A TEXT-BOOK OF PHYSIOLOGY. 



INTRODUCTION. 



1. DISSECTION, aided by microscopical examination, teaches us that 

 the body of man is made up of certain kinds of material, so differing from 

 each other in optical and other physical characters and so built up together 

 as to give the body certain structural features. Chemical examination 

 further teaches us that these kinds of material are composed of various 

 chemical substances, a large number of which have this characteristic, that 

 they possess a considerable amount of potential energy capable of being set 

 free, rendered actual, by oxidation or some other chemical change. Thus 

 the body, as a whole, may, from a chemical point of view, be considered as 

 a mass of various chemical substances, representing altogether a considera- 

 ble capital of potential energy. 



2. This body may exist either as a living body or (for a certain time 

 at least) as a dead body, and the living body may at any time become a 

 dead body. At what is generally called the moment of death (but arti- 

 ficially so, for as we shall see the processes of death are numerous and 

 gradual) the dead body so far as structure and chemical composition are 

 concerned is exceedingly like the living body ; indeed the differences be- 

 tween the two are such as can be determined only by very careful examina- 

 tion, and are still to a large extent estimated by drawing inferences rather 

 than actually observed. At any rate the dead body at the moment of death 

 resembles the living body in so far as it represents a capital of potential 

 energy. From that moment onward, however, the capital is expended ; by 

 processes which are largely those of oxidation, the energy is gradually dis- 

 sipated, leaving the body chiefly in the form of heat. While these chemical 

 processes are going on the structural features disappear, and the body, with 

 the loss of nearly all its energy, is at last resolved into " dust and ashes." 



The characteristic of the dead body then is that, being a mass of sub- 

 stances of considerable potential energy, it is always more or less slowly 

 losing energy, never gaining energy ; the capital of energy present at the 

 moment of death is more or less slowly diminished, is never increased or 

 replaced. 



3. When on the other hand we study a living body we are struck with 

 the following salient facts : 



1. The living body moves of itself, either moving one part of the body 

 on another or moving the whole body from place to place. These move- 

 ments are active ; the body is not simply pulled or pushed by external 

 forces, but the motive power is in the body itself, the energy of each move- 

 ment is supplied by the body itself. 



2. These movements are determined and influenced, indeed often seem to 

 be started, by changes in the surroundings of the body. Sudden contact 

 between the surface of the body and some foreign object will often call 



