Mediastinum. 151 



the right sterno-clavicular articulation and passing, 

 almost vertically downwards, to the lower border of the 

 first right costal cartilage, about half an inch to the right 

 of the sternum. Superior vena cava, by a line drawn from 

 the lower border of the first costal cartilage on the right 

 side, to the middle line of the sternum on a level with the 

 upper border of the third costal cartilage. The inferior 

 vena cava enters the right auricle at the level of the fifth 

 right interspace and the adjoining portion of the sternum. 



3. The Trachea. The trachea is about four and a 

 half inches in length and three-quarters of an inch wide, 

 and extends from the lower border of the cricoid car- 

 tilage, on a level with the upper border of the sixth cervical 

 vertebra, to the lower border of the fourth, or, to the mid- 

 dle of the fifth dorsal vertebra. The principal relations 

 in front of the trachea are the arch of the aorta and its 

 branches, the latter lying directly in front of the trachea 

 at their origins, but diverging above, so as to lie partially 

 on either side, at their terminations, and the former, i.e., 

 the arch itself passing obliquely across the bifurcation, 

 the deep cardiac plexus lying between. Behind, the tra- 

 chea rests on the oesophagus and divides into its two 

 bronchi, right and left ; the former, one inch in length, is 

 nearly horizontal in direction, and enters the lung, about 

 the fifth dorsal vertebra, while the latter, two inches long, 

 passes downwards and to the left and enters the lung at 

 about the level of the sixth vertebra, after running under 

 the left portion of the arch of the aorta. 



4. The Oesophagus, about nine inches long, begins 

 at the upper border of the cricoid cartilage, and about on 

 a level with the intervertebral disk between the fifth and 

 sixth cervical vertebrae. It passes down the neck deviat- 

 ing to the left, and, on entering the superior mediasti- 

 num, inclines towards the middle line again, which it 



