 hypotonic solutions the physical conditions concur with the vital 

 activity of the cells in bringing about a very rapid transference of 

 fluid from the gut into the blood-vessels. Among these physical 

 conditions we must reckon the nature of the salts present in the 

 solution. If these can pass easily into and through the cells, e.g. 

 ammonium salts, sodium chloride, absorption is carried out rapidly. 

 If, on the other hand, the salts in the intestinal contents are but 

 slightly diffusible or have very little power of penetrating into the 

 cells, the absorption of water by the cells causes an increased con- 

 centration of the salts, and therefore an increased osmotic pressure, 

 which offers a resistance to any further absorption , and the process 

 comes to an end, when the absorptive power of the cells is exactly 



