COMPARATIVE PHYSIOLOGY. 



An animal is a machine, and the preceding laboratory 

 exercises are intended to give a student a knowledge of the 

 different kinds of mechanism in the several animal types. 

 Our knowledge of a machine is not complete when we know 

 its structure ; we must also understand the way the differ- 

 ent parts perform their work. The study of the structure 

 of an animal is the province of anatomy, while that branch 

 of science which deals with the action of the various parts 

 the working of the whole is called physiology. 



It is a far more difficult task to ascertain from the speci- 

 mens themselves the function of the parts and the action of 

 the animal machine as a whole, than it is to make out the 

 details of structure and so a general summary is given here. 



Any and every machine, in order that it may perform 

 work, must be supplied with energy, and the animal obtains 

 this energy by the slow combustion (oxidation) of food, just 

 as the steam-engine gets its energy from the rapid combus- 

 tion of coal. In the case of a steam-engine there is an 

 engineer who supplies the fuel, regulates the action of the 

 parts, and disposes of the waste. The animal must be its 

 own engineer. It must have the means of obtaining fuel 

 (food), of putting it in such position that all the energy pro- 

 duced by its oxidation can be utilized to its fullest extent, 

 and all waste can be properly disposed of. This has led, in 

 the first place, to the formation of a digestive tract, in which 

 the food is put in such shape as to be most advantageously 

 used by the organism. 



324 



