THE 



CYCLOPAEDIA 



ANATOMY AND PHYSIOLOGY 



PLEURA is the name given to the serous 

 sac of the lung and the cavity containing it. 

 There are two pleural sacs, one for the right 

 lung and right side of the thorax, the other for 

 the left lung and left side of the thorax. These 

 two sacs being apposed and adherent to one 

 another in the middle line, form there a median, 

 an tero-posterior, vertical septum, called the me- 

 diastinum, which divides the thoracic cavity 

 into two lateral compartments. Each pleura 

 is, like all other serous membranes, with one 

 exception, a shut sac ; and there being but one 

 organ contained in each pleural cavity, and that 

 organ being of a tolerably simple form, the 

 well-known comparison of a double nightcap, 

 expressive of the manner in which a serous sac 

 lines the interior of a cavity and invests the 

 exterior of the viscus contained in it, is ex- 

 tremely apt in the case of these sacs lining the 

 chest and covering the lungs. Of the two sur- 

 faces of the sacs, the inner one is everywhere 

 free and the outer everywhere adherent ; such 

 in fact is universally the case with all serous 

 membranes. Each pleura invests its respective 

 lung, and lines the moiety of the thoracic cavity 

 to which it belongs, in the simplest manner pos- 

 sible, as simply and accurately as though it 

 were a coating of paint, dipping into the 

 fissures of the lungs and into the acute angles 

 formed by the costae with the arching diaphragm 

 in the most neat and accurate manner. It only 

 remains then, in order to complete our descrip- 

 tion of the course of these membranes, to ex- 

 amine the manner in which they pass from the 

 parietes to the viscus. It is thus: the two 

 pleurae, above, below, behind, and in front, 

 meet one another in or near the middle line, 



VOL. IV. 



and form the mediastinum above-mentioned; 

 between the layers of the mediastinum are 

 situated the heart and great vessels and the 

 termination of the trachea ; from these issue on 

 each side a bronchus, pulmonary artery, pul- 

 monary veins, &cc. destined to the lung, which, 

 bound loosely together by areolar tissue, have 

 received the appellation of the root of the lung; 

 this root of the lung emerges from the media- 

 stinum at about the middle of its posterior 

 upper quarter, and is covered with a layer of 

 the pleura, which thereby becomes conducted 

 from this point of the mediastinum to the lung. 

 The term mediastinum is applied by some 

 writers to the antero-posterior vertical septum of 

 the chest, by others to the spaces occupied by the 

 viscera situated between its layers ; in the latter 

 senseof the term three mediastina are enumerated 

 anterior, posterior, and middle; the anterior, 

 which is very large, is the space occupied by the 

 heart in its pericardium, thymus gland, or its re- 

 mains, and phrenic nerves ; the middle contains 

 the bifurcation of the trachea, the arch of the 

 aorta, the pulmonary and other great vessels ; the 

 posterior contains the aorta, oesophagus, &c. 

 All these organs, their position, &c. will be 

 found described in other parts of this work ; 

 their right and left aspects are invested by the 

 right or left pleura respectively. It is in their 

 larger interspaces only that the two pleurae 

 come into actual contact and adhesion with 

 one another. The smaller interspaces are not 

 intruded upon by the pleurae, but are occupied 

 with areolar tissue and fat. In most of the 

 lower (mammalian) animals, where the chest is 

 deep and narrow, and in the human fetus, the 

 two pleurae come into adhesion with one ano- 



