Fig. 69.—Left scapula, posterior surface
(after Toldt).
The [scapula] or shoulder blade, so called from its shape, is a large, flat, triangular bone with a prominent ridge, the spine, crossing its dorsum or posterior surface near its upper edge. It extends from the second to the seventh rib, with its posterior margin parallel to and about one inch from the dorsal vertebræ. The head, in which is situated the glenoid cavity for articulation with the humerus or upper arm bone, is surrounded by a slight constriction, the neck. Above it projects the [coracoid process], so called from its fancied resemblance to a crow’s beak. This can usually be felt about one inch from the juncture of the outer and middle thirds of the clavicle and from it arise the short head of the biceps and the coraco-brachialis muscle. The [acromion process] at the end of the spine extends out beyond the glenoid cavity posteriorly and affords attachment to the deltoid and trapezius muscles. It forms the summit of the shoulder. Numerous other muscles are attached to the surface of the scapula, the only parts which are truly subcutaneous being the whole length of the spine and the acromion process, though the lower angle and the coracoid process can generally be felt. The muscles bulge so much that the spine in the living appears as a slight depression extending back almost to the vertebræ. The large number of the muscles on the shoulder and arm is due to the great flexibility and strength required for the various uses to which the arms are put.
Shoulder Muscles.—The most important shoulder muscle is the [deltoid], a large triangular muscle, which surrounds and protects the shoulder-joint and gives the shoulder its rounded form. It rises from the outer third of the [clavicle], from the acromion process, and from the whole length of the spine of the scapula, and is inserted by a tendon into a rough prominence on the middle of the outer side of the humerus. It serves to raise the arm and to draw it somewhat forward or back, according as the anterior or posterior fibers are used. The [pectoralis major] rises from the inner half of the clavicle, the front of the sternum, and the cartilages of the true ribs and its fibers converge to form a fan-shaped muscle, which is inserted by a flat tendon into the edge of the bicipital groove on the humerus. It draws the arm forward and inward and helps considerably in forced inspiration. The [serratus magnus] rises from the outer surface and upper border of the eight upper ribs and from an aponeurosis covering the upper intercostal spaces, and is inserted along the whole length of the posterior border of the scapula. It carries the scapula forward and is used in pushing.
The scapula is seldom broken because it is quite movable and is covered with large muscles and because it lies on the chest, which serves as an elastic cushion. The acromion process is the part most frequently broken and occasionally the neck is fractured. Tumors occur and may necessitate the amputation of the whole upper extremity.
The Humerus.—The bone of the upper arm, the humerus, is the largest bone in the upper extremity and articulates with the scapula above and with the ulna and radius below. At its upper end are the head and the anatomical neck, with the greater tuberosity external to and the lesser tuberosity in front of them. The constriction of the surgical neck is below the tuberosities, and extending from between them downward and inward along the upper third of the bone is the bicipital groove for the long head of the biceps. Though round above, below the shaft becomes flattened from before backward and curves slightly forward, terminating in the internal and external condyles, from the former of which the flexors and the round pronator arise and from the latter the extensors and supinators. From the external condyle also there projects in front the radial head or capitellum for articulation with the radius. Internally to the capitellum in front and in a corresponding position on the back of the bone are the trochlear surfaces for articulation with the ulna, there being a depression in front called the coronoid fossa for the reception of the coronoid process of the ulna in flexion of the forearm, and another depression behind, the olecranon fossa, to receive the tip of the olecranon process during extension. On the lower half of the humerus at the back is the spiral groove for the musculo-spiral nerve and the superior profunda artery, while the ulnar nerve runs in a groove back of the internal condyle.
The humerus is almost completely covered with muscles, the only part that is subcutaneous being a small portion of the external and internal condyles. The head can be felt under the muscles and the greater tuberosity forms the point of the shoulder. When the arm is at the side, the biceps appears at the front and inner side and the brachialis anticus on either side below, while on the back of the arm, with its largest swelling above, is the triceps.
Fig. 70.
Fig. 70.—Superficial muscles of shoulder and arm (from before): 1, Pectoralis major; 2, deltoid; 3, biceps brachii; 4, brachialis anticus; 5, triceps; 6, pronator radii teres; 7, flexor carpi radialis; 8, palmaris longus; 9, flexor carpi ulnaris; 10, supinator longus; 11, extensor ossis metacarpi pollicis; 12, extensor brevis pollicis; 13, flexor sublimis digitorum; 14, flexor longus pollicis; 15, flexor profundus digitorum; 16, palmaris brevis; 17, abductor pollicis. (Dorland’s Dictionary.)