302 FASCIA AND MUSCLES OF THE HORSE 



Origin. — The scapula, close to the rim of the glenoid cavity. 



Insertion.— The posterior surface of the shaft of the humerus, a short distance 

 below the head. 



Action.— It has been held that it tenses the capsule of the shoulder joint and 

 prevents its being pinched during flexion, but there does not appear to be any 

 attachment of the muscle to the joint capsule. 



Structure. — It is fleshy and usually about the breadth of a finger. It may, 

 however, consist of only a few bundles of fibers; sometimes it is double. It passes 

 through the brachialis muscle to reach its insertion. 



Relations. — Laterally, the teres minor and triceps muscles; medially, the teres 

 major and subscapularis muscles, and the capsule of the joint. 



Blood-supply. — Posterior circumflex artery. 



Nerve-supply. — Axillary nerve. 



m. THE MUSCLES OF THE ARM 



This group consists of five muscles (Mm. brachii) which are grouped around the 

 humerus. They arise from the scapula and the humerus, and are inserted into the 

 forearm. They act on the elbow joint and the fascia of the forearm. 



1. Biceps brachii.' — This is a strong fusiform muscle, which hes on the anterior 

 surface of the humerus (Fig. 277). 



Origin. — The tuber scapulse. 



Insertion. — (1) The radial tuberosity; (2) the medial ligament of the elbow 

 joint; (3) the fascia of the forearm and the tendon of the extensor carpi radialis. 



Action. — To flex the elbow joint; to fix the shoulder, elbow, and carpus in 

 standing; to assist the extensor carpi radialis, and to tense the fascia of the fore- 

 arm. 



Structure. — The muscle is inclosed in a double sheath of fascia, which is attached 

 to the tuberosities and the deltoid ridge of the humerus. The tendon of origin 

 is molded on the intertuberal or bicipital groove; it is very strong and dense and 

 is partly cartilaginous. Its play over the groove is facilitated by the large inter- 

 tuberal or bicipital bursa (Bursa intertubercularis).^ The synovial membrane 

 covers not only the deep face of the tendon, but extends somewhat around the edges 

 to the superficial face. A well-marked tendinous intersection runs through the belly 

 of the muscle and divides distally into two portions. Of these, the short, thick 

 one is inserted into the radial tuberosity and detaches fibers to the medial collat- 

 eral ligament of the elbow joint. The long tendon (Lacertus fibrosus) is thinner, 

 blends with the fascia of the forearm, and ends by fusing with the tendon of the 

 extensor carpi radialis; thus the action is continued to the metacarpus. 



Relations. — Laterally, the brachiocephalicus and brachialis muscles; medially, 

 the posterior deep pectoral and the superficial pectoral muscles; in front, the an- 

 terior deep pectoral muscle; behind, the humerus, the coraco-brachialis muscle, 

 the anterior circumflex and anterior radial vessels, and the musculo-cutaneous 

 nerve. 



Blood-supply. — Branches of the brachial and anterior radial arteries. 



Nerve-supply. — Musculo-cutaneous nerve. 



2. Brachialis.^ — This muscle occupies the musculo-spiral groove of the humerus. 

 Origin. — The proximal third of the posterior surface of the humerus. 

 Insertion. — The medial surface of the neck of the radius (under cover of the 



long collateral ligament) and the transverse radio-ulnar ligament. 

 Action. — To flex the elbow joint. 



' Also known as the ooraco-radialis or flexor brachii. 



^ In some cases the bursa communicates with the cavity of the shoulder joint. 



' Also known as the humeralis obhquus s. externus and as the brachialis anticus. 



