hi ANIMAL LIFE 121 



is, indeed, extremely complex. In the first place it serves 

 to repair tissues, then it . provides the animal with the 

 heat necessary to render it as independent as possible 

 of changes in external temperature. Thus it preserves, 

 supports, and maintains the organism in whieh the nervous 

 system is set and on which the nervous elements have to 

 live. But these nervous elements would have no reason 

 for existence if the organism did not pass to them, and 

 especially to the muscles they control, a certain energy 

 to expend; and it may even be conjectured that there, 

 in the main, is the essential and ultimate destination of 

 food. This does not mean that the greater part of the food 

 is used in this work. A state may have to make enormous 

 expenditure to secure the return of taxes, and the sum 

 which it will have to dispose of, after deducting the cost 

 of collection, will perhaps be very small: that sum is, 

 none the less, the reason for the tax and for all that has 

 been spent to obtain its return. So it is with the energy 

 which the animal demands of its food. 



Many facts seem to indicate that the nervous and mus- 

 cular elements stand in this relation towards the rest of 

 the organism. Glance first at the distribution of ali- 

 mentary substances among the different elements of the 

 living body. These substances fall into two classes, one 

 the quaternary or albuminoid, the other the ternary, 

 including the carbohydrates and the fats. The albumi- 

 noids are properly plastic, destined to repair the tissues — 

 although, owing to the carbon they contain, they are 

 capable of providing energy on occasion. But the function 

 of supplying energy has devolved more particularly on 

 the second class of substances: these, being deposited 

 in the cell rather than forming part of its substance, 

 convey to it, in the form of chemical potential, an ex- 

 pansive energy that may be directly converted into either 



