CHAPTER it 
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 f2o master activities in 
the animal body, those of muscular and those of nervous 
parts. To these the other internal activities—digestion, 
respiration, excretion, and the like—are subsidiary. 
