UPPER EXTREMITY AND THORAX, POSTERIOR. 363 



Insertion. Into the posterior portion of the upper surface 

 and border of the outer third of the clavicle ; into the upper 

 border of the spine and acromion process and the slight 

 tubercle near the base of the spine of the scapula. 



Nerve Supply. The spinal accessory and deep branches 

 of the third and fourth cervical nerves. They are distrib- 

 uted to the anterior surface of the muscle. 



Action. The spine and head being the fixed points and 

 both muscles acting, the shoulders will be drawn backward 

 and the points of the shoulders slightly elevated. The 

 muscles acting singly and in parts from the fixed spine : if 

 the upper portion acts, the clavicle and acromion will be 

 raised (shoulder elevated) ; if the middle portion of the 

 muscle acts, the scapula will be drawn toward the spine, a 

 slight rotation taking place at the same time ; if the lower 

 portion acts, the vertebral border of the spine will be drawn 

 downward (the shoulder carried downward and backward), 

 at the same time the point of the shoulder is very slightly 

 elevated. Acting from the shoulder as the fixed point, 

 both muscles will retract the head and neck and elevate the 

 chin ; one muscle acting, will carry the chin to the opposite 

 side and flex the neck laterally. 



The muscle fixing the shoulders gives the accessory 

 muscles of inspiration a better chance for acting. 



The Deltoid. Fig. 60. 



Origin. From the anterior border of the outer third of 

 the clavicle, from the outer margin of the acromion, 

 lower lip of the spine of the scapula, and the infraspinatus 

 fascia. 



Insertion. For a distance of two or three inches into 

 the outer surface of the humerus just above its middle, at 

 a rough triangular surface. 



