THE GROWTH OF ANIMALS 23 



complicated, both in structure and in the work that they 

 do, than the fore legs of a grasshopper. 



The animal as a machine. — We have seen that the body 

 of an animal is made up of many parts, or organs, each with 

 its own particular work, or function, to perform. With 

 respect to its structure, then, an animal's body is very similar 

 to a machine. In truth, the animal body may be considered 

 a machine, in which each part performs a special kind of 

 work, and although dependent on the other parts, con- 

 tributes its share to the labor of the machine as a whole. 

 For example, a locomotive has a fire box into which the 

 fuel is thrown that is to produce the energy to run the 

 engine. Similarly, the animal body has an alimentary 

 canal which receives and assimilates the food from which 

 energy is derived. The locomotive is provided with a 

 smokestack and exhaust valves to conduct away the waste 

 materials, while the lungs, skin, and kidneys act in a similar 

 capacity for the body. The engine has its steam cylinders 

 in which the transformed energy of the fuel, through the 

 medium of steam, sets up motion. In a corresponding 

 manner, the animal body is furnished with muscles, through 

 which the transformed energy of the food expresses itself 

 in the form of motion. Moreover, the engine must be sup- 

 plied with fuel and water to keep it running; and similarly, 

 the animal body demands food and water to maintain its 

 activity. If the steam cyhnders of the engine wear out, 

 the locomotive becomes useless. If the kidneys of the 

 animal body become incurably diseased, the body dies. 



Of course, it must be remembered that the animal body 

 has that subtle and indefinable characteristic we caU hfe 

 which the locomotive has not; and this makes a very 

 great and real difference between them. 



