The joint formed by the upper end of the HUMERUS and the shoulder-blade or SCAPULA. The acromion process of the scapula and the outer end of the collar-bone (see CLAVICLE) form a protective bony arch above the joint, and from this arch the wide and thick deltoid muscle passes downwards, protecting the outer surface of the joint and giving to the shoulder its rounded character. The joint itself is of the ball-and-socket variety, the rounded head of the humerus being received into the hollow glenoid cavity of the scapula, which is further deepened by a rim of cartilage. One tendon of the biceps muscle passes through the joint, grooving the humerus deeply, and being attached to the upper edge of the glenoid cavity. The joint is surrounded by a loose fibrous capsule, strengthened at certain places by ligamentous bands. The main strength of the joint comes from the powerful muscles that unite the upper arm with the scapula, clavicle and ribs.
or scapula. A flat bone, about as large as the flat hand and fingers, placed on the upper and back part of the thorax. Many of the large muscles that move the arm are attached to it. It is not in contact with the ribs, and its only attachment to the trunk of the body is through a joint between its acromion process and the clavicle on the tip of the shoulder and by the powerful muscles which suspend it from the backbone and ribs. With the arm hanging by the side, the scapula extends from the second to the seventh rib, but, as the arm is raised and lowered, it slides freely over the back of the chest. On the rear surface of the bone is a strong process, the spine of the scapula. This arches upwards and forwards into the acromion process. The latter forms the bony prominence on the top of the shoulder, where it unites in a joint with the outer end of the clavicle.
Posterior view of musculoskeletal structure of left shoulder.