THE MUSCULAR SYSTEM 



All movements of an animal are the results of muscular activ- 

 ity. The skeletal muscles constituting a great portion of the body 

 mass are, in most cases, attached to some part of the skeleton. 

 These are the muscles to which reference is made when the term 

 "muscular system" is used. The skeletal muscles, because of a 

 pecuHar banded appearance of their component fibers, are also 

 termed striated muscles. There is another type of muscle asso- 

 ciated with internal organs such as the walls of blood vessels 

 and the intestine. Since these muscles are so arranged that they 

 extend in a circular fashion about cavities they are known as 

 hollow muscles. Their fibers are not crossed by bandlike stria- 

 tions, and thus they present a smooth appearance. They are 

 therefore termed non-striated or smooth muscles. The skeletal 

 muscles for the most part can be controlled by the organism and 

 are therefore often termed voluntary muscles in contrast to the 

 hollow or involuntary muscles which are not so controlled. 



The middle portion of a muscle which shortens and increases 

 in diameter during contraction is called the belly. From this 

 part tendons extend to the bones for attachment. The two ends 

 of the muscles are attached to different bones. Otherwise the 

 contraction would come to naught as far as moving parts of 

 the body are concerned. The bones serve as levers and fulcra, the 

 muscles as the power. A thin, transparent, glistening membrane, 

 fascia, adheres closely to the muscle. The less movable or fixed 

 attachment of a muscle is called its origin; the movable end, the 

 insertion. 



The primary reason for the study of a muscle is that of deter- 

 mining its action, its name and attachments serving as a means to 

 this end. After the attachments are known it is not ordinarily 

 a difficult matter to determine the action. 



Muscles exhibit force or perform work during contraction only. 

 When a body part has been moved in one direction by a given 

 muscle or group of muscles acting in unison there will be found 

 another muscle or group of muscles for the movement of the body 

 part in the opposite direction. This is called the antagonistic 



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