THE METABOLIC MACHINERY OF ANIMALS 293 



and is under the direct control of the will, while the internal sphincter 

 is derived from one of the coats of the rectum and consists of un- 

 striated or involuntary muscle. 



The process of absorption is thought to be continued to a limited 

 extent in the large intestine as its contents are retained for a consider- 

 able time. The secretions of this region are alkaline, containing much 

 mucus l)ut apparently no enzymes. By the time the contents reach 

 the large intestine the water content is considerably reduced through 

 absorption. Bacteria, which compose nearly 50 per cent of the human 

 feces, carry on putrefactive protein fermentation in the large intestine. 



The Digestive Glands and Their Enzymes 



The chemical processes of dige.stion occur largely through the activ- 

 ity of enzymes which are produced in a variety of different glands. 

 Practically all vertebrates possess salivary and gastric glands, a liver, 

 pancreas, and various intestinal glands. 



v/^^*>,*.^i^^^ SLcblirj^t^cd duct 



submaxillary ^^itblii' 

 glancC gl^^c 



Salivary glands in man. What enzyme do these glands secrete ? (After Walter.) 



The Salivary Glands. Saliva, which acts as a lubricant in the 

 mouth, is manufactured in the cells of three pairs of glands that 

 empty into the mouth by ducts, and which are called, according to 

 their position, the parotid (beside the ear), the submaxillary (imder 

 the jawbone), and the sublingual (under the tongue). In addition, 

 the salivary glands, which are absent in most aquatic forms, secrete 

 a digestive enzyme, ptyalin, that acts upon starch in an alkaline 

 medium, splitting it partially or entirely into a disaccharide sugar 

 known as maltose. Ptyalin is present in all mammals except those 

 which are entirely carnivorous. 



