NUTRITION OF ANIMALS 257 



281. Digestion in the stomach and intestines. An acid (weak 

 hydrochloric acid) and an enzyme (pepsin) are produced in 

 the stomach. Since ptyalin cannot act in an acid liquid, diges- 

 tion of starch is thought to be stopped while the food is in the 

 stomach, but the proteins are acted upon by the pepsin. At the 

 same time, by the constant movements of the stomach its con- 

 tents are thoroughly mixed. When the food leaves the stomach 

 and enters the small intestine, it is an acid emulsion. At this 

 point, through ducts which open into the intestine, two other 

 important secretions enter. They are the bile and the pan- 

 creatic secretion. The bile is produced by the liver, and the 

 pancreatic secretion by the pancreas. The bile is alkaline and 

 counteracts, or neutralizes, the acidity of the food. When 

 the acidity is counteracted, ptyalin that has been carried with 

 the food may again carry on the digestion of starchy sub- 

 stances. One element in the pancreatic secretion may also 

 digest starchy foods. Other parts of the pancreatic secretion, 

 as well as secretions from other small glands in the walls of 

 the stomach and intestines, act upon various parts of the 

 foods. 



Since the food has been changed into liquid form, it may 

 pass into the blood by osmosis and be carried through the 

 body. Materials that are not digested, or other waste mate- 

 rials, are carried from the digestive system and eliminated. If 

 waste or undigested materials accumulate in undue quanti- 

 ties, the work of the digestive system and eventually of the 

 whole body is seriously interfered with. 



282. Circulation. It is a matter of common knowledge that 

 in the higher animals and man the heart is an organ which, by 

 means of its beats, or contractions, forces the blood through 

 the body. The blood flows away from the heart (fig. 124) 

 through arteries, which divide and redivide into smaller and 

 smaller branches. They terminate in many extremely small 

 tubes, known as capillaries, which are interwoven so as to 

 constitute a fine network. These capillaries are directly in 



