88 THE PRINCIPLES OF MYODYNAMICS. 



(3.) The pectoralis major, for instance, makes 

 the humerus a lever of the third order. 



80. The motions of the arm may be arranged 

 under four heads : 



(i.) The arm can move inward and outward 

 about 90 degrees — adduction and abduction. 



(2.) The arifi can move backward and forward 

 about 135 A^^x^^^—extensio7i and flexion. 



(3.) The arm can rotate inward and outward 

 about 90 degrees — i7i-rotation and oitt-i^otation. 



(4.) The arm can move around to the right 

 or to the left 360 degrees — cii'cumditction. 



81. The muscles that move the arm may be 

 grouped according to the direction of these 

 motions : 



(i.) The addzcctoj^s of the arm are the pector- 

 alis major, the teres major, the teres minor, the 

 subscapularis, the infraspinatus, and the latissi- 

 mus dorsi. 



(2.) The flexors of the arm are the biceps 

 brachii, the coraco-brachialis, the pectoralis ma- 

 jor, and the anterior part of the deltoid. 



(3.) The abductors of the arm are the deltoid 

 and the supraspinatus. 



