THE MUSCLES OF THE THORAX 403 



Intercostalis interims; and a third, more delicate layer, is interposed between the 

 two planes of muscular fibers. They are best marked in those situations where 

 the muscular fibers are deficient, as between the Iirtercostales externi and sternum 

 in front, and between the Intercostales interni and vertebral column behind. 



The Intercostales (Intercostal muscles] (Fig. 411) are two thin planes of muscular 

 and tendinous fibers occupying each of the intercostal spaces. They are named 

 external and internal from their surface relations the external being superficial 

 to the internal. 



The Intercostales externi (External intercostals) are eleven in number on either 

 side. They extend from the tubercles of the ribs behind, to the cartilages of the 

 ribs in front, \vhere they end in thin membranes, the anterior intercostal membranes, 

 which are continued forward to the sternum. Each arises from the lower border 

 of a rib, and is inserted into the upper border of the rib below. In the two lower 

 spaces they extend to the ends of the cartilages, and in the upper two or three 

 spaces they do not quite reach the ends of the ribs. They are thicker than the 

 Intercostales interni, and their fibers are directed obliquely downward and lateral- 

 ward on the back of the thorax, and downward, forward, and medialward on the front. 



Variations. Continuation with the Obliquus externus or Serratus anterior: A Supracostalis 

 muscle, from the anterior end of the first rib down to the second, third or fourth ribs occasionally 

 occurs. 



The Intercostales interni (Internal intercostals} are also eleven in number on 

 either side. They commence anteriorly at the sternum, in the interspaces between 

 the cartilages of the true ribs, and at the anterior extremities of th6 cartilages of 

 the false ribs, and extend backward as far as the angles of the ribs, whence they 

 are continued to the vertebral column by thin aponeuroses, the posterior intercostal 

 membranes. Each arises from the ridge on the inner surface of a rib, as well as 

 from the corresponding costal cartilage, and is inserted into the upper border 

 of the rib below. Their fibers are also directed obliquely, but pass in a direction 

 opposite to those of the Intercostales externi. 



The Subcostales (Infracostales) consist of muscular and aponeurotic fasciculi, 

 which are usually well-developed only in the lower part of the thorax; each arises 

 from the inner surface of one rib near its angle, and is inserted into the inner 

 surface of the second or third rib below. Their fibers run in the same direction 

 as those of the Intercostales interni. 



The Transversus thoracis (Triangularis sterni) is a thin plane of muscular and 

 tendinous fibers, situated upon the inner surface of the front wall of the chest 

 (Fig. 390). It arises on either side from the lower third of the posterior surface 

 of the body of the sternum, from the posterior surface of the xiphoid process, 

 and from the sternal ends of the costal cartilages of the lower three or four true 

 ribs. Its fibers diverge upward and lateralward, to be inserted by slips into the 

 lower borders and inner surfaces of the costal cartilages of the second, third, fourth, 

 fifth, and sixth ribs. The lowest fibers of this muscle are horizontal in their direc- 

 tion, and are continuous with those of the Transversus abdominis; the intermediate 

 fibers are oblique, while the highest are almost vertical. This muscle varies in its 

 attachments, not only in different subjects, but on opposite sides of the same 

 subject. 



The Levatores costarum (Fig. 389), twelve in number on either side, are small 

 tendinous and fleshy bundles, which arise from the ends of the transverse pro- 

 cesses of the seventh cervical and upper eleven thoracic vertebrae; they pass 

 obliquely downward and lateralward, like the fibers of the Intercostales externi, 

 and each is inserted into the outer surface of the rib immediately below the 

 vertebra from which it takes origin, between the tubercle and the angle (Levatores 

 costarum breves). Each of the four lower muscles divides into two fasciculi, one 



