LAWS OF NUTRITION 199 



284. The matter of the digested food, including water 

 and oxygen, is exactly equal to that stored in the body or 

 in milk, or both, plus that in waste products feces, 

 water, carbonic acid, and urine solids. Such a balance 

 may not be maintained for any particular day, but will 

 ultimately be found to exist. 



285. Under given conditions of species, sex, climate, 

 and use, a definite amount of digested organic matter is 

 necessary to maintain a particular animal without gain 

 or loss of body substance. This means simply that tis- 

 sue wastes must be replaced, and the fuel-supply must 

 be kept up. 



If the animal receives no food, or less than the amount 

 needed for maintenance purposes, tissue waste and the 

 production of energy do not cease, but go on wholly or 

 in part at the expense of the body substance. 



286. Food supplied above a needed maintenance quan- 

 tity may be utilized for the production of new substances 

 or work or may be eliminated in .part increasing the 

 waste. Within limits, both things generally occur. In 

 the proper sense of the term, no production ever occurs 

 without an excess of food above maintenance require- 

 ments. Milk formation may sometimes go on at the 

 expense of the body substance, but with proper feeding, 

 milk, flesh or muscular work are produced at the expense 

 of food supplied in excess of that needed for maintenance. 



287. Regard must be had to the supply of particular 

 nutrients as well as of total food. Even with an animal 

 doing no work and giving no milk a certain amount of 

 protein will be broken up constantly into urea and simi- 

 lar compounds, an amount which will be withdrawn from 

 the body tissues to the extent that it is not supplied by 

 the food. In addition to this, a milch cow, for instance, 



