204 UTILISATION OF FOOD BY THE ANIMAL [chap. 



classes of still more similar substances, yet it is probable 

 that each species of animal and plant builds up a protein 

 special to itself and differing somewhat from all others. 

 In order to build up its special body protein an animal 

 must be supplied with the right units, 2>., the simple 

 amino-acids, amides, etc., which are produced by the 

 splitting-up of proteins by enzymes, and those units 

 which the animal does not require will be useless to it as 

 nitrogenous food, though they will be burnt as usual to 

 supply energy. Some food proteins also may be with- 

 out a unit that is essential to the building-up of the 

 body protein ; thus it has been shown that zein, the 

 particular protein of maize, will not maintain the 

 nitrogen equilibrium of rats or mice. However, much 

 of it is fed, they eventually die of nitrogen starvation, 

 because the zein molecule lacks a particular constituent 

 which is an essential part of the body protein of these 

 animals. If, however, a very small quantity of this 

 particular substance is also supplied, then the animal 

 begins to utilise the zein split products and form its 

 necessary body protein, because it has also obtained the 

 necessary keystone of the structure. Thus we see that 

 the animal must have more products of protein digestion 

 presented to it than it actually uses, because it has to 

 pick them over in order to select the indispensables in 

 the right proportions ; moreover, the diet should contain 

 some variety in the proteins supplied, in order to make 

 certain that no essential constituent is deficient. 



The food of an animal not increasing in weight should 

 therefore be considered from two points of view — it 

 must supply sufificient energy for the calls upon the 

 animal for work, and at the same time it must contain 

 sufificient protein to repair the tissue waste that an 

 animal always experiences even during starvation. 

 Energy is supplied by all kinds of food that are com- 



