HUMAN PHYSIOLOGY: INTRODUCTION 289 



alike in being chiefly made up of the one substance proto- 

 plasm. This is the living substance of the cell. The life 

 and activity of the body depend upon the life and activities 

 of the protoplasm of the many cells of the body that make 

 up the various organs. A diseased condition of body means 

 a diseased condition of the protoplasmic cells of the body. 



Metabolism. — In their functional activity, the cells of 

 the body provide heat and do some kind of work. This 

 work is done by reason of the energy generated by the cell. 

 In providing energy the cell itself wears out or wastes away. 

 It has, however, the power of self-renewal or self-repair. 

 This double process of "waste and repair" is known as 

 metabolism. 



The air we breathe, the water we drink and the food we 

 eat supply the cells of the body with the three essentials 

 for their metabolism. 



Protoplasm is a very complex substance built of simpler 

 substances. The foods we eat are first reduced to simple 

 substances. Each protoplasmic cell acts as a little chemical 

 laboratory in laying hold of these simple food substances, 

 and recombining their elements into its own complex sub- 

 stance. It then reverses the process and, with the aid of 

 oxygen, breaks up this complex substance into simple sub- 

 stances. These are then thrown out of the cell as waste 

 products. 



The whole problem of the body, as a mechanism, is, there- 

 fore, to obtain air, water, and food and to carry these to the 

 cells; then to carry the waste away from the deep-seated 

 cells, and eliminate it from the body. 



It is to this end that all the systems of the body work 

 together. 



Systems of the human body and their functions. — In 

 the human body, as in the higher animals already studied, 

 there is a digestive system, consisting of the alimentary canal 

 and all of its parts. This system supplies the body with 



