298 FASCIiE AND MUSCLES OF THE HORSE 



serratus dorsalis, longissimi, the ribs and external intercostal muscles, and branches 

 of the deep cervical and dorsal arteries. 



Blood-supply. — Deep cervical, dorsal, vertebral, and intercostal arteries. 



Nerve-supply. — Fifth to eighth cervical nerves. 



n. THE MUSCLES OF THE SHOULDER 



The muscles of this group (Mm. omi) arise on the scapula and end on the arm; 

 they may be divided into two groups — one covering the lateral, the other the costal, 

 surface of the scapula. 



The superficial fascia of the shoulder and arm contains the cutaneous muscle 

 of this region {vide p. 254), and may be considered to be continued on the medial 

 side of the limb by the subscapular fascia. 



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

 tendinous, and is intimately adherent to the muscles on the lateral 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 underlying muscles, for which it forms sheaths; it is attached 

 to the proximal and deltoid tuberosities of the humerus. A specially strong part 

 extends from the deltoid tuberosity to the lateral surface of the elbow; it furnishes 

 insertion to part of the brachiocephalicus and gives origin to fibers of the lateral 

 head of the triceps and of the extensor carpi radialis. The fascia blends dis'tally 

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

 fascia. 



A. Lateral Group (Figs. 267, 268) 



1. Deltoideus. — This lies partly on the triceps in the angle between the scap- 

 ula 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 which covers the infraspinatus; the posterior part 

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

 triceps. The belly of the muscle lies for the most part in a cavity formed in the 

 triceps. It is widest about its middle. 



Relations. — Superficially, the skin, fascia, cutaneus, and brachiocephalicus; 

 deeply, the infraspinatus, teres minor, triceps, and brachialis muscles, and branches 

 of the posterior circumflex artery and axillary nerve. 



Blood-supply. — Subscapular artery (chiefly through the posterior circumflex). 



Nerve-supply. — Axillary nerve. 



2. Supraspinatus. — This muscle occupies the supraspinous fossa, which it 

 fills, and beyond which it extends, thus coming in contact with the subscapularis. 



Origin. — The supraspinous fossa, the spine, and the lower part of the car- 

 tilage of the scapula. 



Insertion. — The anterior parts of the proximal tuberosities of the humerus. 



Action. — To extend the shoulder joint. It also assists in preventing dislo- 

 cation. 



Structure. — ;The surface of the muscle is covered by a strong aponeurosis 

 from the deep face of which many fibers arise. The muscle is thin at its origin 

 from the cartilage, but becomes considerably thicker distally. At the neck of the 

 scapula it divides into two branches, between which the tendon of origin of the 



