4 Life and Health 



combines with oxygen, slowly gives out heat, and therefore 

 slowly loses its store of energy. Even a slight change in 

 the surroundings of living bodies may rapidly, profoundly, 

 and in special ways, affect not only the amount but the kind 

 of energy set free. Thus the mere touch of a hair on some 

 particular surface may lead to such a discharge of energy 

 that a. body previously at rest may be suddenly thrown 

 into violent convulsions. This is especially true in certain 

 diseases, for example, lockjaw. 



The main problem we have to solve in the study of 

 physiology is to ascertain how it is that our living bodies 

 can renew their substance and replenish the energy which 

 they are continually losing, and can, according to the nature 

 of their surroundings, vary not only the amount but the 

 kind of energy which they set free. 



Hence, in all our future work, we must keep steadily in 

 mind the two great fundamental divisions of physiology : 



One which has to do with the renewal of substance and the 

 replenishment of energy ; the other which deals with the setting 

 free of energy. 



6. Life is Incessant Change. All living structures are 

 subject to constant decay. Life is a condition of incessant 

 changes, dependent upon two opposite processes, repair and 

 decay. A considerable part of our bodies wastes every 

 day, and the part which is wasted must be replaced. Thus 

 our bodies are not composed of exactly the same particles 

 from day to day, or even from one moment to another, 

 although to all appearance we remain the same individuals. 

 For instance, we are never of the same weight for two 

 minutes together ; but the change is so gradual, and the 

 renewal of that which is lost may be so exact, that no 

 difference can be noticed except at somewhat long inter- 

 vals of time. 



