112 THE HUMAN MACHINE 



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

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

 on the other hand, uses for fuel the same substances as those out 

 of which it is formed ; it may, indeed, use part of its own substance 

 for fuel. The human organism must do more than purely mechan- 

 ical work. It must be so delicately adjusted to its surroundings 

 that it will react promptly and efficiently 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 trans- 

 forming the energy received 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 itseK. 



The Value of Understanding our Bodies. — Since the purpose 

 of this book is to help prepare young people to become good citi- 

 zens, this chapter is of special importance. No boy or girl can 

 go into the big game of life with an insufficient knowledge of the 

 human machine and expect to be a helpful member of society. 

 Neglect or lack of care of our bodies may defeat some of our life's 

 fondest ambitions. The efficient citizen usually is the healthy 

 citizen. 



The Skin. — Covering the body is the protective structure 

 called the skin. Under the epidermis, a layer of dead cells, there 

 are deficate sense organs, lying in the dermis or true skin, which 

 give us sensations of touch, pressure, and temperature. The skin 

 aids also in passing wastes out of the body by means of sweat 

 glands, and it plays an important part in equalizing the tempera- 

 ture of the body. Hairs are present on most parts of the body, 

 growing from tiny outgrowths of the deep living skin, the true 

 skin. They are kept soft by means of oil secreted by glands at 

 the base of each hair. 



The skin is first of all an organ of protection against man's 

 microscopic foes, the bacteria. But a dirty skin harbors bacte- 

 ria. Moreover, the skin pores, through which the sweat passes, 

 are easily clogged with dirt. Frequent washing is necessary if 

 we wish to keep the skin clean. Pride in one's own appearance 



