THE USE OF MUSCLES IN THE BODY 129 



Prehension. In man, and to a less extent in monkeys, the 

 fore limbs are differentiated from organs of locomotion into pre- 

 hensile structures. To a large degree the effectiveness of the 

 fore limb as an organ of prehension depends on peculiarities of its 

 bony framework. These have been described in detail in a pre- 

 vious chapter (p. 66). The chief special features, which it may 

 not be amiss to recall, are three; the attachment of the shoulder 

 girdle to the trunk by muscles rather than by a firm bony articula- 

 tion; the rotation of the radius over the ulna; the opposibility of 

 the thumb to each of the fingers. These skeletal features, which 

 afford a groundwork for great flexibility of action, are made effect- 

 ive by the arrangement of the arm muscles. Although detailed 

 description of this arrangement is outside the scope of this work, 

 some general statements may properly be presented. 



The muscles of the arm fall into three groups, shoulder muscles, 

 muscles of the upper arm, muscles of the fore arm and hand. The 

 muscles of the shoulder are arranged so that the arm can be raised 

 or lowered; drawn forward or backward. Those at the back of 

 the shoulder include within their mass the shoulder blade (scapula] 

 in such a manner that in ordinary upward movements of the arm 

 the rotation is about the shoulder joint, but in extensive upward 

 movements, as in raising the arms above the head, the shoulder 

 blade itself is pulled out of its ordinary horizontal position into a 

 nearly vertical one. 



The muscles of the upper arm are simple flexors and extensors 

 since the elbow-joint is a hinge-joint, permitting no variety of 

 movements. The muscles of the fore arm and hand have a great 

 variety of movements to provide for, and are accordingly numerous 

 and complicated. Only the arrangements for securing flexion 

 and extension will be mentioned here. The front of the fore arm 

 is made up of a number of muscles, of which most are flexors of 

 the wrist or of the fingers. Similarly the back of the fore arm 

 contains the extensors of wrist and of fingers. The tendons of 

 the latter muscles form the conspicuous cords at the back of the 

 hand. These flexors and extensors interact in an interesting 

 fashion. Thus the flexors of the finger aid in flexion of the wrist. 

 If flexion of the fingers without accompanying flexion of the wrist 

 is desired the latter must be prevented by simultaneous contrac- 

 tion of the wrist extensors. Similarly the extensors of the fingers 



