234 LIVESTOCK ON THE FARM 



use different nutrients or materials for different purposes, but 

 this substitution is by no means unlimited. An animal cannot 

 use anything else in place of protein, mineral matter, or water. 

 The substitution, therefore, applies only to carbohydrates and 

 fat, and, as seen by the writer, this is only applicable to the use 

 of these materials after they are digested, absorbed from the 

 digestive tract, and taken into the circulation. Since these 

 two different materials are handled in the processes of digestion 

 and absorption by entirely different parts of the mechanism in 

 the body, it is clear that best results can be obtained only if the 

 proper amount of each is being fed. 



The animal body as a whole, therefore, in the use of its food 

 materials is somewhat similar to an imaginary threshing 

 machine that would be made to thresh oats, wheat, beans and 

 clover seed all at the same time. If the largest amount of work 

 is to be done by such a machine it must be fed to the fullest 

 extent in all parts. That is, if only beans and wheat are being 

 threshed and the oats and clover seed parts of the machine are 

 allowed to run empty, maximum results will not be obtained. 

 The parts of the mechanism of the animal body that are not 

 in use apparently also seem gradually to lose their powers and 

 later in life are not able to do the required amount of work. 



During the growing period an animal is necessarily develop- 

 ing a considerable amount of protein material in the form of 

 muscles as well as in the bones and other tissues. The bones, 

 however, are largely (two-thirds, in the mature animal) 

 composed of mineral matter, therefore, a large quantity of 

 material that can be used for this purpose must also be 

 supplied. 



After the structure is once made and the animal has attained 

 its full growth a given amount of material is always necessary 

 for maintenance. That is, the animal body is a living machine 

 and is constantly using up material. For this purpose a small 

 quantity of protein is absolutely necessary. More, of course, 

 can be used, but a minimum requirement of protein supple- 

 mented with carbohydrates and fat with water, is more eco- 

 nomical. In these respects the animal body as a machine is 

 quite dissimilar from a mechanical machine. As soon as a 

 piece of work is done an engine will not use any fuel for main- 



