_Insertion._—(1) The outer tuberosity of the humerus, distal to the outer insertion of the supraspinatus; (2) the posterior eminence of the outer tuberosity. _Action._—To abduct the arm and rotate it outward.[57] It also plays the part of a lateral ligament. _Structure._—This muscle is also covered by a strong aponeurosis, from which many fibers arise, and by means of which the deltoid is attached to the spine of the scapula. A thick tendinous layer partially divides the muscle into two strata, and, coming to the surface at the shoulder joint, constitutes the chief means of insertion. This tendon, an inch or more (3 cm.) in width, passes over the posterior eminence of the external tuberosity of the humerus; it is bound down by a fibrous sheet, and a =synovial bursa= is interposed between the tendon and the bone. When the long insertion is cut and reflected, the short insertion, partly tendinous, partly fleshy, is exposed. _Relations._—Superficially, the skin, fascia, panniculus, trapezius, and deltoid; deeply, the scapula and its cartilage, the shoulder joint and capsule, the long head of the triceps, the teres minor, and the nutrient artery of the scapula. _Blood-supply._—Subscapular artery. _Nerve-supply._—Suprascapular and axillary nerves. 4. =Teres minor.=—This is a much smaller muscle than the foregoing. It lies chiefly on the triceps, under cover of the deltoid and infraspinatus. _Origin._—(1) The rough lines on the distal and posterior part of the infraspinous fossa; (2) a small part of the posterior border of the scapula, about its middle; (3) a tubercle near the rim of the glenoid artery. _Insertion._—The deltoid tuberosity and a small area just above it. _Action._—To flex the shoulder joint and to abduct the arm; also to assist in outward rotation. _Structure._—The muscle is not rounded in the horse and ox, but flat and triangular. Its origin from the posterior border of the scapula is by means of an aponeurosis which also gives origin to fibers of the infraspinatus and triceps. A =bursa= is commonly found between the terminal part of the muscle and the capsule of the shoulder joint, and is often continuous with that of the infraspinatus. _Relations._—Superficially, the deltoid and infraspinatus muscles; deeply, the scapula, the shoulder joint, and the triceps muscle. _Blood-supply._—Subscapular artery (circumflex branches). _Nerve-supply._—Axillary nerve. B. INTERNAL GROUP 1. =Subscapularis.=—This muscle occupies the subscapular fossa, beyond which, however, it extends both before and behind. _Origin._—The subscapular fossa. _Insertion._—The posterior eminence of the internal tuberosity of the humerus. _Action._—To adduct the humerus. [Illustration: FIG. 187.—MUSCLES OF THORACIC LIMB OF HORSE, INTERNAL VIEW.