386 TEXTBOOK OF ZOOLOGY 



be found. The tissues of the animal body must not only have avail- 

 able a wide range of amino acids but must also select in the proper 

 proportion the ones needed to reconstruct their specific protein con- 

 stituency. 



Amylopsin (amylase) is the pancreatic diastase, and it is able to 

 bring about hydrolysis of carbohydrates in the alkaline medium of 

 the intestine without activation. It produces dextrin and maltose 

 (malt sugar). The pancreatic lipase, steapsin, brings about the split- 

 ting of fats into glycerin (glycerol) and one or more fatty acids, 

 such as stearic acid, oleic acid, butyric acid, etc. The alkaline salts 

 which are introduced by the bile, combine with these fatty acids to 

 form soaps which help in emulsifying the remaining fats, thus mak- 

 ing them more readily split. 



Intestinal secretions or succus entericus which are produced by 

 glands in the mucous membrane of the small intestine include five 

 enzymes. Enterokinase, which activates trypsinogen to form tryp- 

 sin, has been mentioned already. Erepsin, the intestinal protease, 

 supplements the activity of trypsin by converting proteoses and pep- 

 tones into amino acids. Maltase converts maltose and dextrin into 

 dextrose. Invertase changes sucrose (cane sugar) into dextrose and 

 levulose. Lactase converts milk sugar (lactose) into galactose and 

 dextrose, both simple sugars. 



The undigested residue passes into the large intestine where prob- 

 ably no enzyme digestion occurs. Certain bacteria (B. coli and 

 others) attack any undigested protein and bring about putrefactive 

 fermentation. Products of this action may be absorbed; some of 

 them are frequently toxic and must be eliminated in either the urine 

 or the feces. Certain other bacteria here feed upon cellulose and 

 may produce some sugar from it. When the chyme reaches the large 

 intestine it is about the consistency of thick cream, but it becomes 

 more and more solid by absorption of water here until finally only 

 concentrated fecal matter remains. 



Functions of the Liver. — The secretion of the liver is bile and is 

 discharged into the duodenum of the small intestine by way of the 

 common bile duct. This is an alkaline solution which serves to help 

 neutralize the acidity of the chyme as it comes from the stomach. This 

 with the pancreatic juice brings about the emulsification of fats men- 

 tioned above. Cholesterin and two pigment materials are excreted in 



