THE DIGESTIVE SYSTEM 81 



lated to production by the presence of the acid chyme from the 

 stomach, which acts on a product of the epithehal cells of the 

 duodenum known as prosecretin, changing it into secretin. When 

 secretin is absorbed by the blood and reaches the pancreas it 

 stimulates the gland to secrete the pancreatic juice. It also acts 

 on the liver, causing that gland to secrete bile. Secretin is not 

 an enzyme, for it is not destroyed by heat. It cannot be syn- 

 thetized; in fact, little is known about its properties. To date 

 nothing has been found that can be substituted for it. The 

 average daily secretion of pancreatic juice has not been deter- 

 mined in the horse, but it must be considerable. It has a 

 specific gravity of 1010. Normally, the secretion contains three 

 enzymes — trypsin, amylopsin, and steapsin. 



Trypsin is the most important pancreatic enzyme. It is se- 

 creted in pro-enzymic form as trypsinogen, but is at once activated 

 by the enterokinase of the intestinal juice. It has the power to 

 change proteins into proteoses, peptones, and many other com- 

 paratively simple products of protein digestion. Amylopsin, or 

 diastase, acts on the starchy constituents of the food in much the 

 same way as does the ptyalin of the saliva, converting them into 

 dextrins and sugars. Steapsin derives its name from the fact 

 that it splits the fats by hydrolysis into glycerin and their fatty 

 acids. These fatty acids combine with the alkaline salts to form a 

 soapy emulsion. 



Intestinal juice is secreted by the glands in the intestinal walls, 

 so has been termed succus entericus. It contains several enzymes of 

 much importance to the welfare of the animal. These enzymes 

 include erepsin, which is the last agent to act on the products of 

 protein digestion and prepare them for absorption; invertase, 

 which has the power to invert dextrose, maltose, and lactose; 

 enterokinase, which converts trypsinogen into trypsin. Colin has 

 shown that the intestinal juice of the horse has a distinct action on 

 starch. Furthermore, through the carbonate of soda that is 

 found in it the acid chyme is partially neutralized and made ready 

 for absorption. 



Ingesta pass very rapidly through the small intestine. This is 

 evident when a postmortem examination is held. Only a very little 

 content is found, and it is fluid in consistency. The movement of 

 materials from one part of the intestine to another is due, in large 

 measure, to the contraction of the involuntary muscles of the 



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