THE MUSCLES. 



-B 



marked by parallel wavy lines, about ten thousand to an 

 inch, 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, opposite to it, another 

 muscle, or group of muscles, which 

 brings the limb back to its place. When 

 they act alternately, a to-and-fro move- 

 ment results. When a joint is bent, the 

 motion is called flexion; and when it is 

 made straight again, it is called exten- 

 sion. When both sets act equally, and 

 at the same moment, no motion is pro- 

 duced, 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 pow- 

 erful than those opposite to them. 



4. The muscles are also distinguished, 

 on the other hand, 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 involuntary 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 contin- 

 ues to beat without cessation. The voluntary muscles, 

 on the other hand, are such as are used only when we wisn 

 or will to use them as the muscles of the hand or arm 

 (Pig. 9). 



FIG. 9. A, Biceps mus- 

 cle of the arm: B, C, 

 Its tendons. 



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

 tion of the muscles when we stand erect ? 



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

 example. The hand ? Arm ? 



