CHAPTER XV 



SPECIAL USES OF MUSCLES 



145. Review. In the last two chapters we have 

 learned that the bones are so joined together as to 

 permit freedom of movement in various ways ; that 

 there are ball-and-socket, pivot, hinge, saddle, and glid- 

 ing joints, all of which allow more or less movement of 

 the bones ; and that most of the voluntary muscles are 

 fastened to bones. These muscles are called skeletal 

 muscles. The contraction of a skeletal muscle always 

 causes some bone to move. It is the action of the 

 muscles upon the bones that gives rise to motion of any 

 part of the body and to movement of the body as a 

 whole. 



Standing, walking, running, swimming, gesturing, 

 speaking, whispering, and singing are each accomplished 

 by the action of different groups of muscles stimulated 

 in a definite order. In this chapter we shall learn 

 something about these special functions of the muscles. 



146. Bones as Levers. When a strong stick or a 

 crowbar is used to move a heavy load, as in prying a 

 stone out of the ground, it becomes a lever. There are 

 three classes of simple levers in common use. They are 

 known as levers of the first, second, and third class. In 



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