114 GENERAL ZOOLOGY 



like gastric secretin, is an example of a hormone, a chemical 

 agent that stimulates the activity of organs other than the one 

 in which it is produced. The chemical control of pancreatic 

 secretion provides an automatic mechanism for regulating the 

 secretion of pancreatic juice. 



The peptones formed by the gastric digestion of proteins are 

 hydrolyzed by trypsin and erepsin into amino acids, the final 

 product of protein digestion. 



Amylase is similar to ptyalin in its action on starches. Thus 

 the amylolytic action by ptyalin begun in the mouth and con- 

 tinued in the stomach until interrupted by hydrochloric acid, is 

 renewed and carried to completion by amylase in the intestine. 

 Maltose, the product of starch digestion is then converted by 

 maltase into dextrose. Most of the maltase is produced in the 

 intestine, but saliva also contains small amounts, so that some 

 inverting action on maltose may take place before the food reaches 

 the intestine. Cane sugar, taken in a pure form with food is 

 converted in the intestine by sucrase into dextrose and levulose. 

 Milk sugar or lactose is changed by lactase into dextrose and 

 galactose. Dextrose, levulose, and galactose are all monosacchar- 

 ides and represent the end products of carbohydrate digestion. 



It is usually agreed that the digestion of fats takes place in the 

 small intestine. The lipolytic enzyme involved is lipase, pro- 

 duced by the pancreas. Lipase is capable of hydrolyzing or 

 saponifying neutral fats. Its action on beef fat, tristearin, may 

 be illustrated as follows: 



C 3 H 5 (C 17 H 3 5COO) 3 + 3H 2 -> C 3 H 5 (OH) 3 + 3C 17 H 35 COOH 



tristearin water glycerin stearic acid 



Since this reaction takes place in an alkaline medium, the free 

 acid is at once neutralized to form a soap, sodium stearate, as 

 follows : 



C 17 H 35 COOH + NaOH -> C 17 H 35 COONa + H 2 



stearic acid sodium hydroxide sodium stearate water 



Or both steps may perhaps be combined in a single reaction as 

 follows : 



C 3 H 5 (C 17 H 35 COO) 3 + 3NaOH -> C 3 H 5 (OH) 3 + 3C 17 H 35 COONa 



tristearin sodium hydroxide glycerin sodium stearate 



