PERISTALSIS 317 



enzymes now commence (Chap. IX.). Polysaccharides are 

 broken down to maltose by ptyalin and the native proteins first 

 converted to metaprotein by the action of the hydrochloric acid 

 of the gastric juice and reduced in size and complexity to the 

 proteose stage by the action of pepsin. 



Even a casual examination of the stomach will show that it 

 is divided into two parts, each with a distinct function. The 

 upper or cardiac portion is a reservoir or hopper where the food 

 pulp is stored for a short time without mixing. % It is during its 

 stay here that salivary digestion reaches its maximum. By 

 the steady pressure of the walls in the cardia, the mushy mass 

 is fed little by little through the throat of the hopper (prepyloric 

 sphincter) into the lower or pyloric part of the stomach. By 

 peristaltic contractions of its walls, this pyloric section of the 

 stomach mixes food and gastric juice most thoroughly. The 

 acid of the juice aids peptic while inhibiting diastatic action. 

 Cathcart showed that the prepyloric sphincter was controlled 

 by the hydrogen ion concentration of the duodenum and of the 

 pyloric part of the stomach. Acid entering the intestine un- 

 doubtedly causes constriction of the sphincter. While the C H 

 of the pyloric portion of the stomach does not seem to effect the 

 closure of the sphincter, the above worker demonstrated that a 

 sufficient reduction of the C H brought about a rapid opening of 

 the sphincter. The introduction of an extra alkaline juice, e.g. 

 by regurgitation from the intestine, leads to a smart flow of acid 

 chyme into the antrum pylori. 



The rate of exit from the antrum is controlled by the hydrogen 

 ion concentration of the duodenum. As long as the duodenal 

 contents are markedly acid, the pyloric sphincter remains firmly 

 closed and only opens to admit more acid chyme when its receptors 

 are no longer stimulated by acid. 



IV. The intestines have three functions to perform : (a) trans- 

 porting, (b) mixing and digesting, (c) absorbing. 



(a) Transporting. This is carried on by means of a series of 

 peristaltic waves, i.e. a section of the muscular wall adjacent 

 to the anterior end of the food-mass undergoes relaxation 

 while a corresponding posterior section contracts. This double 

 wave of relaxation and contraction passes along the tube and 

 acts as a piston with a central orifice. In this way, the chyme 

 is passed along at the rate of about 1 inch a minute, and churned 

 meanwhile. 



(b) These driving peristaltic waves are not the only movements 



