418 ALIMENTARY CANAL 



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 pre- 

 pyloric sphincter w^as controlled by the hydrogen ion concentration 

 of the duodenum and of the pyloric part of the stomach. Acid 

 entering the intestine undoubtedly causes constriction of the 

 sphincter. While the pH 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 H ion 

 concentration 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. 



Pyloric Sphincter. 



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, {h) 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 distal end of the food-mass undergoes relaxation while a 

 corresponding proximal 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. 



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

 of the intestinal musculature. While these movements are going 

 on in some loops of the small intestine, in other loops the chyme is 

 kneaded and its surface broken by the rhythmic segmented 

 contractions of the circular muscles of the bowel. By this means 

 (i.) the various digestive juices of the intestine are thoroughly 

 mixed wdth the chyme, (ii.) fresh surfaces are exposed to the 

 absorbing surfaces of the wall, and (iii.) the capillary blood-vessels 

 of the lining membrane are compressed rhythmically, so helping 

 to drive the blood laden with the products of digestive activity on 

 to the liver, etc. 



The work of digestion, begun in the mouth and stomach, is 

 completed in the intestine. Carbohydrates are reduced to single 

 sugars and proteins are broken down to amino acids, etc. In 

 addition to this, the fats are attacked by lipase, which resolves 



