72 THE PHILOSOPHY OF BIOLOGY 



mechanism which does work. The source of its 

 energy is the potential chemical energy of its food- 

 stuffs, which latter reduce down to those substances 

 known as proteids, fats, and carbohydrates. The 

 energy- value of these compounds is considerable, that 

 is to say, if they are burned in a stream of oxygen 

 a large quantity of heat is obtained from their com- 

 bustion. They are ingested by the animal, broken 

 down chemically, and rearranged. The proteids eaten 

 by the animal (say those of beef or mutton or vv^heat) 

 are acted upon by the enzymes of the alimentary canal 

 and are decomposed into their immediate constituents, 

 animo-acids, and then other enzymes rearrange these 

 animo-acids so as to form proteid again, but proteids 

 of the same kinds as those characteristic of the tissues. 

 This decomposition and re-synthesis is carried out also 

 with respect to the fats and carbohydrates ingested. 

 The result is that the food taken into the alimentary 

 canal, or at least a part of it, is built up into the living 

 substance of the animal's body. The energy expended 

 upon these processes of digestion and assimilation is 

 probably inconsiderable. During these processes the 

 animal absorbs available chemical energy. 



The energy thus taken into the animal is then 

 transformed. The major part of it appears as mechani- 

 cal energy — that of bodily movement, the movements 

 of heart, lungs, blood, etc. — and heat. Some part of 

 it becomes nervous energy, by which rather vague term 

 we mean the energy involved in the propagation of 

 nervous impulses. Some of it is used in glandular 

 reactions, in the formation of the digestive juices, for 

 instance. The most of it, however, transforms to 

 mechanical energy and heat. Just how these energy 

 transformations are effected we do not know. The 

 heat is, of course, the result of chemical changes, oxida- 



