BREEDS OF BEEF CATTLE 295 



things being equal, a better source of protein than Indian corn, car- 

 rying about 10 per cent. 



In the second place, however, the protein of the feeding stuff 

 must be capable of being digested by the animal. Of two feeding 

 stuffs containing equal amounts of protein, that one is the more 

 valuable as a source of supply in which the larger proportion of the 

 protein is digestible. The second column of the table herein shows 

 the average percentage of digestible protein contained in a number 

 of the more common feeding stuffs. These figures are the average 

 results of a considerable number of analyses of the feeding stuffs 

 and a smaller number of determinations of their digestibility. In- 

 dividual samples may vary more or less, 'and sometimes considerably, 

 from the average. 



A third question is at once suggested, viz, whether the digest- 

 ible protein from different feeding stuffs is equally valuable to the 

 animal. It is unlikely that this is exactly the case, but whether 

 these probable differences are of much practical significance, espe- 

 cially in rations containing a number of feeding stuffs, seems doubt- 

 ful. At any rate, the only course open at present is to assume them 

 to be of substantially equal value. 



The Demand for Fuel Material. Since the animal machinery 

 is running continually, it requires a continual supply of fuel ma- 

 terial, the amount which is necessary depending upon the amount 

 of work done. This fuel material consists chiefly of the carbohy- 

 drates and fats of the food, although if more protein be fed than is 

 required for repair and construction purposes it, too, may be used as 

 fuel, while the worn-out portions of the protein tissues are also util- 

 ized that is, the bodily engine can burn up its own waste products 

 as fuel. The unnecessary use of protein as fuel material, however, 

 is wasteful, because protein is ordinarily more expensive to buy or 

 to produce on the farm than are carbohydrates and fat. 



If the fuel materials supplied in the food are just adequate to 

 the work to be done, they are virtually burned up as a source of 

 power. If more are supplied than are immediately needed, the body 

 is able to store away the surplus for future use, much as we may fill 

 up the gasoline tank of an engine. To a small extent the body stores 

 up carbohydrates (in the form of glycogen), but most of its surplus 

 fuel it converts into fat. The fat of the body, therefore, is its re- 

 serve of fuel. In fattening, the body is accumulating a surplus 

 against future needs which man diverts to his own use as food. If 

 the food becomes insufficient, this store is drawn upon and the ani- 

 mal gradually becomes lean. Similarly, in growth and in milk pro- 

 duction, the animal sets aside a part of the supply of both repair 

 and fuel material in its food for its own growth or for the use of its 

 young, and man appropriates the resulting meat or milk as repair 

 and fuel material for his own body. 



Fecrl. as (i Source of Fuel Material. We can run an engine with 

 various kinds of fuel. For the steam engine we may use coal, wood or 

 petroleum ; for the internal-combustion motor, gas, alcohol, or gaso- 

 line may be employed. Similarly we supply the animal body with 



