440 



DISSECTION OF THE THOEAX. 



Dissection 

 of inl ernal 

 mammary 



vessels. 



Triangularis 

 sterni : 



origin ; 



insertion ; 



relations ; 



Internal 



mammary 



artery 



courses 

 through 

 thorax to 

 abdomen. 



the pleural sacs. To bring into view the triangularis sterni muscle 

 and the internal mammary vessels, the left half of the sternum 

 with the cartilages of the true ribs, except the first and 

 seventh, are to be taken away with the intervening muscles ; but 

 the two ribs mentioned are to be left untouched for the benefit of 

 the dissectors of the abdomen and of the head and neck. Small 

 arteries to each intercostal space and the surface of the thorax, and 

 the intercostal nerves are to be preserved. If the piece of sternum 

 and the costal cartilages are divided and removed carefully these 

 can be shelled off the subjacent structures without injury to them. 

 The surface of the triangularis sterni will be apparent when the loose 

 tissue and fat are removed. 



The TRIANGULARIS STERNI (fig. 160) is a thin muscle beneath the 

 costal cartilages. It arises internally from the side of the ensiform 

 process, from the back of the sternum as high as the third costal 



cartilage, and, usually, 

 from the inner ends 

 of the cartilages of the 

 lower two or three 

 true ribs. Its fibres 

 are directed outwards, 

 the upper ones also 

 ascending consider- 

 ably, and are inserted 

 by fleshy slips into 

 the true ribs except 

 the first and last, at 

 the junction of the 

 bone and cartilage : 

 some of the fibres 

 frequently end in an 

 aponeurosis in the in- 

 tercostal spaces. 



In front of the 

 muscle are the rib- 

 cartilages and the internal intercostals, with the internal mammary 

 vessels and intercostal nerves. Behind, it lies on the pleura. Its 

 lower part is continuous with the transversalis muscle of the 

 abdomen (fig. 160, B). The size of this muscle varies greatly, and 

 one or more of the upper slips are frequently wanting. 



Action. The triangularis sterni assists in depressing the anterior 

 ends of the ribs, and acts with the interosseous part of the internal 

 intercostals in expiration. 



The INTERNAL MAMMARY ARTERY is a branch of the subclavian, 

 and enters the thorax, beneath the cartilage of the first rib. It is 

 continued through the thorax, lying behind the costal cartilages and 

 about half an inch from the sternum, as far as the sixth intercostal 

 space ; here it gives externally a large muscular branch (musculo- 

 phrenic), and then passing beneath the seventh cartilage, enters 

 the sheath of the rectus muscle in the wall of the abdomen. In 



FIG. 160. VIEW FROM BEHIND OP, A, 

 TRIANGULARIS STERNI MUSCLE. 



