THE THORAX. 



465 



Independently of these four serous layers, the right pleura furnishes a 

 special membranous fold, which arises from the inferior wall of the thoracic 

 cavity, and ascends to envelop the posterior vena cava. This fold also 

 sustains the right diaphragmatic nerve. 



In order to study the various portions of the pleuras collectively, with 

 their reciprocal relations, and their connections with the organs contained 

 in the thoracic cavity, we will suppose three transverse sections of this 

 cavity one passing behind the heart ; the other at the roots of the lungs, 

 and dividing the left ventricle of the heart , the third traversing the anterior 

 mediastinum, a little in front of the right ventricle. 



If in the first section (Fig. 235), we take the costal pleura at the point 

 a and follow it up to 6, we will see it folded downwards to form the 



Fig. 235. 



Fig. 236. 



Fig. 237 



THEORETICAL SECTIONS OF THE THORACIC CAVITY; INTENDED TO SHOW THE 

 DISPOSITION OF THE PLEURA. 



mediastinal layer, to be applied to the aorta, c, and the oesophagus, d ; then 

 reflected at e on the lung,/, enveloping every part of the organ ; returning to 

 the point e, it leaves the lung, is again reflected to achieve the formation 

 of the mediastinal septum, b g, and finally regains the point it started from. 

 On the right side, with only a slight variation, it has the same arrangement. 

 After being carried from the point a' to fe', then to e', and after enveloping 

 the lung, returning to e, and being reflected in the median plane to the 

 point <?', the right pleura leaves the inferior thoracic wall to pass around 

 the posterior vena cava, and come back to a', its point of departure. 



The second section, (represented by Fig. 236) shows the pleura arrived 

 at the point fe, descending on the root of the lung, c, covering that organ 

 and returning to c, and reflected on the pericardium, d, to gain the point a. 



In the third section (Fig. 237), we see the parietal pleura, a b, without 

 any points of continuity with the visceral pleura, c. It is at the level of 

 this section that the lung forms two perfectly free lobes, which are not 

 attached to the anterior mediastinum. 



STRUCTURE. Like all the serous membranes, the pleurae have a free face 

 covered by a simple tesselated ep^elium ; it is perfectly smooth, always in 

 contact with itself, and constantly lubricated by a serous fluid which 

 facilitates the gliding of the lung on the parietes of the thoracic cavity, 



