CHAPTER II. 



THE FUNCTIONS OF ANIMALS. 



(PHYSIOLOGY.) 



MOST animals live an active life, in great part ruled by the 

 two motives of love and hunger in their widest sense ; they 

 are busy finding food, avoiding enemies, wooing mates, 

 making homes, and tending the young. These and other 

 forms of activity depend upon internal changes within the 

 body. Thus the movements of all but the very simplest 

 animals are due to the activity of contractile parts known 

 as muscles, which are controlled by nervous centres and by 

 impulse-conducting fibres, and the energy involved in these 

 movements, and in most other vital activities, is supplied 

 by the oxidation or combustion of the complex carbon- 

 compounds which form a substantial part of the various 



organs. 



The work done means expenditure of energy, and is 

 followed by exhaustion (muscular, nervous, etc.), so that 

 the necessity for fresh supplies of energy is obvious. This 

 recuperation is obtained through food, but before this can 

 restore the exhausted parts to their normal state, or keep 

 them from becoming, in any marked degree, exhausted, it 

 must be rendered soluble, diffused throughout the body, 

 and so chemically altered that it is readily incorporated 

 into the animal's substance. In other words, it has to be 

 digested. A fresh supply of oxygen and a removal of waste 

 are also obviously essential to continued activity. 



We may say, then, that there are two master activities in 

 the animal body, those of muscular and those of nervous 

 parts. To these the other internal activities are subsidiary 



