CHEMISTRY OF DIGESTION AND NUTRITION. 299 



account it will be seen that, speaking generally, the digestive functions of the 

 stomach are in part to act chemically upon the proteids, and in part, by the 

 combined action of its secretion and its muscular movements, to get the food 

 into a physical condition suitable for subsequent digestion in the intestine. 

 The material sent out from the stomach (chyme) must be quite variable in 

 composition, but physically the action of the stomach has been such as to 

 reduce it to a liquid or semi-liquid consistency. The extent of the true 

 digestive action of gastric juice on proteids is not now believed to be so 

 complete as it was formerly thought to be. Examination of the chyme 

 shows that it may contain quantities of undigested or only partially digested 

 proteid, complete digestion being effected in the intestines. Moreover, arti- 

 ficial peptic digestion of proteids under the most favorable conditions shows 

 that only a portion is ever converted to peptone, most of it remaining in the 

 proteose stage. It has been suggested, therefore, that gastric digestion of 

 proteids is largely preparatory to the more complete action of the pancreatic 

 juice, whose enzyme (trypsin) has more powerful proteolytic properties. In 

 accordance with this idea, it has been shown that an animal can live and 

 thrive without a stomach. Several cases 1 are on record in which the stomach 

 was practically removed by surgical operations, the oesophagus being stitched 

 to the duodenum. The animals did well and seemed perfectly normal. Exper- 

 iments of this character do not, of course, show that the stomach is useless in 

 digestion ; they demonstrate only that in the animals used it is not absolutely 

 essential. The reason for this will better be appreciated after the digestive 

 properties of pancreatic secretion have been studied. 



D. Intestinal Digestion. 



After the food has passed through the pyloric orifice of the stomach and has 

 entered the small intestine it undergoes its most profound digestive changes. 

 Intestinal digestion is carried out mainly while the food is passing through 

 the small intestine, although, as we shall see, the process is completed during 

 the slower passage through the large intestine. Intestinal digestion is effected 

 through the combined action of three secretions — namely, the pancreatic juice, 

 the bile, and the intestinal juice. The three secretions act together upon the 

 food, but for the sake of clearness it is advisable to consider each one separately 

 as to its properties and its digestive action. 



Composition of Pancreatic Juice. — Pancreatic juice is the secretion of 

 the pancreatic gland, in man the main duet of the gland opens into the 

 duodenum, in common with the bile-duet, about 8 to 10 cm. below the opening 

 of the pylorus. In some of the other mammals the arrangement is different : 

 in dogs, for example, there arc two duets, one opening into the duodenum, 

 together with the bile-duct, about 3 to 5 cm. below the opening of the 

 pylorus, and one some 3 to 5 cm. farther down. In rabbits the principal 

 duct opens separately into the duodenum about 35 em. below the opening 

 of the bile-duct. For details as to the act of secretion, its time-relations to 



1 Ludwig and Ogata/ Archiv far Anatomie urnl I'ln/sinlogie, 1883, S. 89; and Carvallo and 

 Paction : Archives tic Physiologie normal* et pathologique, L894, p. 100. 



