840 PHYSIOLOGY 



followed at the usual interval by the increased output of urea which 

 is the invariable concomitant of protein absorption and assimilation. 

 We must therefore conclude that the products of protein digestion 

 are taken up by the epithelial cells and passed on by these into the 

 blood-vessels. During the absorption of a protein meal changes have 

 been described by various observers in the structures of the villus. 

 In nearly every case there is marked increase in the number of mitotic 

 figures in the epithelium lining the follicles of Lieberkiihn. According 

 to Hofmeister there is during absorption an increase in the number 

 of leucocytes in the villi, and this observer ascribed an important 

 function to these leucocytes in the absorption of protein. Heidenhain 

 showed that this increase of leucocytes was not constant in all animals, 

 and bore no relation to the amount of absorption that was taking 

 place, and was quite inadequate to account for the total absorption 

 that was being carried on. On the other hand, several observers 

 have described changes in the epithelium as the result of protein 

 digestion. According to Reuter the epithelial cells become swollen, 

 their protoplasm stains less deeply, and at their basal ends the cells' 

 limits disappear, the protoplasm being apparently distended with 

 hyaline coagulable material (Fig. 350). Reuter regards this appearance 

 as a direct expression of the taking up of proteins in a dissolved 

 form and their conversion near the bases of the cells into coagu- 

 lable proteins ; but further evidence on this subject is necessary 

 before we can attach much importance to such an interpretation 

 of the appearances observed. 



Under the influence of the gastric juice the proteins of the 

 food are resolved during their stay in the stomach into albumoses 

 and peptones. In the small intestine the process of hydration 

 is carried further, the trypsin of the pancreatic juice carrying the 

 proteins through the stage of secondary albumoses and peptones, 

 and converting them into a mixture of amino-acids and polypeptides. 

 The same end-products result from the action of the erepsin of the 

 intestinal wall on the albumoses and peptones produced by gastric 

 digestion. The digestive juices finally reduce the proteins therefore 

 to a mixture of amino-acids, with a certain remainder of polypeptides 

 consisting of two or three of the amino-acids associated together, 

 which do not undergo further disintegration under the action of the 

 intestinal ferments. The final products give no biuret test. The first 

 question we have to decide is to what extent the proteins are reduced 

 to their ultimate hydration products before absorption. We have 

 evidence that protein may be absorbed by the small intestine without 

 having undergone any hydration whatsoever. The absorption of 

 serum protein has been discussed already in dealing with the mechanism 

 of absorption of salt solutions from the gut. In a series of experi- 



