26 THE FUNCTIONS OF ANIMALS 



the aid of catalysts, which may be compared to lubricants. 

 Thus pure hydrogen does not combine with oxygen 

 except at very high temperatures, when it does so with 

 explosive violence ; but the two gases can be got to com- 

 bine at a low temperature by the introduction of finely- 

 divided platinum as a catalyst. The essential features 

 of a catalyst are that it is not used up in the course of 

 the reaction which it promotes, that accordingly a small 

 quantity of catalyst is sufficient for a great mass of reactant, 

 and that only reactions in which energy is set free can 

 be promoted — a catalyst cannot supply energy. 



Within living cells oxidations are aided by catalysts of 

 different kinds. In the first place, the enormous surfaces 

 of the microscopic and ultra-microscopic constituents of 

 the protoplasm are of importance, since in their neigh- 

 bourhood are local peculiarities of concentration and 

 electrical forces. In the second place, there are certain 

 fairly simple organic substances, such as glutathione, 

 which are easily oxidised and reduced again, and in their 

 transmutations assist in the oxidation of more resistant 

 compounds. In the third place, there are catalysts of a 

 special type, peculiar to living organisms, known as 

 enzymes or ferments. The chemical nature of these 

 enzymes is obscure ; they are very sensitive to changes 

 in the medium in which they act (acidity, etc.), and are 

 easily destroyed ; and they are markedly specific. Each 

 enzyme — a large number being known — catalyses only 

 one reaction or a small group of reactions. Their activity 

 is not restricted to oxidations ; as we shall see, they play 

 a most important part in the processes of digestion. 



Chief functions of the animal body. — There are two 

 master activities in animals, those of muscular and of 

 nervous structures ; the other vital processes, always ex- 

 cepting growth and reproduction, are subservient to these. 

 Let us now consider these master and subsidiary functions, 

 as they occur in some higher organism, such as man. 



Nervous activities. — Life has been described as action 

 and reaction between the organism and its environment, 

 and it is evident that an animal must in some way become 

 aware of surrounding influences. 



An external influence stimulates a sensory cell or its 



