1 84 NUTRITION OF FARM ANIMALS 



plants transform the radiant energy of the sun into the chemical 

 energy of their various constituents, to be yielded up by the 

 latter to the animal organism through the processes of metab- 

 olism. This conception has become a familiar one and much 

 emphasis has been laid upon it in recent years. 



Second, the feed supplies the specific materials required for 

 building and maintaining all the complex structures of the 

 body and for their harmonious functioning, i.e., it is the source 

 of structural and repair material. 



That the proteins, fats and mineral ingredients which make 

 up by far the larger part of the dry matter of the body (99, 280) 

 are derived ultimately from the feed needs no special demon- 

 stration, but the importance of many substances present in the 

 body in only minute amounts tends to be overlooked. For 

 example, the enzyms of the body, both extra- and intra-cellular, 

 form no considerable portion of its mass, yet they are essential 

 to its vital activities. So, too, the various hormones and se- 

 cretions of the ductless glands, while ignored in the broad scheme 

 of metabolism just presented, are essential to the vital processes. 

 Clearly, the feed must supply material for the production of 

 these and other similar substances. 



In other words, no amount of energy-yielding material will 

 suffice to support life in the absence of those specific substances 

 which are necessary in order that the machinery of conversion 

 shall operate properly, much as *LO amount of coal under the 

 boiler will enable an electric plant to furnish a normal amount 

 of current if the insulation of the generator is defective. For 

 example, if tryptophan is necessary for the formation of some 

 essential internal secretion, a diet lacking that substance, 

 however much energy it might furnish, would fail to support the 

 organism permanently unless the body can manufacture trypto- 

 phan from other substances. 



The latter qualification is a very important one. The animal 

 is very far from being dependent upon the presence in its feed 

 of all the varied chemical compounds required for its operation. 

 Indeed, quite the reverse is the case. As has appeared in pre- 

 vious sections, the actual substances resorbed are comparatively 

 simple and uniform and upon them the animal body executes 

 a great variety of chemical changes, both analytic and synthetic. 

 What is necessary is that the resorbed feed shall include sub- 



