A. External Group (Figs. , )

1. Deltoid (M. deltoideus; long abductor of the arm; scapular portion of the deltoid of man).—This lies partly on the triceps in the angle between the scapula 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 infraspinatus.

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 of 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, panniculus, and mastoido-humeralis; 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.—Suprascapular and axillary nerves.

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 cartilage of the scapula.

Insertion.—The inner and outer lips of the bicipital groove.

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

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 below. At the neck of the scapula it divides into two branches, between which the tendon of origin of the biceps emerges. These branches, fleshy superficially, tendinous deeply, are inserted into the lips of the bicipital groove. They are united by a fibrous membrane already mentioned in connection with the deep pectoral muscle; some fibers are attached to this membrane and the capsule of the shoulder joint.

Relations.—Superficially, the skin, fascia, panniculus, trapezius, and mastoido-humeralis; deeply, the scapula and its cartilage, the subscapularis muscle, and the suprascapular vessels and nerve; in front, the anterior deep pectoral muscle; behind, the spine of the scapula and infraspinatus muscle.

Blood-supply.—Suprascapular and posterior circumflex arteries.

Nerve-supply.—Suprascapular nerve.

3. Infraspinatus.—This muscle occupies the greater part of the infraspinous fossa.

Origin.—The infraspinous fossa and the scapular cartilage.

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. 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.