THE BALANCE OF NUTRITION 1 93 



the products of its oxidation excreted. In addition, the activi- 

 ties of the body, like those of the engine, require a supply of 

 fuel material containing available chemical energy equivalent 

 to the work to be done. For this purpose the body utilizes 

 in the first instance the substances contained in its own cells 

 and tissues. As shown in Chapter V, all the organic ingredients 

 of the body protein, fat and carbohydrates undergo 

 katabolism, giving rise to carbon dioxid, water and compara- 

 tively simple nitrogenous products, accompanied by a trans- 

 formation of their chemical energy into other forms. In other 

 words, the body is a storehouse of chemical energy as well as a 

 mechanism. This stored-up energy of the body is contained 

 particularly in its fat, and to a minor degree in its glycogen, 

 while the body protein, although it likewise yields energy when 

 katabolized, especially through the oxidation of its non-nitrog- 

 enous residue (229), usually plays a small part quantitatively. 

 The fat of the body constitutes its great reserve of energy. The 

 store of reserve material in the body may be compared, for the 

 sake of illustration, to the gasoline in the tank of an automobile, 

 with the difference, however, that the body derives more or 

 less energy from the combustion of the material (protein) of 

 the engine itself. 



276. The feed. Neither the automobile nor the animal can 

 long depend entirely upon its own stock of material without 

 disaster. Sooner or later it must obtain supplies from the out- 

 side. The supplies required in both cases are obviously of two 

 classes, corresponding to the two classes of materials consumed 

 in the operation of the machine, and may be briefly designated 

 as repair material and fuel. 



In the automobile, parts of the machinery, the tires, etc., as 

 they wear out must be replaced by new ones of the same kind, 

 while the gasoline tank must be filled at intervals and the work- 

 ing parts must be suitably lubricated. The case of the animal 

 is precisely similar. In the first place, it must be supplied in 

 its feed with materials from which, by the processes of digestion 

 and resorption, it can secure the particular atomic groupings 

 (amino acids, peptids, ash ingredients, etc.) which will exactly 

 fit into its protoplasm and replace those eliminated by the vital 

 activities. In the second place it must also derive from its 

 feed molecules which it may, according to circumstances, break 

 o 



