498 PLEURJE. 



the form of the cavity of the thorax resembles that of the hoof 

 of the ox when its posterior part is presented forwards. 



Of the Arrangement of the Pleura. 



The thorax contains the two lungs and the heart, as well as 

 several very important parts of smaller size. 



The lungs occupy the greatest part of the cavity ; ana to 

 each of them is appointed a complete sac, called Pleura, which 

 is so arranged that it covers the surface of the lungs, and is 

 continued from it to the contiguous surface of the thorax, 

 which it lines. After covering the lung, it is extended from it 

 to the spine posteriorly : so that in tracing the pleura in a 

 circular direction, if you begin at the sternum, it proceeds on 

 the inside of the ribs, to the spine ; at the spine it leaves the 

 surface of the thorax, and proceeds directly forwards towards 

 the sternum. In its course from the spine to the sternum, it soon 

 meets with the great branch of the windpipe and blood-vessels, 

 which go to the lung : it continues on these vessels and round 

 the lung until it arrives at the anterior side of the vessels, when 

 it again proceeds forwards until it arrives at the sternum. Each 

 sac being arranged in the same way, there is a part of each 

 extended from the spine to the sternum. These two laminae 

 form the great vertical septum of the thorax, called Medias- 

 tinum. They are situated at some distance from each other; 

 and the heart, with its investing membrane or pericardium, is 

 placed between them. 



The pericardium is also a complete sac or bladder, which, 

 after covering perfectly the surface of the heart, is extended 

 from it so as to form a sac, which lies loose about it, and 

 appears to contain it. This loose portion adheres to those parts 

 of the laminae of the mediastinum, with which it is contiguous ; 

 and thus three chambers are formed within the cavity of the 

 thorax: one for each lung, and one for the heart. 



The two laminae of the pleura, which constitute the mediasti- 

 num, are at different distances from each other, in different 

 places. At the upper part of the thorax, they approach each 

 other from the internal edges of the first ribs ; and as these 



