148 NUTRITION OF FARM ANIMALS 



207. Purpose of metabolism. As implied at the opening 

 of this chapter, the vital activities of the body are essentially 

 transformations of energy. The living body is continually 

 doing work upon its surroundings and continually loosing heat 

 to them and the energy for the production of work and the main- 

 tenance of the body temperature is derived, as already stated, 

 from the transformation of the chemical energy contained in 

 the substances broken down, tiiis transformation being indeed 

 the essence of the whole process. This fact is familiarly, if not 

 altogether accurately, expressed in the statement that the 

 feed is the fuel of the body. 



There will be occasion later to consider this aspect of the 

 matter in detail, but it is important at the outset to grasp the 

 conception that the final end and aim of metabolism is to sup- 

 ply energy for the vital activities and that the demand for en- 

 ergy is the controlling factor in all its processes. It is these 

 transformations of energy which, if not synonymous with life, 

 are at least its objective manifestation. 



But while it is essential to hold fast to this broad general con- 

 ception of metabolism, it is also important to understand clearly 

 that the processes by which this end is reached are exceedingly 

 complex. A volume would be required for any adequate dis- 

 cussion even of existing knowledge regarding the details of the 

 metabolic processes. Such a discussion lies outside the scope 

 of the present work. All that is attempted in this chapter is 

 to outline the metabolism of the principal groups of feed sub- 

 stances and, as preliminary to a subsequent consideration of 

 their values as sources of matter and energy to the body, to 

 indicate the functions which they perform in the building up 

 and maintenance of the organism and the support of its activ- 

 ities. 



2. ENZYMS AS AGENTS IN METABOLISM 



Enzym action has come to play so large a part, even if a more 

 or less hypothetical one, in the current conceptions of the pro- 

 cesses of metabolism that a brief outline of the prevailing 

 views seems called for. 



208. Extracellular enzyms. The enzyms of the digestive 

 tract are those which are most familiar in physiology. As has 

 been seen (114), the digestion of all three of the chief classes 



