UPPER EXTREMITY AND THORAX, POSTERIOR. 369 



Nerve Supply. The lowest or third subscapular, bringing 

 filaments from the sixth and seventh cervical nerves. 



Action, To rotate the arm inward and adduct it, also to 

 draw the arm backward when it has been carried forward. 



The Triceps. Fig. 77. 



Origin. The long or middle head rises from the axillary 

 border of the scapula just below the glenoid cavity (and 

 from the lower portion of the capsule of the shoulder-joint). 



The external head comes from the posterior surface of 

 the humerus above, and external to the musculospiral 

 groove (reaching up as high as the insertion of the teres 

 minor). 



The internal head from the surface of the humerus below 

 and internal to the same groove (reaching up to the inser- 

 tion of the teres major). 



The external and internal heads also arise from the pos- 

 terior surface of the external and internal intermuscular 

 septa opposite their bony origins. 



Insertion. Into the tip of the olecranon process of the 

 ulna, and into the deep fascia of the forearm. 



Nerve Supply. The musculospiral. The source of the 

 filaments is from the seventh and eighth cervical nerves. 



Action. Primarily, upon the forearm to extend it ; 

 secondarily, upon the arm to adduct and support it. 



The Subanconeus. This is not a separate muscle, 

 merely some fibres of the triceps which are inserted into 

 the posterior part of the ligament of the elbow-joint. Its 

 function is to raise the ligament in extension of the joint 

 and keep it out of the way of the bones. 



DISSECTION. 



Find the suprascapular artery and nerve, trace them over the upper border 

 of the scapula. Divide the supraspinatus muscle and follow the artery and 

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