io THORAX 



Musculus Transversus Thoracis (O.T. Triangularis Sterni). 

 This is a thin muscular layer placed on the deep surface of 

 the sternum and costal cartilages. It is continuous below 

 with the transversus abdominis, and 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. 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. 3). 



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 other more important structures in their proper relations. 



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 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. 



The mediastinum is formed by the heart, enveloped in a 

 fibre-serous sac called the pericardium ; the great vessels 

 passing to and from the heart, i.e. the pulmonary artery and 

 veins, the aorta, and the vena cava superior ; the oesophagus ; 

 the trachea and the commencements of the bronchi; the 

 thoracic duct ; the azygos, hemiazygos and accessory hemi- 

 azygos veins ; the vagi and phrenic nerves ; numerous lymph 

 glands ; and the areolar tissue in which these structures are 

 embedded and by which they are bound together. For con- 

 venience of description the mediastinum is divided into a 

 superior and an inferior portion, by an imaginary plane 

 which passes from the lower border of the manubrium sterni 

 anteriorly, to the lower border of the fourth thoracic vertebra 



