442 THE MUSCLES AND FASCIA 



The External Intercostals are eleven in number on each side. They extend 

 from the tubercles of the ribs, behind, to the commencement of the cartilages of 

 the ribs, in front, where they terminate in a thin membranous aponeurosis, which 

 is continued forward to the sternum. They arise from the lower border of each 

 rib, and are inserted into the upper border of the rib below. In the two lowest 

 spaces they extend to the end of the cartilages. 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. 



Relations. By their outer surface, with the muscles which immediately invest 

 the chest viz. the Pectoralis major and minor, Serratus magnus, and Rhomboideus 

 major, Serratus posticus superior and inferior, Scalenus posticus, Ilio-costalis, 

 Longissimus dorsi, Cervicalis ascendens, Transversalis colli, Levatores costarum, 

 and the Obliquus externus abdominis ; by their internal surface, with the middle 

 intercostal fascia, which separates them from the intercostal vessels and nerve, and 

 the Internal intercostal muscles, and, behind, from the pleura. 



The Internal intercostals are also eleven in number on each side. They 

 commence anteriorly at the stei'num, in the interspaces between the cartilages 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. They arise from the ridge on the 

 inner surface of each rib, 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. 



Relations. By their external surface, with the intercostal vessels and nerves, 

 and the middle intercostal fascia, which separates them from the External inter- 

 costal muscles ; by their internal surface, with the internal intercostal fascia, 

 which separates them from the pleura costalis, Triangularis sterni, and Diaphragm. 



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

 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 is most usually oblique, like the Internal intercostals. 

 They are most frequent between the lower ribs. 



The Triangularis sterni is a thin plane of muscular and tendinous fibres, 

 situated upon the inner wall of the front of the chest. It arises from the lower 

 part of the side of the sternum, from the inner 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 fleshy digitations 

 into the lower border and inner surfaces of the costal cartilages of the second, 

 third, fourth, and fifth 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, whilst 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. 



Relations. In front, with the sternum, ensiform cartilage, costal cartilages, 

 Internal intercostal muscles, and internal mammary vessels ; behind, with the 

 pleura, pericardium, and anterior mediastinum. 



The Levatores Costarum (Fig. 288), twelve in number on each side, are small 

 tendinous and fleshy bundles, which arise from the extremities of the transverse 

 processes of the seventh cervical and eleven upper dorsal vertebrae, and, passing 

 obliquely downward and outward, are inserted into the upper border of the rib 

 below them, between the tubercle and the angle. That for the first rib arises 

 from the transverse process of the last cervical vertebra, and that for the last from 

 the eleventh dorsal. The Inferior levatores divide into two fasciculi, one of which 

 is inserted as above described ; the other fasciculus passes down to the second rib 



