HISTOEY OF THE AMERICAN MENHADEN. 253 



lirodacts are similar. Lean meat or muscle and tlie casein (curd) dT 

 milk, like the albumen of the egg, are albuminoid substances and con- 

 tain nitrogen. The fat of the body and the fat (butter) in the milk, like 

 the oils and fats of plants, contain no nitrogen. 



The ingredients of the body are built up from those of the food. The 

 nitrogenous materials, muscle, connective tissue, i^kiu, &c., are formexl 

 from albuminoids. The carbohydrates and fats of the food, which have 

 no nitrogen, cannot be transformed into nitrogenous tissues of the body. 



To form the fats, both the fats and albuminoids of the food contribute. 

 A large part of the fat meat stored in the body and of the butter given 

 off with the milk is made and must be made of the albuminoids of the 

 food. 



Just what work the carbohydrates do in the animal economy is not 

 yet fully settled. They certainly cannot make flesh, and probably do 

 but little at most to make fat. They act as fuel to keep up the animal 

 heat, and doubtless contribute to the generation of muscular force. 

 Just how much of the heat and force produced in the body comes from 

 the consumption of albuminoids, how much from carbohydrates, and 

 how much from fats is still an unsettled problem. 



The animal has been compared to a machine. It is, however, a 

 machine that must be kept running whether it produces anything or 

 not. A horse, or cow, or sheep needs food even at rest in the stall. 

 The machine is peculiar also in that it is wearing out continually and 

 very rapidly, and consumes its own material for both fuel and repairs. 

 The tissues of the body are all the while being used up and rebuilt. 

 In the process of using up, heat and force are produced. The animal 

 consumes food to make its flesh and fat and to give it warmth and 

 strength, but it gets warmth and strength from the consumption of its 

 own flesh and fat at the same time. 



Now to make up for the continued wasting away of tissues and to 

 maintain the supply of heat, food is necessary. But for this purpose 

 but little of albuminoids is required. Carbohydrates will serve for 

 fuel to keep the body warm. The horse or sheep at rest will get on 

 with comparatively little nitrogen. Maintenance fodder may be poor 

 in albuminoids if it furnish carbohydrates in plenty. Stock may be 

 kept in the barn and even wintered on poor hay, cornstalks, and straw. 

 But when iirodnction is required the case is very different. To make 

 lean meat the animal must have albuminoids. Fat meat may be pro- 

 duced from the fat of the food, if there be enough, but practically a 

 large part of the fat must come from albuminoids. The casein and fat 

 (butter) of the milk likewise come from the albuminoids of the food, 

 and for work also more or less of albuminoids are used. The growing 

 colt or lamb, the working horse or ox, the milch cow and the fattening 

 sheep or swine or steer must all have rich food and food rich in nitro- 

 gen. The nitrogenous ingredients, the albuminoids of the food, are 

 its most important constituents. They may take the i)lace of the carbo- 



