rNTKODUCTION 



Physiology is a study of activity. It deals with the 

 activity by which a living plant or animal manifests 

 the fact that it is alive, and the activity of the various 

 parts of its body which do the work required to keep 

 it alive. 



Physiology is related to anatomy in very much the 

 same way that the work that a machine is capable of 

 doing is related to its structure. When we study the 

 anatomy of an animal we study the structure of its body, 

 and we may do this through dissection after it has been 

 killed. But when we study physiology we study the 

 manifestation of life, the curious incomprehensible thing 

 which marks the infinite, possibly the infinitesimal, dif- 

 ference between inert matter and a sensitive organism. 



It has been customary to devote a large amount of 

 space in text-books of human physiology to a considera- 

 tion of anatomy. Structure and function have been 

 presented together and causal relations have been estab- 

 lished between them. This has led to confusion of 

 thought. Structure has come to be regarded by young 

 students as the underlying cause of function ; and char- 

 acteristics common to all animals as peculiar to human 

 beings. This is fundamentally wrong. Structure can 

 not initiate activity. It can only control or modify it. 

 Although the activity of living organisms may be regu- 



