24 FEEDS AND FEEDING 



attacked. The small intestine receives near its upper part digestive 

 fluids from two outside organs, the liver and the pancreas, and another 

 digestive juice is secreted by the wall of the intestine itself. Immediately 

 on entering the small intestine the inpouring material is changed from 

 an acid to an alkaline character thru the rapid addition of bile and 

 pancreatic juice, both alkaline. This stops the action of the pepsin, 

 which works only when acid is present. The intestinal contents are moved 

 along by waves of muscular contraction of the intestinal walls. 



42. The pancreatic juice. The pancreatic juice is produced by the 

 pancreas, or sweetbread, a slender gland lying just beyond the stomach 

 and connected with the small intestine by a duct or by two ducts in certain 

 animals, as the horse. In some animals it is thin, clear, and watery and in 

 others thick and slimy. The chief enzymes it contains are trypsin, 

 amylase and lipase. 



Trypsin, like pepsin, changes protein into proteoses and peptones, 

 but is also able to split some of these partially digested substances into 

 amino acids. It is believed that before the food protein can be absorbed 

 and used by the animal body it must all be cleaved into amino acids, 

 which, as has been previously shown (11), are the simple "building 

 stones " from which the proteins are formed. 



The digestion of protein brought about by trypsin in the small intestine 

 is thus much more complete than that occurring in the stomach thru 

 the action of pepsin. It is most interesting that trypsin is secreted by 

 the pancreas in an inactive form, which will not digest protein. As soon 

 as the pancreatic juice comes in contact with the intestinal wall, intestinal 

 juice is produced, which contains a substance that changes the trypsin 

 into the active digesting form. 



Amylase, formerly called amylopsin, changes starch into malt sugar. 



Lipase, formerly called steapsin, splits fats into fatty acids and 

 glycerin. The fatty acids unite with the alkalies in the bile to form 

 soaps, and are absorbed from the intestine in this form. 



Ordinarily, when digestion is not going on in the small intestine there 

 is no secretion by the pancreas. However, the secretion of this digestive 

 fluid begins promptly after the partially digested food enters the small 

 intestine from the stomach. This is brought about by the following 

 action : The partially digested food as it enters the intestine is acid, due 

 to the hydrochloric acid of the gastric juice. The acid acts on the lining 

 of the small intestine, at once producing something which is absorbed 

 into the blood and then causes the pancreas to pour forth the pancreatic 

 juice just when needed, a forceful illustration of how the organs of the 

 complicated digestive system work in harmony. 



43. The liver. The liver, the largest organ in the body, has several 

 important duties in digestion and other vital processes. In addition 

 to the secretion of bile, which is discussed in the following paragraph, the 

 liver is a storehouse for carbohydrates (60), and has important functions 

 in regulating the composition of the blood. It also protects the body 



