48 PHYSIOLOGY OF FARM ANIMALS [CH. 



Pancreatic jiiice, which is coloiirless and alkahne in reaction, 

 in an animal with a pancreatic fistula makes its apjjearance 

 between five and twenty minutes after a meal. Two or three 

 hours later its quantity is considerably increased and after five 

 hours it ceases to flow. The time of maximal flow, therefore, is 

 when the contents of the stomach are being passed out into the 

 duodenum. Bayliss and Starling have shown that the main 

 factor in producing pancreatic secretion is chemical, but it is 

 possible that nervous action may play a very small part in the 

 initial stages of secretion. The pancreas is activated by a 

 hormone, called secretin, which is liberated from the wall of the 

 duodenum (where it is present as prosecretin), through the 

 action of hydrochloric acid, the normal stimulus being derived 

 from the passage of the gastric contents into the duodenum. 

 Bayliss and Starling have sho^vn that after the nerve supply of 

 the pancreas has been cut off injection of acid extract of duodenal 

 mucous membrane into the circulation results in an almost 

 immediate flow of pancreatic juice. The acid converts the 

 precursor prosecretin into the activating hormone secretin. 



The Ferments of the Pancreas and Small Intestine. The fer- 

 ments present in the pancreatic juice are of great importance. 

 They are as follows: (1) trypsin, which is a proteolytic ferment 

 like pepsin but has the power of breaking down the protein 

 molecule still further, and can change proteins into amino- 

 acids of which leucine, tyrosine, and tryptophane are examples; 



(2) amylopsin, which, like ptyalin, acts on starches and converts 

 them into sugars, and (3) steapsin, which splits fats into fatty 

 acids and glycerin. The fatty acids are then able to unite with 

 the alkalis of the bile salts to form digestible soaps. 



The succus entericus contains the following ferments : (1) mal- 

 tase, which converts maltose into dextrin and dextrose ; (2) inver- 

 tase, which converts cane sugar into dextrose and levulose; 



(3) lactase, which converts lactose into dextrose, and galactose, 

 and (4) erepsin, which converts peptones into amino-acids. It 

 also contains (5) enterokinase, which is a necessary factor for 

 changing the trypsinogen present in the juice secreted by the 

 pancreas into the active ferment trypsin. For trypsin does not 

 exist as such in the juice produced by the pancreas, but is 

 only present in pancreatic juice which has been brought under 

 the influence of enterokinase. 



