66 PHYSIOLOGY 



tion of food. The series of cnemical changes that occur 

 in the process set free heat, which furnishes the motive 

 power for the activity of the body. The foods fall into 

 five classes: water, salts, proteids, fats and starches. 

 The last three must be digested, or made soluble, before 

 they can be assimilated. This is accomplished through 

 the activity of enzymes manufactured by the protoplasm. 

 In the lowest animals the entire process of assimilation 

 occurs in a single cell ; in higher animals a special organ 

 is set apart for digestion which has the form of a tube 

 of varying width open to the outside at both ends. A 

 series of muscular contractions forces the food to pass 

 through the tube. On the way, the food is acted on by 

 enzymes, and the nutritious substances pass into the 

 blood and are carried by it to every part of the body. 

 Undigested material and material that can not be di- 

 gested pass off from the other end of the tube. Wastes 

 from the disintegration of protoplasm are given off 

 through special excretory organs. 



