MUSCLES AND PHENOMENA OF CONTRACTION. 113 



propel the re-productive elements. The real organs of 

 movement are, however, the muscles. A careful examina- 

 tion of the entire muscular system of the body shows at 

 once three tolerably distinct kinds of muscles. First, the 

 voluntary or skeletal muscles ; or, on account of their mi- 

 croscopic appearance to be described later, cross-striated 

 muscles. Second, the involuntary, visceral or plain muscles. 

 Third, the muscles which form the heart, and which differ 

 materially from both of the preceding, and occupy from 

 an anatomical point of view an intermediate position. 



Voluntary Muscles. 



As the name indicates, these muscles are under the con- 

 trol of the will. This is, however, not absolutely true, as 

 there are a number of muscles over which the control of the 

 will has been lost. Thus, there are some muscles of the ear 

 which, if they could be used, would serve to move that 

 organ, but the power to move them has been entirely lost. 

 That nearly all of these muscles are attached to bones has 

 given them the name of skeletal muscles. But there are 

 exceptions to this as well. Thus the circular mucles enclos- 

 ing the mouth or the eyes, are not attached at all to any 

 bone, and yet are voluntary in the highest degree. The 

 real classification is based upon their minute structure, and 

 in this all agree in showing under the high power of the 

 microscope peculiar cross striations to be discussed later. 



TYPES OF VOLUNTARY MUSCLES. 



A typical muscle is composed of a thick, central portion 

 called the belly, which tapers at each end into a thick 

 string of white fibrous tissue familiar as the tendon. From 

 this type, however, there are many deviations. Thus the 

 muscle which serves to bend the arm forward, ends at the 

 shoulder in two tendons instead of one. This has given 

 the name of biceps to this muscle. Immediately under the 



