418 THE MUSCLES AND FASCIA 



Intercostal Fascia. A thin but firm layer of fascia covers the outer surface of 

 the External intercostal and the inner surface of the Internal intercostal muscles; 

 and a third layer, more delicate, is interposed between the two planes of muscle 

 fibres. These are the intercostal fasciae, external, middle, and internal; they are best 

 marked in those situations where the muscle fibres are deficient, as between the 

 External intercostal muscles and sternum, in front, and between the Internal 

 intercostals and vertebral column, behind. 



The Intercostal muscles (Figs. 321 and 349) are two thin planes of muscle 

 and tendon fibres, placed one over the other. They fill up the intercostal spaces, 

 and are directed obliquely between the margins of the adjacent ribs. They 

 have received the name external and internal from the position they bear to each 

 other. Between them in the grooved under surface of the rib run the intercostal 

 vessels and nerve. The tendon fibres are longer and more numerous than the 

 muscle fibres; hence the walls of the intercostal spaces possess very considerable 

 strength, to which the crossing of the muscle fibres materially contributes. 



The External intercostals (mm. intercostales eccterni) are eleven in number on 

 each side. They extend from the tubercles of the ribs, behind, to the commence- 

 ment of the cartilages of the ribs, in front, where they terminate in a thin mem- 

 brane, the anterior intercostal membrane, which is continued forward to the sternum. 

 They arise from the lower border of the rib above, and are inserted into the upper 

 border of the rib below. In the two lowest spaces they extend to the ends of the 

 cartilages, and in the upper two or three spaces they do not quite extend to the 

 ends of the ribs. Their fibres are directed obliquely downward and forward, in 

 a similar direction with those of the External oblique muscle of the abdomen. 

 They are thicker than the Internal intercostals. 



The Internal intercostals (mm. intercostales interni) are also eleven in number 

 on each side. They commence anteriorly at the sternum in the interspaces between 

 the cartilage of the true ribs, and from the anterior extremities of the 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 a thin aponeurosis, the posterior intercostal 

 membrane. They arise from the ridge on the inner surface of the rib above, as well 

 as from the corresponding costal cartilage, and are inserted into the upper border 

 of the rib below. Their fibres are directed obliquely downward and backward, 

 passing in the opposite direction to the fibres of the External intercostal muscle. 



The Infracostales (mm. subcostales) consist of muscular and aponeurotic fas- 

 ciculi, which vary in number and length; they are placed on the inner surface of 

 the ribs, where the Internal intercostal muscles cease; they arise from the inner 

 surface of one rib, and are inserted into the inner surface of the first, second, or 

 third rib below. Their direction, like the Internal intercostals, is usually oblique. 

 They are most frequently found between the lower ribs. 



The Triangularis sterni (m. transversus thoracis) (Fig. 314) is a thin plane con- 

 sisting of muscle and tendon fibres, and is situated upon the inner wall of the front 

 of the thorax. It arises from the lower third of the posterior surface of the sternum, 

 from the posterior surface of the ensiform cartilage, and from the sternal ends of 

 the costal cartilages of the three or four lower true ribs. Its fibres diverge upward 

 and outward, to be inserted by digitations into the lower borders and inner surfaces 

 of the costal cartilages of the second, third, fourth, fifth, and sixth ribs. The 

 lowest fibres of this muscle are horizontal in their direction, and are continuous 

 with those of the Transversalis; those which succeed are oblique, while the 

 superior fibres are almost vertical. This muscle varies much in its attachment, 

 not only in different bodies, but on opposite sides of the same body. The internal 

 mammary artery runs between this muscle and the costal cartilages. 



The Levatores costarum (mm. levatores costarum) (Fig. 313), twelve in number 

 on each side, are small tendinous and fleshy bundles which arise from the extrem- 



