CHAPTER II. 



THE FUNCTIONS OF ANIMALS. 



(PHYSIOLOGY.) 



MOST animals live a conscious and active life, busied with 

 the search for food, the wooing of mates, the building of 

 homes, and the tending of young. These and other forms 

 of activity depend upon internal changes within the body. 

 For 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. 



But as the work done means expenditure of energy, 

 and is followed by muscular and nervous exhaustion, the 

 necessity for fresh supplies of energy is obvious. This 

 recuperation is obtained from 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, t In other words, it has to be 

 digested. 



We may say then that there are two master activities in 

 the animal body, those of muscular and those of nervous 

 parts, to which the other internal activities are subsidiary 

 conditions, turning food into blood and thus repairing the 

 waste of matter and energy, keeping up the supply of 

 oxygen and the warmth of the body, sifting out and 

 removing waste products. 



Besides the more or less constantly recurrent activities or 

 functions, which are summed up under the general term 

 " metabolism," there are the processes of growth and repro- 



