294 DIGESTION 



action of the gastric juice, is that of transforming the food into a gruelly 

 mass; but that its work can be replaced by careful comminution of the food 

 before eating. And yet this role of the stomach is of very great importance ; 

 for it is owing to the gastric digestion that we can utilize all possible kinds 

 of food for our nourishment, and can limit our eating to a few meals per 

 day. If the food were to be introduced immediately into the intestine, we 

 would be compelled to eat only fluid or semifluid foods, and it would be 

 necessary to eat much more frequently than we do. More than that, the 

 stomach protects the intestine from excesses of temperature whether high or 

 low and from all kinds of harmful substances. It brings all the food to the 

 temperature of the body and dilutes harmful substances with the gastric 

 juice before allowing them to pass into the intestine. In short, the stomach 

 is a protecting organ for the intestine, and permits us to derive our sus- 

 tenance from a very great variety of foods. 



3. DIGESTION IN THE INTESTINE 



Comparative anatomy teaches us that the length and diameter of the 

 intestine are intimately related to the character of the food of the animal 

 species. In carnivorous animals the intestine is considerably shorter than 

 in herbivorous animals; while in man its length is intermediate between 

 these two extremes. 



The most important part of the work of digestion is carried out in the 

 intestine, and, as it appears, chiefly under the influence of the pancreatic 

 secretion. 



When the chyme enters the intestine from the stomach it is subjected to 

 the action of this secretion, of the bile and of the intestinal juice. 



The pancreatic secretion continues the transformation of proteids begun 

 in the stomach. As we have already seen, the proteolytic enzyme of the pan- 

 creas is essentially different from that of the stomach. We may add to what 

 was said before that the pancreatic juice acts rather feebly on the gelatin- 

 forming substances (cf. page 291), whereas it acts very powerfully on the 

 true proteids. This fact is in perfect agreement with the condition already 

 emphasized, that the food must be of a gruelly nature in order to be adapted 

 for digestion in the intestine. The proteolytic, the amylolytic and especially 

 the lipolytic action of the pancreatic juice are assisted in some way not fully 

 understood by the bile (Rachford and Southgate, Bruno, Ussow). 



The action of pepsin-HCl on proteid is soon stopped in the intestine. In 

 the first place the bile hinders the swelling of proteid necessary for pepsin diges- 

 tion; moreover it has the property of precipitating proteids in acid solution, 

 whence the pepsin is removed from the fluid with the precipitate. This precipi- 

 tation of proteids by the bile can be very prettily demonstrated in vitro ; but in 

 natural digestion it appears to transpire only to a slight extent; for the bile- 

 acid precipitate is easily redissolved by the bile salts, and by other salts like 

 sodium chloride, lactate or acetate. It is stated also that one never finds any 

 such precipitate in the intestine of an animal killed during digestion. 



Hydrochloric acid in. small quantities has no harmful effect on tryptic diges- 

 tion and is even said to favor it in the presence of bile. 



