138 The Vertebrate Organ Systems 



Chemical Digestion. — Three major types of food supply energy 

 for work and repair in the organism. These are proteins, carbohydrates, 

 and jats. As taken in, they are complex molecules which must be 

 changed chemically into smaller, simpler molecules. This change is 

 accomplished by means of the organic catalysts, enzymes, which are con- 

 tained in the various digestive juices. 



By means of the various enzymes, the proteins are split into their 

 component amino acids, the carbohydrates into simple soluble sugars, 

 and fats into glycerols and fatty acids. Since enzymes are so specific 

 in their action, there are different ones not only for each food, but for 

 each step in the gradual process of the breakdown of the complex 

 molecules. 



Chemical digestion begins in the mouth in many mammals. Here 

 the saliva, with its enzyme ptyalin, breaks the starches into double 

 sugars. In addition to its digestive function, saliva performs other nec- 

 essary tasks. It lubricates the food for easier passage, it moistens and 

 dissolves some materials thus making possible the sensation of taste, and 

 finally it helps to hold the macerated particles together by means of 

 the mucin present in it. The esophagus contains no digestive enzymes. 



The numerous glands in the walls of the stomach secrete the gas- 

 tric juice which contains the enzyme pepsin, and, in addition, hydro- 

 chloric acid (HCl). Pepsin starts the digestion of proteins by split- 

 ting them into forms intermediate between the intact protein and the 

 amino acids. The pepsin works most efficiently in an acid medium ; thus 

 hydrochloric acid is indispensable. In addition, hydrochloric acid is 

 somewhat germicidal and hence gives some protection against bacterial 

 infections. In the stomachs of some young mammals such as the calf, 

 another enzyme, rennin, is demonstrable. This coagulates milk. 



The digestion of foods is continued and completed in the small in- 

 testine. Here there are three main digestive juices: bile, pancreatic 

 juice, and intestinal juice. Each of the latter two contains a whole 

 complex of enzymes. The bile contains no enzymes but has other im- 

 portant functions. 



Food entering the small intestine from the stomach is acid in 

 its reaction ; but this is soon changed to a basic condition due to the 

 reaction of the bile and pancreatic juice. This is important because the 

 enzymes of the small intestine perform their work most efficiently in 

 a basic medium. 



Bile is made up of waste products from the red blood cells, and, 

 in addition, contains the important bile salts. These latter break the 



