THE MUSCLES OF THE SHOULDER — EXTERNAL GROUP 255 



action, however, it seems desirable to distinguish the two portions. The cer- 

 vical part is thick and almost entirely fleshy. The thoracic part has on its 

 superficial face a thick, tendinous layer which may sustain the weight of the 

 trunk when the muscle substance relaxes. The ventral edge presents distinct 

 digitations, the last four of which alternate with those of the obliquus externus 

 abdominis, and are covered by the abdominal tunic. The fourth, fifth, and sixth 

 digitations extend nearly to the distal ends of the ribs. The last digitation is 

 small and may be absent. Exceptionally additional digitations may be attached 

 to the tenth or eleventh rib or to the fascia over the intercostal muscles. The 

 fibers converge to the insertion, which is thick and is intersected by elastic lam- 

 ellae derived from the dorso-scapular ligament. 



Relations. — Superficially, the mastoido-humeralis, trapezius, deep pectoral, 

 subscapularis, teres major, latissimus dorsi, panniculus carnosus, the abdominal 

 tunic, the brachial vessels, and the long thoracic nerve; deeply, the splenius, 

 complexus, longissimus, transversalis costarum, the ribs and external intercostal 

 muscles, and branches of the superior cervical and dorsal arteries. 



Blood-sup pit/. — Superior cervical, dorsal, vertebral, and intercostal arteries. 



Nerve-supply. — Brachial plexus. 



II. THE MUSCLES OF THE SHOULDER 



Under this head will be described those muscles which arise on the scapula 

 and end on the arm; they may be divided into two groups — one covering the 

 dorsum, the other the venter of the scapula. 



The superficial fascia of the shoulder and arm contains the panniculus carno- 

 sus of this region, and may be considered to be continued on the inner side of the 

 limb by the subscapular fascia. 



The thoracic or scapulo-humeral portion of the panniculus (M. cutaneus 

 scapulae et humeri) arises by a thin aponeurosis from the ligamentum nuchae at 

 the withers. The fleshy portion begins over the upper part of the scapula and 

 extends to the elbow. Its fibers have in general a dorso-ventral direction. It is 

 continuous behind with the abdominal portion. 



The deep fascia of the shoulder and arm (Fascia omobrachialis) is much more 

 developed and important. It is strong and tendinous, and is intimately adherent 

 to the muscles on the outer surface of the scapula, between which it detaches 

 intermuscular septa, which are attached to the spine and borders of the scapula. 

 The brachial portion is, for the most part, only loosely attached to the underly- 

 ing muscles, for which it forms sheaths; it is attached to the humerus, especially 

 to the lips of the bicipital groove and the deltoid tuberosity. It blends distally 

 with the tendon of insertion of the biceps, and is continued by the antibrachial 

 fascia. 



A. External Group (Figs, us, 179) 



1. Deltoid (M. deltoideus; long abductor of the arm; scapular portion of 

 the deltoid of man). — This lies partly on the triceps in the angle between the 

 scapula and humerus, partly on the infraspinatus and teres minor. 



Origin. — (1) The upper part of the posterior border of the scapula; (2) the 

 spine of the scapula, by means of the strong aponeurosis which covers the infra- 

 spinatus. 



Insertion. — The deltoid tuberosity of the humerus. 



Action. — To flex the shoulder joint and abduct the arm. 



Structure. — The origin of the muscle is partly aponeurotic, partly fleshy. 

 The aponeurosis fuses with that of the infraspinatus; the posterior part is 

 attached to the scapula immediately in front of the origin of the long head of the 



