1 68 ATLAS AND TEXT-BOOK OF HUMAN ANATOMY. 



The Second Layer. The Pectoralis Minor and the Subclavius. 



The pectoralis minor (Fig. 245) is a flat triangular muscle which is completely concealed 

 by the pectoralis major and, at its insertion, also by the deltoid. It arises by thin tendinous slips, 

 frequently indistinctly separated, from the costochondral articulations of the second or third to 

 the fifth ribs; it passes upward and outward and becomes markedly narrower toward its short 

 tendinous insertion into the tip of the coracoid process. 



It covers the upper portion of the serratus anterior and bridges over the axillary vessels and 

 the brachial plexus. 



The pectoralis minor, like the major, is supplied by the anterior thoracic nerves. It draws down the scapula, or, 

 if the scapula be fixed, elevates the ribs, and it can also aid in fixing the scapula. 



The subclavius (Figs. 245 and 269) is a small, elongated, somewhat flattened muscle which 

 arises by a tendon from the first costal cartilage alongside of the costoclavicular ligament, and is 

 inserted into the under surface of the acromial end of the clavicle between the two portions of 

 the coracoclavicular ligament (see page 120). In this situation there is usually a shallow groove 

 in the bone. 



The nerve supplying the muscle is the subclavian from the brachial plexus. 



The muscle, by its contraction, fixes the clavicle in the sternoclavicular joint, and when the shoulder girdle is fixed 

 the muscle elevates the first rib. 



The Third Layer. The Serratus Anterior. 



The serratus anterior (serratus magniis) (Figs. 247, 250) is covered in its upper portion 

 by both pectoral muscles; its lower portion is situated in the lateral pectoral region and, immedi- 

 ately below the pectoralis major, is covered only by the integument and fascia, the most inferior 

 portion of the muscle, however, being placed beneath the anterior margin of the latissimus. 



The muscle is flat throughout, irregularly quadrilateral in shape, and its middle portion is 

 very thin; it forms a muscular plate which is adapted to the curved surface of the thorax. It 

 arises from the first to the ninth ribs by means of individual serrations, the lower five of which 

 are distinctly separated and interdigitate with the serrations of the origin of the obliquus exter- 

 nus abdominis. In the broad muscle sheet formed by the union of the serrations, three por- 

 tions, distinctly differentiated by the direction of their fibers, may be recognized. The fibers 

 of the upper and lower portions converge toward the insertion of the muscle, while those of the 

 middle segment pass in the same direction but in a diverging manner. The upper converging 

 portion (Fig. 258) arises as a rather strong muscular mass from the first and second ribs and 

 from an intervening tendinous arch; it inserts into the superior angle of the scapula; the middle 

 diverging portion is by far the thinnest and weakest part of the muscle and it arises from the 

 second * and third ribs and diverges markedly to be inserted into the entire length of the vertebral 

 border of the scapula; and the lower converging portion, which is the strongest part of the entire 

 muscle, arises from the fifth to the ninth ribs and passes to the inferior angle of the scapula. The 

 fibers of the lower portion are the longest and those of the upper portion are the shortest. The 



* The second rib consequently gives origin to two serrations. 



