MAMMALIA— PHYSIOLOGY 431 



man the stomach is about ten inches long, four inches wide and 

 deep, and holds about three pints. 



Gast7-ic juice is a pale yellow fluid, rich in HCl. It arrests the 

 process of changing starch to sugar, producing chyme. It contains 

 pepsin^ which changes albumen and gelatin to a soluble form called 

 peptone; rennin, which curdles milk; and lipase^ which emulsifies 

 neutral fats and fatty acids, but has no effect on non-emulsified fats. 



Digestive Action in the Stomach. — The action of the saliva upon 

 starch is arrested, and the larger lumps of masticated food are 

 broken up into a thick, grayish soup-like chyme which consists of 

 starch unchanged by saliva; oils and fats set free in the gastric 

 digestion; undissolved proteins; dissolved proteins; and debris of 

 indigestible materials such as cellulose. 



The small intestine consists of the duodenum situated on the 

 right side of the body with a wide curve embracing pancreatic 

 and bile ducts., t\\& jejunufn situated on the left side, and the some- 

 what constricted ileum which leads to the cecum and the large 

 intestine. The human small intestine is twenty-three feet in length. 



The semi-digested acid food from the stomach, as it passes over 

 the bile duct, causes a flow of bile and pancreatic juices which tend 

 to neutralize the acid chyme, but the contents of the duodenum do 

 not become distinctly alkaline until they have reached a point at 

 some distance from the pylorus. The valve of the common duct is 

 closed when the chyme is rendered alkaline, and is opened when the 

 acid chyme arrives again. 



The liver is of a dark red-brown color, situated just below the 

 diaphragm. It is the largest gland in the body. The principal 

 function of the liver is the formation of glycogen, sometimes called 

 " animal starch." The glycogen is given out into the blood in the 

 form of dextrose, into which it is changed by an enzyme in the hepatic 

 cells. The liver thus acts as a reservoir for food, storing it up when 

 it is in excess and expending it gradually to tide over periods of 

 fasting. The liver forms bile and urea and is most important in the 

 purification of the blood. The gall bladder, present in the cat as in 

 man, is not found in all mammals, some rodents lacking it entirely. 

 It stores up the bile and liberates it periodically, through the bile 

 duct which enters the anterior portion of the duodenum as the com- 

 mon bile duct, together with or close to the pancreatic duct. The 

 liver weighs between three and four pounds in man and secretes 

 nearly three pounds of bile in twenty-four hours. Bile is alkaline, 



