- toid 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
