ANIMAL PHYSIOLOGY 55 



theless the essential facts are the same in both cases. The 

 muscle-engine, like the steam-engine, produces a form of 

 energy and apphes it to machines so as to lift weights or to 

 move things from place to place. But we learn in physics 

 that we can get any form of energy only by changing some 

 other form into the one desired. The forms of energy are 

 heat, light, electricity, chemical energy, and tnat of a body 

 in motion. Now the only way to get heat, for example, is 

 by a change from one of the others. We can make a piece 

 of iron hot by striking it with a hammer; here the energy 

 of a moving body is converted into heat. Or the energy of 

 the electric current may be converted into heat or motion. 

 Man's most common way of getting heat is to take coal, 

 wood, or oil, and apply some heat to start with, when the 

 oxygen of the air will unite with carbon and hydrogen, sub- 

 stances in the coal, wood, or oil, to make two new substances, 

 one of these being carbon dioxide, the other water. This is 

 chemical action; it results in changing chemical energy into 

 heat. In ordinary language this union of oxygen with car- 

 bon or hydrogen is spoken of as "burning" or "combustion." 



An animal cannot make the least motion without using 

 a certain amount of energy. And it has been shown by 

 investigation that the energy possessed by an animal is 

 derived from the chemical energy resulting from the union 

 of oxygen with the carbon, hydrogen, and nitrogen in other 

 substances. The muscles are the engines in which this 

 energy is made use of for motion. This brings us now to 

 see how essential it is that the animal should have in its 

 body oxygen and substances for the oxygen to combine with. 



Respiration. — Respiration is the name commonly used 

 in books for the process of obtaining oxygen. The pro- 

 cess has, however, another object in addition to procuring 

 oxygen. When oxygen combines with carbon a poisonous 

 gas, carbon dioxide, is formed. If this remains in the 

 muscle or other tissue cells it interferes with the activity 



