THE DIGESTIVE SYSTEM. 325 



the intestinal juice. Erepsin acts upon proteoses and peptones, splitting 

 them up into amino-acids. There is some ground for believing that 

 the hydrolysis is not quite complete, and that, in the final result, in 

 addition to amino-acids, there are some more complex substances called 

 polypeptides, groupings of amino-acids which are less complex than 

 peptones, and which do not undergo complete hydrolysis. Erepsin is 

 also contained in the epithelial cells covering the villi, and it is possible 

 that the final stages of hydrolysis may occur in these cells. 



The chief amino-acids resulting from the digestion of proteins are 

 leucine, tyrosine, aspartic acid, glutaminic acid, tryptophane, and the 

 hexone bases, lysine, arginine, and histidine. These, and other sub- 

 stances belonging to the same group, are linked together to form the 

 protein molecule, and the differences found to exist between the various 

 proteins are associated with differences in the proportions of their 

 constituent amino-acids. 



The intestinal juice is produced by the crypts of Lieberkiihn, 

 tubular glands lined by columnar epithelium, occurring in the mucous 

 membrane of the small intestine, and opening between the bases of the 

 villi. There is no evidence that the secretion is influenced by a nervous 

 factor. If three adjacent loops of intestine are separated from each 

 other by ligatures, and the nerves to the middle loop are divided, the 

 latter is found full of fluid after four to sixteen hours, while the 

 adjacent loops are empty. Two days later, however, all three loops are 

 empty. It appears probable, therefore, that the production of fluid 

 following the section of the nerves is due to dilatation of blood-vessels 

 resulting from the division of vaso-constrictor nerves, and that absorp- 

 tion occurs and the production of fluid ceases as the vessels regain their 

 tone. No conclusions can therefore be drawn as to an inhibitory or 

 other influence of the nervous system on the secretion from such an 

 experiment. The normal stimulus for the secretion of the intestinal 

 juice is undoubtedly secretin, and possibly also other hormones. This 

 possibility is supported by the fact that intestinal juice is produced 

 in the dog about ten minutes after the ingestion of a meal of meat, 

 and that the flow is increased in the third hour after the food has been 

 taken. The secretion of intestinal juice can be brought about by 

 mechanical stimulation, probably by means of a local nervous mechanism. 



THE PROGRESS OF DIGESTION IN THE SMALL 

 INTESTINE. 



Experiments have been made on dogs in which the intestinal con- 

 tents were withdrawn, by means of appropriate fistulae, at different 

 stages of their passage along the bowel. It was fo'und that after a 



