174 THE UPPER EXTREMITIES. 



toid and trapezius muscles. It forms the sunlmit 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 cqracoid 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 vertebrae. 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 mus- 

 cle is the deltoid, a large triangular muscle, which sur- 

 rounds 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 in- 

 serted by a flat tendon into the edge of the bicipital 

 groove on the humerus. It draws the arm forward and in- 

 ward 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 aponeuro- 

 sis 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 mov- 

 able 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 



