PART IV. THE BIOLOGY OF MAN 



UNIT X 

 HOW DOES THE HUMAN MACHINE DO ITS WORK? 



Preview. I suppose every boy and girl who reads these pages 

 has seen an interscholastic track meet and perhaps envied the 

 perfect coordination of every part of the bodies of the men who 

 run hurdles, high jump, or pole vault. Perhaps you have tried 

 some of these feats yourself and have discovered how difficult it 

 is to make the different muscles coordinate at exactly the right 

 time. It is a very wonderful machine, this body of ours, and we 

 cannot help but feel a real reverence for it when we think of the 

 delicate mechanism which, with its numerous adjustments and 

 adaptations, can do work so efficiently. Unl ik e a man-made 

 machine, the body is seK-directed, and with care will far outlast 

 most machines made of iron and steel. 



In all animals, and the human organism is no exception, the body 

 has been likened to a machine in that it turns over the latent or 

 potential energy stored up in food into kinetic energy f 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 sub- 

 stances as those out of which it is formed; it may, indeed, use 

 part of its own substance for fuel. The human organism does 

 more than purely mechanical work. It is so delicately adjusted 

 to its surroundings that it will react promptly and efficiently to 

 stimuli from without ; it is able to utilize its fuel (food) in the most 

 economical manner ; it is fitted with machinery for transforming 

 the energy received from food into various kinds of work; it 



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