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 bodies 

 of the animals. For all movements are due to the activity 

 of contractile parts known as muscles, which are controlled 

 by centres of thought and by impulse-conducting threads, 

 in other words, by nerve-ganglia and nerve-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. 



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 may be summed up under the general 

 term "metabolism," or change, there are two other pro- 

 cesses which should be ranked on a different platform, 



