112 THE VITAL FUNCTIONS. 



structure of the organs, will also be regulated by 

 a great variety of other circumstances in the 

 condition of the animal, independent of the me- 

 chanical or chemical nature of the food. The 

 greater the energy with which the more pecu- 

 liarly animal functions of sensation and muscular 

 action are exercised, the greater must be the 

 demand for nourishment, in order to supply the 

 expenditure of vital force created by these exer- 

 tions. Compared with the torpid and sluggish 

 reptile, the active and vivacious bird or quadruped 

 requires and consumes a much larger quantity 

 of nutriment. The tortoise, the turtle, the toad, 

 the frog, and the chamelion, will, indeed, live 

 for months without taking any food. Fishes, 

 which, like reptiles, are cold-blooded animals, 

 although at all times exceedingly voracious when 

 supplied with food, can yet endure long fasts 

 with impunity. 



The rapidity of developement has also great 

 influence on the quantity of food which an ani- 

 mal requires. Thus the caterpillar, which grows 

 very quickly, and must repeatedly throw off its 

 integuments, during its continuance in the larva 

 state, consumes a vast quantity of food compared 

 with the size of its body ; and hence we find it 

 provided with a digestive apparatus of consi- 

 derable size. 



