| The body of an animal may well be compared with(1) some machine like a locomotive engine. Indeed, the animal body is a machine. It is a machine composed of(2) many parts, each part doing some particular kind of work for which a particular kind of structure fits it(3); and all the parts are dependent on each other and work together for the accomplishment of the total business of the machine. The locomotive must be provided with fuel, such as coal or wood or other combustible substance, the consumption of which furnishes the force or energy of the machine. The animal body must be provided with fuel, which is called food, which furnished similarly the energy of the animal. Oxygen must be provided for the combustion of the fuel in the locomotive and of the food(4) in the body. The locomotive is composed of special parts: the fire box for the reception and combustion of fuel; the steam pipes for the carriage of steam; the wheels for locomotion; the smokestack for throwing off waste. The ani-mal body is similarly composed of parts: the alimentary canal for the reception and assimilation of food; the excretory organs for the throwing off of waste matter; the arteries and veins for the carriage oxygen and food-holding blood; the legs or wings for locomotion. |