SOURCES OF ENERGY AND MATERIALS 105 



charge their secretions even after the nerves which innervate them 

 are cut. 



The Pancreatic Juice. — The pancreas produces a thin watery secre- 

 tion containing three enzymes, which act upon proteins, carbohydrates, 

 and fats, respectively. The protein-spHtting enzyme is inactive when 

 it emerges from the pancreatic duct and is then known as trypsinogen; 

 but, when it comes in contact with the duodenal surface, it is quickly 

 rendered active. The conversion of trypsinogen is initiated by the 

 enzyme enter okinase , produced in the lining of the duodenum. This 

 enzyme acts upon the inactive trypsinogen, changing it to the active form 

 called trypsin. The trj^psin splits proteins, proteoses, and peptones from 

 the stomach into simpler and simpler compounds. The end products of 

 protein digestion are amino acids (page 41) and several other compounds. 

 Trypsin works in alkaline, neutral, or even acid media. It completes the 

 work begvin by the pepsin and works more rapidly and breaks up the 

 protein more completely than does the pepsin. 



The carbohydrate-splitting enzyme of the pancreas is amylopsin. 

 Unlike trypsinogen, it requires no activation. It converts starches, dex- 

 trins, and complex sugars (with the aid of so-called inverting enzymes) 

 into simple sugars (glucose and others), which are in condition to be 

 absorbed. 



The fat-splitting enzyme of the pancreatic juice is steapsin. Steapsin 

 splits fats into glycerol (glycerin) and one or more fatty acids (page 41).. 

 These substances are soluble and are absorbed in this condition. 



The Secretion of the Liver. — Bile, the secretion of the liver, contains 

 no enzyme. It contains water, bile salts, and certain excretory materials. 

 The discharge of bile is stimulated, as explained above, by the hormone 

 secretin in the same manner as is the secretion of pancreatic juice. Bile is 

 ordinarily stored in the gall bladder until the partially digested acid food 

 is ejected by spurts from the stomach, but it has been shown in some 

 animals that such a temporary storage place is not essential to the 

 proper production and ejection of the bile. Each ejection of food into the 

 intestine stimulates a flow of bile through the bile duct. The bile salts 

 break up the fats into very fine droplets, thus greatly increasing the 

 surface through which the fat-splitting enzyme may attack them. If 

 the bile is artificially prevented from entering the intestine, a large 

 share of the ingested fat is not digested and may be recovered in the 

 feces. 



The Intestinal Secretion. — The secretion of the small intestine is 

 produced in small tubular glands which are local evaginations of the lining 

 layer. This secretion consists of enterokinase, erepsin, several other 

 enzymes, and secretin. Enterokinase, as stated above, converts inactive 

 trypsinogen into active trypsin. Erepsin is a protein-splitting enzyme 



