18 INTRODUCTION. 



forth a movement. The body is sensitive to changes in its surroundings, 

 and this sensitiveness is manifested not only by movements but by other 

 changes in the body. 



3. It is continually generating heat and giving out heat to surrounding 

 things, the production and loss of heat, in the case of man and certain 

 other animals, being so adjusted that the whole body is warm, that is, of a 

 temperature higher than that of surrounding things. 



4. From time to time it eats, that is to say takes into itself supplies of 

 certain substances known as food, these substances being in the main similar 

 to those which compose the body, and being like them chemical bodies of 

 considerable potential energy, capable through oxidation or other chemical 

 changes of setting free a considerable quantity of energy. 



5. It is continually breathing; that is, taking in from the surrounding 

 air supplies of oxygen. 



6. It is continually, or from time to time, discharging from itself into its 

 surroundings so-called waste matters, which waste matters may be broadly 

 described as products of oxidation of the substances taken in as food, or of 

 the substances composing the body. 



Hence the living body may be said to be distinguished from the dead 

 body by three main features. 



The living body like the dead is continually losing energy (and losing it 

 more rapidly than the dead body, the special breathing arrangements per- 

 mitting a more rapid oxidation of its substance), but unlike the dead body 

 is by means of food continually restoring its substance and replenishing its 

 store of energy. 



The energy set free in the dead body by the oxidation and other chem- 

 ical changes of its substance leaves the body almost exclusively in the form 

 of heat, whereas a great deal of energy leaves the living body as mechanical 

 work, the result of various movements of the body, and as we shall see a 

 great deal of the energy which ultimately leaves the body as heat, exists for 

 a while within the living body in other forms than heat, though eventually 

 transformed into heat. 



The changes in the surroundings affect the dead body at a slow rate and 

 in a general way only, simply lessening or increasing the amount or rate 

 of chemical change and the quantity of heat thereby set free, but never 

 diverting the energy into some other form such as that of movement ; 

 whereas changes in the surroundings may in the case of the living body 

 rapidly, profoundly, and in special ways affect not only the amount but also 

 the kind of energy set free. The dead body left to itself slowly falls to 

 pieces, slowly dissipates its store of energy, and slowly gives out heat ; a 

 higher or lower temperature, more or less moisture, a free or scanty supply 

 of oxygen, the advent of many or few putrefactive organisms, these may 

 quicken or slacken the rate at which energy is being dissipated but do not 

 divert that energy from heat into motion ; whereas in the living body so 

 slight a change of surroundings as the mere touch by a hair of some par- 

 ticular surface, may so affect the setting free of energy as to lead to such a 

 discharge of energy in the form of movement that the previously appar- 

 ently quiescent body may be suddenly thrown into the most violent con- 

 vulsions. 



The differences, therefore, between living substance and dead substance 

 though recondite are very great, and the ultimate object of physiology is 

 to ascertain how it is that living substance can do what dead substance 

 cannot that is, can renew its substance and replenish the energy which it 

 is continually losing, and can, according to the nature of its surroundings, 

 vary not only the amount but also the kind of energy which it sets free. 



