.132 DIGESTION 



Many experiments prove beyond doubt that for the open- 

 ing of the sphincter the presence of free acid on its gastric 

 side is essential. It is equally proved that its closure is 

 due to the presence of the same acid on its duodenal side — 

 a local reflex mediated through Auerbach's plexus. But 

 the action of the acid on the duodenal side is much the 

 stronger, so much so that the presence of a very small 

 amount of acid on this side is sufficient to counterbalance 

 the antagonistic action of the large amount of acid in the 

 stomach. The pylorus therefore opens only after the 

 acid in the duodenum has been neutrahsed by the alkah 

 secreted by intestine, pancreas and liver. 



But the rate of emptying of the stomach varies with the 

 nature of the food. It is shghtly more rapid with carbo- 

 hydrates than with proteins, and much more rapid with 

 these than with fats. This difference has been shown by 

 Cannon to be due to the effect of these foodstuffs upon the 

 amount of free acid formed. Fats, as we have seen, inhibit 

 the secretion of gastric juice. The shght difference between 

 protein and carbohydrate is attributed to the combination 

 of part of the HCl with the former, the effective acidity 

 being thus reduced. 



Absorption from the Stomach 



The only substances known to be absorbed, and these 

 only in small amounts, are peptones, sugars and alcohol. 

 There is no absorption of water. 



We may now summarise the digestive changes that 

 occur in the stomach. 



1. The digestion of starch continues in the fundus until 

 the ptyahn is destroyed by HCl. 



2. Proteins are hydrolysed incompletely to proteoses 

 and peptones. 



3. Milk is clotted. 



