OF THE THORAX. 367 



vessels and nerve, and the Internal Intercostal muscles, and, behind, from the 

 pleura. 



The Internal Intercostals, also eleven in number on each side, are placed on 

 the inner surface of the preceding, commencing anteriorly at the sternum, in the 

 interspaces between the cartilages of the true ribs, and from the anterior extre- 

 mities of the cartilages of the false ribs ; and extend backwards as far as the 

 angles of the ribs, where they are continued to the vertebral column by a thin 

 aponeurosis. They arise from the inner lip of the groove on the lower border 

 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 

 downwards and backwards, decussating with the fibres of the preceding. 



Relations. By their external surface, with the External Intercostals, and the 

 intercostal vessels and nerves. By their internal surface, with the pleura costalis, 

 Triangularis Sterni, and Diaphragm. 



The Intercostal muscles consist of muscular and tendinous fibres, the latter 

 being longer and more numerous than the former ; hence the walls of the inter- 

 costal spaces possess very considerable strength, to which the crossing of the 

 muscular fibres materially contributes. 



The Infracostales consist of muscular and aponeurotic fasciculi, which vary in 

 number and length ; 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 upwards and outwards, to be inserted by fleshy digita- 

 tions 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 hori- 

 zontal 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, In- 

 ternal Intercostal muscles, and internal mammary vessels. Behind, with the 

 pleura, pericardium, and anterior mediastinum. 



The Levatores Costarum (Fig. 228), twelve in number on each side, are small 

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

 processes of the dorsal vertebrae, and passing obliquely downwards and out- 

 wards, are inserted into the upper rough surface 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 in- 

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

 below its origin ; thus, each of the lower ribs receives fibres from the transverse 

 processes of two vertebra. 



Nerves. The muscles of this group are supplied by the intercostal nerves. 



Actions. The Intercostals are the chief agents in the movement of the ribs 

 in ordinary respiration. The External Intercostals raise the ribs, especially 

 their fore part, and so increase the capacity of the chest from before backwards; 

 at the same time they evert their lower borders, and so enlarge the thoracic 

 cavity transversely. The Internal Intercostals, at the side of the thorax, de- 

 press the ribs, and invert their lower borders, and so diminish the thoracic cavity ; 

 but at the fore part of the chest these muscles assist the External Intercostals 

 in raising the cartilages. The Levatores Costarum assist the External Inter- 



