544 



RESPIRATORY APPARATUS IN MAMMALIA. 



membrane. Its deep face adheres intimately, in Solipeds, to the proper tissue of 

 the kings. 



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

 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 pleurae 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. 322), we take the costal pleura at the point a, 

 and follow it up to b, we shall see it folded downwards to form the mediastmal 

 layer, to be applied to the aorta, r, and the oesophagus, d ; then reflected at e on 

 the lung, /, enveloping every part of the organ ; returning to the point e, it 



Fig. 322. 



Fig. 323. 



Fig. 324. 



THIiORETICAL SKCTIONS UV lllE THORACIC CAVITV, INTENDED TO SHOW THE DISPOSITION OF 



THE PLEURA. 



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 b', then to e', and after enveloping the lung, returning to e', 

 and being reflected in the median plane to the point g', 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. 323), shows the pleura arrived at the 

 point h, descending on the root of the lung, c, covering that organ and returning 

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



In the third section ( Fig. 324), we see the parietal pleura, a b, without any 

 points of continuity with the visceral pleura, r. 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 pleurte have a free face 

 covered by a perfectly smooth endothelium, always in contact with itself, and 



