THE LUNGS* 149 



of the vertebrae. This duplicature is the Ligamentum Pul mo- 

 m's. It is longer on the left lung than on the right, by reason of 

 the greater vertical diameter of the former. 



From what has been said it will now be readily understood, 

 that the whole cavity of the thorax is divided vertically into two 

 halves, by that portion of the two pleurae which advances from 

 the spine towards the sternum. This septum is called the Me- 

 diastinum, and the heart, enveloped by the pericardium, is placed 

 in its centre, and separates the two pleuras widely apart. It has 

 been found useful by anatomists, for descriptive purposes, to 

 subdivide the mediastinum ito three portions or regions. One, 

 passing from the front of the pericardium to the posterior face 

 of the middle line of the sternum, is the Anterior Mediastinum; 

 another, passing from the posterior face of the pericardium to 

 the dorsal vertebrae, is the Posterior Mediastinum; and a -third, 

 which is within the circuit of the first ribs, is the Superior Me- 

 diastinum. This division, though evidently arbitrary, is indis- 

 pensable to a correct account of the relative situation of very 

 important organs placed between the twa pleurae. 



1. The Anterior Mediastinum is less important than the other 

 two; the portions of the two pleurae of which it consists are al- 

 most in contact, and contain between them some loose cellular 

 substance by which they adhere together, and by cutting through 

 which, after a longitudinal section of the sternum, they are 

 easily separated from one another. The upper part of this sep- 

 tum contains the remains of the thymus gland ; its lower part 

 leaves the middle line of the sternum, and inclines' to the left 

 side; and when the sternum is narrow below, it is attached to 

 the anterior ends of the cartilages of the lower true ribs- 



2. The Posterior Mediastinum, where it leaves the vertebras 

 to reach the pericardium, passes off from a line nearer the 

 heads of the ribs on the left side, than on the right. The de- 

 scending portion of the thoracic aorta is contained within this 

 septum, on the left side of the dorsal vertebrae. The oesopha- 

 gus is- in its middle in front of the vertebrae above, but, in de- 

 scending it crosses in front of the aorta, and inclines to the 

 left side of the dorsal vertebrae to reach the foramen cesopha- 

 geum of the diaphragm. The vena azygos occupies the right: 



14* 



