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.