XVIII. MUSCLES. 

 Kinds, Stkuctuee, and Composition. 



In the skeleton we have a well-jointed framework that 

 of itself has no power of movement. In order to move 

 the different bones and produce the different contractions 

 and expansions of the organs of the body, a special tissue, 

 called the muscles, is devised. Muscles, then, are respon- 

 sible for all movement in our bodies. 



Kinds of muscle tissue. — Every boy knows that when 

 he bends his forearm at the elbow a sweUing appears on 

 the upper side of the upper arm. This swelling, which the 

 boy speaks of as his " muscle," is, in fact, a separate mass 

 of muscle tissue and known technically as the biceps 

 muscle. Its action illustrates two characteristics of 

 muscle tissue. First, the production of motion by con- 

 traction. Second, that contractions are under control, 

 for in this case the boy determines when they shall contract. 

 The muscles which form the walls of the intestines and 

 produce the grinding movements that we call peristalsis, 

 do so by contraction also, but their action differs from the 

 action of the biceps and triceps in that it is not under the 

 control of the will. We cannot stop the action as we wish. 



It was formerly customary to make this matter of con- 

 trol the basis of classification of muscles. Muscles whose 

 contractions required no act of will on our part were called 

 involuntary muscles. Those which arc subject to our 

 direct control, like the muscles of the fingers, arms, and 

 legs, were called voluntary muscles. The difficulty of this 



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