INTRODUCTION 3 



peculiar to each kind. The carbon compounds have definite 

 structures, as Kekule's demonstration of the benzole-ring 

 proved; they form under certain conditions and their be- 

 havior is characteristic of the kind. A machine has defi- 

 nite structure, it operates in a fashion characteristic of its 

 kind. Physiology has long been conceived to be the study 

 of the structure and operations of peculiar machines, the 

 study of functions as based upon a knowledge of ana,tomy. 

 The physiologist is now striving not only to know the func- 

 tions which are the manifestations of the life possessed by 

 complicated living structures or organisms, but also to 

 determine the causes both of structure and of functions. In 

 an engine we have a structure which, under certain know- 

 able conditions, does certain kinds of work. The materials 

 and the construction of the engine are lifeless ; the engine 

 is the result of human intelligence acting in harmony 

 with physical laws upon lifeless material; the working of 

 the engine is the result of energy (physical force) applied 

 through human intelligence to it ; the structure itself and its 

 working are the result of physical forces acting upon inert 

 materials in harmony with physical laws comprehended by 

 the designer and driver of the engine. These are all exter- 

 nal to the structure and are consciously taken advantage of 

 and applied to and through the engine by the living organ- 

 isms concerned in its construction and operation. 



A living organism is a structure existing in harmony with 

 physical forces and laws and because of them. Few living 

 organisms strive to ascertain, and none fully knows, what 

 these forces are. The materials employed in the construc- 

 tion of a living organism are inert, lifeless; they are ar- 

 ranged in harmony with, and through the operation of, 

 physical laws and forces ; but the inert materials are acted 

 upon and the physical forces employed by the organism 

 itself, and for the most part unconsciously. 



Between the engine and the living organism there is then 

 a radical difference; the engine and the organism, though 

 both machines, differ in that life, external though applied 

 to the one, is internal and possessed by the other. The 

 difference thus stated is one plainly felt but inadequately 



