RI'SPIRATORV SYSTEM. 227 



Pleura. 



The pleura is a fine semi-transparent membrane, lining the 

 cavity of the chest, and giving a covering to the lungs. By that 

 portion of it which is called the niediaslitmm, the cavity is divided 

 into the right and left sides of the thorax. 



General Coiiformaiion. — If the lungs be exposed, by breaking 

 off one or two of the ribs, we shall perceive that their surface, as 

 well as that of the cavity itself, is everywhere smooth, polished, 

 and humid : this is owing to the extensive investment of the 

 pleura, the surface of which is now presented; so that, in reality, 

 without breaking the surface, nothing but pleura can be touched ; 

 although, from its extreme tenuity and pellucidity, the viscera 

 appear, on a superficial view, to present their own bare exterior. 

 Its other side, on the contrary, is rough, having numerous cellular 

 flocculent appendages, by which it is united to the parts it in- 

 vests : and so close and firm are these adhesions, that to cleanly 

 detach it, in the recent subject, is a very ditficult and tedious dis- 

 section. 



The pleura is a rejlected membralie ; by which is meant one 

 that not only lines the cavity in which the viscera lie inclosed, 

 but, by duplicature, or what in anatomical language is called re- 

 flection, gives a partial or complete covering to the contained 

 organs themselves. It is evident, therefore, that such a mem- 

 brane admits of division into two portions : — a li/iing or parietal, 

 and a rejlected portion ; and these, with regard to the pleura, 

 have, for the sake of more definite description, received the names 

 of pleura costalis and pleura pulmonalis : they are both, however, 

 continuous at all points, are precisely similar in structure and 

 function, and, in fact, are still but one and the same pleura. 



Mediastinum. — ^There is yet a third portion of this membrane 

 to which a distinct appellation has been given, and that is the me- 

 diastinum, the membranous partition between the cavities or 

 sides of tlie thorax ; it differs from both the others in being com- 

 posed of ^7^0 layers, which are derived from the two pleurseof the 

 opposite sides. If we conceive the pleurae of the two sides of the 

 thorax to be perfect sacs or bags, with flattened sides turned in- 

 wardly, and closely applied and united together, in such a man- 

 ner that the double membrane formed by their union extends 

 through the middle of the chest, from the dorsal vertebra3 to the 

 sternum, we shall at once have a tolerably correct idea of the for- 

 mation as well as situation of the mediastinum. 



Structure. — The pleura, from the nature of its secretion, is one 

 of those included in the list of st'yo^/-5 uiciubranes, to which it has 

 been demonstrated also to be siunlariu its intimate organization. 



