III. THE MUSCLES OF THE SHOULDER
The deep fascia covering the Deltoid invests the muscle, and sends numerous septa between the fasciculi. In front, it is continuous with the pectoral fascia behind, where it is thick and strong, with the fascia infraspinata; above, it is attached to the clavicle, the acromion; and the crest of the spine of the scapula; below, it is continuous with the brachial fascia.
The Deltoid (fig. 619) is a thick, triangular muscle, which covers the shoulder joint. It arises from the anterior border and upper surface of the lateral one third of the clavicle; from the lateral margin and upper surface of the acromion, and from the lower lip of the crest of the spine of the scapula, as far back as the triangular surface at its medial end. The fibers converge towards their insertion; the middle passing vertically, the anterior inclining backwards, and the posterior forwards; they unite in a thick tendon which is inserted into the deltoid tuberosity on the lateral side of the shaft of the humerus. At its insertion the tendon gives off an expansion to the deep fascia, of the upper arm. This muscle is remarkably coarse in texture, and the part arising from the acromion consists of oblique fibers; these arise in a bipennate manner from the sides of tendinous septa, generally four in number, which pass downwards from the acromion into the muscle. These oblique fibers are inserted into similar tendinous septa, generally three in number, which ascend from the tendon of insertion of the muscle and alternate with the descending septa. The portions of the muscle arising from the clavicle and spine of the scapula are not arranged in this manner, but are inserted into the margins of the inferior tendon.
Relations.-Its superfcial surface is in relation with the skin, the superficial and deep fasciae, Platysma, lateral supraclavicular and upper lateral brachial. cutaneous nerves. Its deep surface covers the coracoid process, coracoacromial ligament, subacromial bursa, Pectoralis minor, Coracobracbialis, both heads of the Biceps, the tendon of the Pectoralis major, the insertions of the Subscapularis, Supraspinatus; Infraspinatus, and Teres minor, the long and lateral heads of the Triceps, the circumflex humeral vessels, the circumflex (axillary) nerve and the surgical neck and upper part of the shaft of the humerus. Its anterior border is separated at its upper part from the Pectoralis major by the infraclavicular fossa in which the cephalic vein and deltoid branch of the acromiothoracic artery lie; lower down the two muscles are in contact. Its posterior border rests on the Infraspinatus and Triceps.
Nerves.-The Deltoid muscle is supplied by the circumflex (axillary) nerve (C. 5 and 6).
Actions.-The Deltoid raises the arm from the side, so as to bring it at right angles with the scapula, while retaining it in the same plane as the body of that hone. The latter point is of importance, because it is only when the humerus lies in the same plane as the body of the scapula that rotation of the latter bone can have its full effect in raising the arm above the head. Its anterior fibers draw the arm forwards; and its posterior fibers draw it backwards.
Applied Anatomy.-The Deltoid atrophies after injury to the circumflex nerve, and in this condition dislocation of the shoulder-joint is simulated, as there is flattening of the shoulder and apparent prominence of the acromion; the distance between the acromion and the head of the bone is increased also, and the tips of the fingers can be inserted between them.
The subscapular fascia is a thin membrane attached to the entire circumference of the subscapular fossa, and giving origin by its deep surface to some of the fibers of the Subacapularis. The Subscapularis (fig. 620) is a large, triangular muscle, which fills the subscapular fossa and arises from its medial two-thirds, including the grooved area adjoining the lateral (axillary) border of the scapula. Some fibers arise from tendinous laminae which intersect the muscle and are attached to ridges on the bone; others from an aponeurosis, which separates the muscle from the Teres major and the long head of the Triceps. The fibers pass laterally; and; gradually converging, end in a tendon which is inserted into the lesser tuberosity of the humerus and the front of the capsular ligament of the shoulder-joint. The tendon of the muscle is separated from the neck of the scapula by a large bursa, which communicates with the cavity of the shoulder-joint through an aperture in the capsular ligament.
Nerves.—–The Subscapularis is supplied by the upper and lower subscapular nerves (C. 5 and 6).
Actions.-The Subscapularis rotates the head of the humerus medially; when the arm is raised, it draws the humerus forwards and downwards. It is a powerful defense to the front of the shoulder-joint.
The fascia supraspinata completes the osseofibrous case in which the Supraspinatus muscle is contained, and its deep surface gives origin to some of the fibers of the muscle. It is thick medially, but thinner laterally under the coraco-acromial ligament.
The Supraspinatus (fig. 621) occupies the supraspinous fossa, arising from its medial two-thirds, and from the fascia supraspinata. The muscular fibers pass under the acromion, and converge to a tendon which crosses the upper part of the shoulder-joint and is inserted into the highest of the three impressions on the greater tuberosity of the humerus; the tendon is intimately adherent to the capsule of the shoulder-joint.
Nerve-supply.-The Supraspinatus is supplied by the suprascapular nerve (C. 5 and 6).
Action.-The Supraspinatus abducts the arm.
The fascia infraspinata covers the Infraspinatus muscle, and is fixed to the circumference of the infraspinous fossa; its deep surface gives origin to some fibers of that muscle. It is attached to the deltoid fascia along the overlapping border of the Deltoid.
The Infraspinatus (fig. 621) is a thick triangular muscle, which occupies the chief part of the infraspinous fossa; it arises by fleshy fibers from the medial two-thirds of the fossa, and by tendinous fibers from the ridges on its surface: it also arises from the fascia infraspinata, which covers it and separates it from the Teres major and minor muscles. The fibers converge to a tendon, which glides over the lateral border of the spine of the scapula, and, passing across the posterior part of the capsule of the shoulder-joint, is inserted into the middle impression, on the greater tuberosity of the humerus. The tendon of this muscle is sometimes separated from the capsule of the shoulder-joint by a bursa which may communicate with the joint-cavity.
Nerve-supply.-The Infraspinatus is supplied by the suprascapular nerve (C. 5 and 6).
Action.-The Infraspinatus rotates the arm laterally.
The Teres minor (fig. 621) is a narrow, elongated muscle, which arises from the upper two-thirds of a flattened strip on the lateral part of the dorsal surface of the scapula, immediately adjoining the lateral (axillary) border, and from two aponeurotic laminae, of which one separates it from the Infraspinatus, and the other from the Teres major. Its fibers run obliquely upwards and laterally; the upper ones end in a tendon which is inserted into the lowest of the three impressions on the greater tuberosity of the humerus; the lower fibers are inserted directly into the humerus immediately below this impression and just above the origin of the lateral head of the Triceps. The tendon of this muscle passes across, and is united with, the lower part of the posterior aspect of the capsular ligament of the shoulder-joint.
Nerve-supply.-The Teres minor is supplied by the circumflex (axillary) nerve (C. 5).
Action.—The Teres minor rotates the arm laterally.
The Teres major (fig. 621) is a thick, somewhat flattened muscle, which a rises from the oval area on the dorsal surface of the inferior angle of the scapula, and from the fibrous septa interposed between the muscle and the Teres minor and lnfraspinatus: the fibers are directed upwards and laterally and end in a flat tendon, about 5 cm. long, which is inserted into the medial lip of the bicipital groove (intertubercular sulcus) of the humerus. At its insertion the tendon lies behind that of the Latissimus dorsi, from which it is separated by a bursa, the two tendons being, however, united along their lower borders for a short distance.
Actions.-The Teres major draws the humerus medially and backwards, and rotates it medially.
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