THE DIGESTION OF THE FOODSTUFFS 133 



Moreover, the peristalsis is influenced by the central 

 nervous system. The vagus is the motor nerve, and the 

 splanchnic nerve the inhibitory nerve for the circular muscles. 

 Stimulation of this nerve brings about inhibition of the 

 peristaltic movements. 



The contraction of the longitudinal muscles of the intestine 

 produces dilation. The splanchnic is supposed to be the motor 

 nerve for the longitudinal muscles, while the vagus is the inhibitory 

 nerve. 



2. Chemistry of intestinal digestion. The food, grad- 

 ually driven from the stomach into the intestine, continues 

 to be acted upon by the pepsin as long as free hydrochloric 

 acid is present. But the free hydrochloric acid is speedily 

 neutralized by the alkali of the secretions which are poured 

 upon the food. There are three secretions: pancreatic juice, 

 bile, and intestinal juice. Of these, the pancreatic juice is 

 the most important for digestion. 



A. Pancreatic digestion. The pancreatic juice changes 

 starcli to sugar, proteids to peptone, and sptits neutral fat 

 into glycerin and fatty acids. These actions are brougJit 

 about by three ferments amylopsin, trypsin, and steapsin. 



1. Action of amylopsin. Amylopsin splits up starch in 

 the same manner as ptyalin of the saliva. The starch forms 

 successively amylo-, erythro-, and achroo-dextrin, maltose, 

 and finally grape-sugar. The quantity of grape-sugar 

 formed is somewhat larger than that formed by ptyalin, per- 

 haps because the pancreatic juice contains more glucase than 

 the saliva. 



2. Tryptic digestion. Trypsin splits up proteids. It can 

 act in an acid medium, but acts best in an alkali medium. 

 This proteid digestion differs from peptic digestion in the 

 following particulars : 



(a) The proteid digestion by trypsin is carried further than 



that by pepsin. Peptic action stops at the formation of 



, . .. 



peptones, but trypsin splits some proteids up into leucine^ 



v atv tyrosine, and aspartic acid (see page 27). The peptones Kg* 

 - which can be split up by trypsin are called hemipeptones ; 



neew 

 i 



