3.H fbt Pleura. [Book IX. 



foration of the diaphragm from the peritoneum. It 

 covers the internal furface of the bones of the thorax 

 and the upper part of the diaphragm, and involves the 

 viicera of the thorax in the fame manner as below it 

 involved thofe of the abdomen. The internal furface 

 of the pleura is conftantly moiftened, and rendered 

 flippery by a ferous' exudation. 



The mediaftinum is formed by two laminae of the 

 pleura including cellular fubftance. Thefe are clofely 

 connected near the fternum and vertebra? -, but in the 

 middle and towards the lower part they are feparated 

 by the pericardium and heart. Before the heart, from 

 the pericardium to the fternum, the two laminse ad- 

 here very clofely j higher up they are divided to re- 

 ceive the thymus. The mediaftinum divides the 

 thorax perpendicularly into two feparate cavities or 

 faicks, which contain the lungs. The mediaftinum is 

 attached in fuch a manner to the anterior part of the 

 bones cf the thorax, as to render the right lack of the 

 pleura larger than the k j ft. Behind, the mediaftinum 

 is attached to the dorfal vertebras, before to the fter- 

 num,, below to the diaphragm and pericardium, and 

 above to the large blood-veflels. 



Behind, tovvruds the vertebras of the back, is left a 

 triangular fpace, in which is placed the windpipe, the 

 cefophagus, the thoracic duft, and feveral large bloud- 

 veflels j before, the gland called the thymus occupies 

 a fimilar ipace. The ufes,of the pleura are to furnifh 

 an internal covering to the banes of the thorax and 

 the diaphragm, and an external covering to the tho- 

 racic viicera. 



The iinian of the two facks of the pleura, forming 

 the mediaftinum, is of ufe, by fupporting the lungs, 

 and by preventing their prefiure on each other when 

 the body is turned to either fide. By the two fides of 



the 



