34 THE MUSCLES 



which, give the fibre its name of the striped muscular fibre. 

 All of the voluntary muscles present this appearance. 



3^ Flexion and Extension. The muscles are, for the most 

 part, so arranged in pairs, or corresponding sets, that when 

 motion is produced in one direction by one set, there is, oppo- 

 site to it, another muscle, or group of muscles, which brings 

 the limb back to its place. When they act alternately, a to- 

 and-fro movement results. When a joint is bent, the motion 

 is called flexion; and when it is made straight again, it is 

 called extension. When both sets act equally, and at the same 



FIG. 13. KAISING THE FOREARM. A, Biceps muscle 



moment, no motion is produced, but the body or limb is main- 

 tained in a fixed position: this occurs when we stand erect. 

 The muscles which produce extension are more powerful than 

 those opposite to them. 



4. The muscles are also distinguished as the voluntary and 

 involuntary muscles, according as they are, or are not, under 

 the control of the will. The heart is an example of the invol- 

 untary variety. We cannot change its action in the least by 

 an effort of the will. When we sleep, and the will ceases to 

 act, the heart continues to beat without cessation. The volun- 

 tary muscles, on the other hand, are such as are used only 

 when we wish or will to use them as the muscles of the hand 

 or arm (Figs. 13 and 14). (Bead Note 1.) 



3. Arrangement of the muscles ? Their action ? Flexion and extension ? Action of 

 the muscles when we stand erect ? 



4. Kinds of muscles ? The voluntary ? Involuntary? The heart? Give the example. 

 The hand? Arm? 



