_Structure._—The long tendon of origin emerges between the subscapularis and the inner branch of the supraspinatus. It passes over the terminal part of the subscapularis and is provided with a =synovial sheath=. The muscular part spreads out and divides into two portions. The smaller and shorter portion is inserted into the proximal third of the shaft of the humerus, close to the origin of the lateral head of the triceps; the larger and longer portion is inserted into the middle third of the humerus, in front of the internal tubercle and the inner head of the triceps. _Relations._—Externally, the subscapularis muscle and the humerus; internally, the deep pectoral and brachialis muscles. The anterior circumflex artery and the nerve to the biceps usually emerge between the two insertions, and the brachial vessels lie along the posterior border of the muscle. _Blood-supply._—Anterior circumflex artery. _Nerve-supply._—Musculo-cutaneous nerve. 4. =Capsularis= (Scapulo-humeralis posticus s. gracilis).—This is a very small muscle, which lies on the back of the capsule of the shoulder joint. _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 may perhaps tense the capsule of the shoulder joint and prevent its being pinched during flexion. _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. Its attachment to the joint capsule is slight. It passes through the brachialis muscle to reach its insertion. _Relations._—Superficially, the teres minor and triceps muscles; deeply, the teres major and subscapularis muscles, and the capsule of the joint. _Blood-supply._—Posterior circumflex artery. _Nerve-supply._—Axillary nerve. III. THE MUSCLES OF THE ARM This group consists of five muscles 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. [Illustration: FIG. 188.—BICEPS AND BRACHIALIS MUSCLES OF HORSE. (After Ellenberger-Baum, Anat. für Künstler.) ] 1. =Biceps brachii= (Coraco-radialis; flexor brachii).—This is a strong, somewhat rounded muscle, which lies on the anterior surface of the humerus. _Origin._—The tuberosity of the scapula. _Insertion._—(1) The bicipital tuberosity of the radius; (2) the internal lateral 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, and to tense the fascia of the forearm.