440 THE HUMAN BODY 



The food enters the stomach in very much the same condition 

 chemically as when taken into the mouth; a small amount of 

 maltose added to it through the action of salivary ptyalin, and a 

 correspondingly diminished amount of starch, being the only 

 differences. That part of the food which is crowded down into 

 the pyloric region begins at once to be churned by the peristaltic 

 waves which sweep over that region; by the churning it is mixed 

 with gastric juice. The food which remains in the fundic end of 

 the stomach does not come into contact with the gastric juice; 

 its reaction, therefore, continues alkaline, and the splitting of 

 starch by pytalin goes on uninterruptedly. In the portion of 

 food (chyme) which becomes impregnated with gastric juice there 

 is an acid reaction and the changes which the gastric enzyms, 

 pepsin and rennin, are capable of producing take place. Rennin 

 clots any milk that may be present; pepsin attacks albuminoids 

 and proteins, converting them into proteoses and peptones. Any 

 fats present are liquefied, not by enzyms but by the stomach 

 warmth. Some of the substances produced during this peptic 

 digestion react with other substances in the mucosa of the pyloric 

 region, forming a hormone, gastric secretin. This hormone is 

 taken up by the blood, passes in the blood-stream to the gastric 

 glands, and stimulates them to further outpouring of juice; thus 

 enough for the whole meal is secured. Finally as the hydrochloric 

 acid of the gastric juice accumulates in excess the pyloric sphincter 

 is stimulated to relax; the mass of chyme next to it is pushed 

 through; and more material from the fundic end comes down to 

 fill its place. Too much chyme is prevented from passing the 

 sphincter at once by the powerful stimulus to contraction which is 

 exerted on the sphincter by the acid chyme in contact with the 

 upper intestine. The acid of this same chyme reacts with the prose- 

 cretin of the intestinal mucosa to form secretin, a hormone which 

 is carried by the blood to the pancreas and excites it to activity. 



The chyme which enters the intestine contains some, at least, of 

 all the food-stuffs originally making up the meal, and in addition 

 maltose, proteose, and peptone. The strongly alkaline bile and 

 pancreatic juice quickly neutralize its acid and the various en- 

 zyms of the intestinal tract act upon it. The amylopsin of the 

 pancreatic juice converts to maltose all starch not affected by 

 ptyalin; the lipase of the same secretion splits the fats to fatty 



