246 



ANIMAL BIOLOGY 



are built, but it furnishes the energy for performing the 

 work of the body. In order to yield this energy the food 

 must undergo chemical decomposition. It is split up by 

 ferments, and then oxidized or burned in the various tis- 

 sues. Some of these chemical changes result in the 

 building up of living matter out of simpler substances. 

 Others result in its tearing down or decomposition. 



J 



FIG. 186. A, the heart; B, the lungs drawn aside to show the internal 

 organs; C, diaphragm; D, liver; F, gall cyst; E, stomach; G, small intes- 

 tine; H, large intestine. (After Le Pileur.) 



In vital activity we therefore have a twofold process 

 of waste and repair a process known as metabolism 

 which goes on continually. Metabolism is the very care 

 and essence of vital activity; all other processes are depend- 

 ent upon it. But as it always involves waste, it always 

 requires new material or food to enable it to keep going. 

 In order that the various organs of the body can re- 



