320 MAN, A MAMMAL 



were used. About this time the subjugation and domestication of 

 animals began to take place. Man then began to cultivate the 

 fields, and to have a fixed place of abode other than a cave. The 

 beginnings of civilization were long ago, but even to-day the earth 

 is not entirely civilized. 



The Races of Man. At the present time there exist upon the 

 earth five races or varieties of man, each very different from the 

 other in instincts, social customs, and, to an extent, in structure. 

 These are the Ethiopian or negro type, originating in Africa; the 

 Malay or brown race, from the islands of the Pacific; the Amer- 

 ican Indian; the Mongolian or yellow race, including the natives 

 of China, Japan, and the Eskimos; and, finally, the highest type 

 of all, the Caucasians, represented by the civilized white in- 

 habitants of Europe and America. 



The Human Body a Machine. In all animals, and the human 

 animal is no exception, the body has been likened to a ma- 

 chine in that it turns over the latent or potential energy stored 

 up in food into kinetic energy (mechanical work and heat), which is 

 manifested when we perform work. One great difference exists 

 between an engine and the human body. The engine uses fuel 

 unlike the substance out of which it is made. The human body, 

 on the other hand, uses for fuel the same substances out of which 

 it is formed ; it may, indeed, use part of its own substance for food. 

 It must as well do more than purely mechanical work. The human 

 organism must be so delicately adjusted to its surroundings that 

 it will react in a ready manner to stimuli from without ; it must 

 be able to utilize its fuel (food) in the most economical manner; 

 it must be fitted with machinery for transforming the energy re- 

 ceived from food into various kinds of work; it must properly 

 provide the machine with oxygen so that the fuel will be oxidized, 

 and the products of oxidation must be carried away, as well as 

 other waste materials which might harm the effectiveness of the 

 machine. Most important of all, the human machine must be able 

 to repair itself. 



In order to understand better this complicated machine, the 

 human body, let us examine the structure of its parts and thus 

 get a better idea of the interrelation of these parts and of their 

 functions. 



