CHAPTER V. 



THE SOURCE OF MUSCULAR ENERGY; COMPOSITION" OF ANIMALS 

 BEFORE AND AFTER FATTENING. 



I. The Source of Muscular Energy and the Production of Force. 



90. Food the basis of life. The ultimate object of most of 

 the food supplied to the animal is the production of work. All 

 the manifestations of life, shown in a thousand ways by the animal, 

 are in some manner derived from the food. The horse is kept for 

 the direct production of labor. The cow, in furnishing milk, 

 supplies a substance used for building up the tissues of the body 

 or for the production of work. Doubtless the manner in which 

 the food is converted into energy will always exceed man's power 

 to definitely determine, but many facts in this connection already 

 known are interesting and highly instructive. 



We have learned that the substances which enter the body as 

 food can be grouped under three great divisions: protein, carbo- 

 hydrates, and ether extract. (7-9) We have further learned 

 that when the protein of the food has entered the body proper 

 and been broken down, the amount of this loss to the body can 

 be measured by nitrogen found in the urea. (57) The carbo- 

 hydrates and ether extract, when absorbed from the alimentary 

 tract to nurture the body, may be built up into body fat or de- 

 composed into carbonic acid and water, heat and energy being 

 given off in this process. The water resulting from the decom- 

 position cannot be used as a measure of the broken-down carbo- 

 hydrates and fat, for the reason that it is mingled with water 

 from other sources when excreted. Scientists are, nevertheless, 

 able to determine the quantity of these substances which has been 

 broken down, by measuring the carbonic acid exhaled in the 

 breath of the animal. 



