i 6 THORAX 



of each artery end in a single internal mammary vein, which 

 joins the corresponding innominate vein in the superior 

 mediastinum. The terminations of the internal mammary 

 veins will be seen when the contents of the superior media- 

 stinum are dissected (see p. 66 and Figs. 4, 27). 



Musculus Trans versus Thoracis (O.T. Triangularis Sterni). 

 The transversus thoracis is a thin muscular layer placed on 

 the deep surface of the sternum and costal cartilages. It 

 arises from the posterior surface of the xiphoid process, the 

 lower part of the body of the sternum, and from the medial 

 ends of the fifth, sixth, and seventh costal cartilages, and it is 

 continuous below with the transversus abdominis. Its fibres 

 radiate in an upward and lateral direction, in the form of 

 five slips, which are inserted into the deep surfaces and 

 lower borders of the second, third, fourth, fifth, and sixth 

 costal cartilages, close to their junction with the ribs (Fig. 4). 

 It is supplied by the fourth, fifth, and sixth intercostal nerves, 

 and probably facilitates expiration by assisting to depress the 

 anterior parts of the ribs. It is supplied by the intercostal 

 nerves. 



In many cases the muscle is feebly developed, and does 

 not show such wide connections. Upon its anterior aspect 

 are placed the internal mammary artery and some of the 

 intercostal nerves. 



It is only a partial view of the muscle which is obtained 

 in the present dissection, but it is not advisable to remove 

 the costal cartilages to expose it further, as this would 

 materially interfere with the subsequent display of the 

 relations of other more important structures. 



THORACIC CAVITY. 



Before the dissection of the interior of the thorax is com- 

 menced it is necessary that the dissectors should have some 

 general knowledge of the cavity and its contents. The shape 

 and the boundaries have been studied already (p. i), and it 

 must now be understood that the cavity is divided into two 

 lateral parts, right and left, by a median septum called the 

 mediastinum, which extends from the sternum anteriorly to 

 the vertebral column posteriorly, and from the upper aperture 

 of the thorax above to the diaphragm below. 



