302 FASCL£ 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. 
Ill. 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 lies on the anterior 
surface of the humerus (Fig. 277). 
Origin.—The tuber scapule. 
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).2. 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. 
1 Also known as the coraco-radialis or flexor brachii. 
? In some cases the bursa communicates with the cavity of the shoulder joint. 
’ Also known as the humeralis obliquus s. externus and as the brachialis anticus. 
