288 THE ANIMALS AND MAN 



the statements of physicians, surgeons, and trained physi- 

 ologists for most of their knowledge of human anatomy and 

 human physiology. 



The health of the human body depends upon the right 

 performance of its proper function by each part of the body. 

 The laws of health or of hygiene are merely those rules 

 that have been proved by experience and experimentation 

 to be best adapted for maintaining the body in its best work- 

 ing condition. 



Our chief purpose in studying human physiology is there- 

 fore to understand the working of the human body as a 

 self -regulated, working machine, that we may know how to 

 give it the protection and care necessary to preserve it as such. 



Structural units of the living body and division of 

 labor. — In a lifeless mechanism like a watch, we find many 

 individual parts, and these parts so nicely adjusted one to 

 the other that they all work together successfully as a whole. 

 So it is in the living mechanism. 



Our study of the amoeba (Chapter V) made us acquainted 

 with a living, feeding and moving animal whose body is 

 composed of a single cell. Our study of the structure of 

 the toad (Chapter III) showed us that the life- functions 

 are performed in this animal by certain large systems of 

 organs; that these organs are made up of groups of tissues 

 formed of masses of similar cells, each cell performing its 

 own special kind of work. The cell is therefore the struc- 

 tural and physiological unit of the body. Each cell does 

 its share of the work of that tissue of which it is a part, 

 and each tissue does its share of the work of the organ of 

 which it is a part. In the same way each organ works in 

 harmony with all the other parts in its system, and all the 

 systems work together to maintain life. 



The living substance of the cell and metabolism. — In 

 Chapter V it was explained that while the cells of the body 

 may differ very much in appearance and function, they are 



