OF THE THORAX. 



351 



costal membrane and the Pectoralis major. By their internal surface, with 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 (Fig. 215) 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 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 



STERNO-MASTOID 



SUBCLAVIUS. 



SUBCLAVIUS. 



Internal mam- 

 mary artery. 



y TRIANGULARIS 

 STERNI. 



TRANSVERSALIS ABDOMINIS. 



FIG. 215. Posterior surface of sternum and costal cartilages, showing Triangularis sterni muscle. (From a 

 preparation in the Museum of the Royal College of Surgeons of England.) 



or four lower true ribs. Its fibres diverge upward and outward, to be inserted by 

 digitations into the lower border 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, 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, 



