MUSCLES 



365 



the body. In man, nearly half the body weight is muscle. 

 Nearly every muscle in the human body is attached to a bone 

 either at one or at both ends. Movement is performed by means 

 of the muscles, leverage being obtained by means of their attach- 

 ment to the bones. Movement is, indeed, the chief function of 

 muscles. In the human body there are over five hundred mus- 

 cles, var}'ing from the size of a pinhead to a band 

 almost two feet in length. Every movement of the 

 body, be it merely a change of expression or change 

 in the pitch of the voice, directly results from con- 

 traction or expansion of a muscle. Muscles also 

 give form to the body, and are useful in protecting 

 the delicate organs and large blood vessels within 

 them. 



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7 



Standing. — Certain muscles of the body attached to 

 bones are used in standing. These are muscles of the legs, 

 trunk, and neck. Certain joints must be kept rigid by 

 the pull of flexor and extensor muscles attached to them; 

 cooperation of muscles with the central nervous system, 

 especially the balancing organ in the ear, must also be had. 



Walking and Running. — Walking is more difficult 

 than standing. Two sets of muscles are always used, one 

 set being extended while the other set is contracted. 

 Walking may be considered as a series of falls in a forward 

 direction, equilibrium being maintained by putting for- 

 ward one foot; the swinging motion of the arms and 

 swaying of the body aids in keeping this equilibrium. Run- 

 ning differs from walking in the fact that, when walking, 

 one foot is always on the ground, while in running, a brief 

 period exists when both feet are off the ground. 



7n 



Diagram of some 

 of the muscles 

 which tend to 

 keep the body 

 erect. 



Work done by Muscles. — The heart is an excel- 

 lent example of an organ which does muscular work. 

 The constant pulsation of the heart during one day 

 releases energy sufficient to raise one pound 434,112 

 feet from the earth's surface. During this period the heart is 

 estimated to rest about one third of the time between the pulsa- 

 tions. Thus, it is evident that muscles may liberate a vast 

 amount of energy; in order to do this, food material, which is 

 brought by the blood to the muscles, must be oxidized. As we 



