CHAPTER I 

 LIFE AND ENERGY 



The Problem 



WE all know what living beings are, and it would be unprofit- 

 able to attempt to define life in such a way as to make an 

 inhabitant of another planet understand what we mean by it on 

 the earth. The most striking thing in the behaviour of living 

 things is their perpetual change they are always doing things. 

 From that aspect, with which physiology has to deal, we may say 

 that they are extraordinarily complicated machines, in which the 

 laws of physics and chemistry are made use of in a way quite 

 different from that in which a machine made by an engineer uses 

 them. What we have to do, then, is to try to find the way in 

 which living machines work (p., p. viii). 



It is natural that our own bodies should be, to ourselves, the 

 most interesting and important of living organisms ; but when we 

 come to investigate them, we find that there is nothing in them 

 which is not to be found in some form in what we call the lower 

 organisms. Certain arrangements, especially those connected with 

 the brain, are more complex, it is true. These would not be 

 understood, however, without a knowledge of the simpler arrange- 

 ments to begin with. 



It must not be forgotten that physiology is not directly con- 

 cerned with the mind. Our thoughts and feelings, when investi- 

 gated with the view of finding out how they depend on one 

 another, is the province of another science, psychology. So that 

 when we speak of living machinery, it is not to be supposed that 

 a denial is made of the existence of anything else. When the 

 functions of the brain are discussed, it is the changes taking 

 place therein, as looked at from the outside, that we are dealing 

 with. 



Plants, as well as animals, are alive, and we shall find that 

 there is very much in common between them. Strictly speaking, 



