46 PHYSIOLOGY 



The body is like a steam engine. Heat furnishes it 

 with the power to move. This heat comes from chemical 

 action. Whenever an animal moves, protoplasm, or some 

 other unstable compound, splits into simpler substances 

 and sets free heat. This heat causes other compounds 

 to disintegrate, or simple substances to combine, until, 

 through a series of chemical changes all of which set 

 free heat, the food is converted into new complex com- 

 pounds which replace the old ones that have split up, 

 and wastes are formed which are given off from the 

 body through an excretory organ. 



Were it not for the assimilation of food the body 

 would waste away as a result of activity. The more 

 active the body then the more food is necessary. A 

 laborer must eat more to keep the body in good condition 

 than a bank clerk ; and a growing child must eat more in 

 proportion than a man, for the child's food must not 

 only replace what is lost through activity but it must 

 provide for growth. 



In One-Celled Animals. — Assimilation is a universal 

 process. It occurs in all organisms. With the com- 

 pound microscope we may see one-celled animals take in 

 food and give off wastes. The food surrounded by a 

 drop of water passes directly into the protoplasm and 

 the part that is unfit for use is eliminated through a 

 weak spot in the body wall. This material never be- 

 comes part of the body, and it must not be confused with 

 waste from the protoplasm. The nutritive part of the 

 food is carried about in the circulating stream until it is 

 converted into protoplasm. Waste from disintegrated 

 protoplasm leaves the body through a pulsating vacuole, 





