Chapter II: The Muscles of the Shoulder

Under this head will be described those muscles which arise on the scapula and end on the arm; they may be divided into two groups—one covering the dorsum, the other the venter of the scapula.

The =superficial fascia= of the shoulder and arm contains the panniculus carnosus of this region, and may be considered to be continued on the inner side of the limb by the subscapular fascia.

The =thoracic= or =scapulo-humeral portion= of the =panniculus= (M. cutaneus scapulæ et humeri) arises by a thin aponeurosis from the ligamentum nuchæ at the withers. The fleshy portion begins over the upper part of the scapula and extends to the elbow. Its fibers have in general a dorso-ventral direction. It is continuous behind with the abdominal portion.

The =deep fascia= of the shoulder and arm (Fascia omobrachialis) is much more developed and important. It is strong and tendinous, and is intimately adherent to the muscles on the outer surface of the scapula, between which it detaches =intermuscular septa=, which are attached to the spine and borders of the scapula. The brachial portion is, for the most part, only loosely attached to the underlying muscles, for which it forms sheaths; it is attached to the humerus, especially to the lips of the bicipital groove and the deltoid tuberosity. It blends distally with the tendon of insertion of the biceps, and is continued by the antibrachial fascia.

A. EXTERNAL GROUP (Figs. 178, 179)

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.[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.

FIG. 187.—MUSCLES OF THORACIC LIMB OF HORSE, INTERNAL VIEW.

_3_, _4_, Rhomboideus; _5_, latissimus dorsi; _8_, posterior deep
pectoral; _9_, anterior deep pectoral; _11_, supraspinatus; _15_,
subscapularis; _16_, teres major; _20_, long head of triceps; _22a_,
_22b_, tensor fasciæ antibrachii; _23_, internal head of triceps.
(After Ellenberger, in Leisering’s Atlas.)
]

_Structure._—The muscle is flat and triangular, with the base upward. The latter is thin and interdigitates with the scapular attachments of the serratus. Below this the belly thickens and becomes narrower. It is covered by an aponeurosis, and contains a considerable amount of tendinous tissue. The tendon of insertion is crossed by the tendon of origin of the coraco-brachialis; it is intimately adherent to the capsule of the shoulder joint, and may be regarded as replacing the internal ligament of the latter. A small bursa may occur here.

_Relations._—Superficially, the scapula and shoulder joint, the supraspinatus, triceps, and teres major muscles; deeply, the serratus magnus muscle, the brachial vessels, and the chief branches of the brachial plexus. The subscapular vessels run along or near the posterior edge of the muscle.

_Blood-supply._—Subscapular artery.

_Nerve-supply._—Subscapular nerves (from the brachial plexus).

2. =Teres major= (Teres internus; adductor of the arm).—This muscle is flat, widest about its middle, and lies chiefly on the deep face of the triceps.

_Origin._—The dorsal angle and the adjacent part of the posterior border of the scapula.

_Insertion._—The tubercle on the inner surface of the shaft of the humerus, in common with the latissimus dorsi.

_Action._—To flex the shoulder joint and adduct the arm.

_Structure._—It is for the most part fleshy, but the origin consists of an aponeurosis which blends with that of the tensor fasciæ antibrachii. The insertion is by a flat tendon which fuses with that of the latissimus dorsi.

_Relations._—Superficially, the triceps, infraspinatus, and deltoid muscles; deeply, the serratus magnus muscle. The subscapular vessels lie in a groove between the anterior edge of this muscle and the posterior border of the subscapularis; near the shoulder joint the posterior circumflex artery and the axillary nerve emerge between the two muscles. The deep face of the muscle is crossed by the thoracic branches of the brachial plexus, and by the branch of the subscapular artery which supplies the latissimus dorsi.

_Blood-supply._—Subscapular artery.

_Nerve-supply._—Axillary nerve.

3. =Coraco-brachialis= (Coraco-humeralis).—This muscle lies on the inner surface of the shoulder joint and the arm.

_Origin._—The coracoid process of the scapula.

_Insertion._—(1) A small area above the internal tubercle of the humerus; (2) the middle third of the anterior surface of the humerus.

_Action._—To adduct the arm and to flex the shoulder joint.

_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.