4G4 THE RS8PTRATOR7 .1 /'/' I /,' 177 s I\ MAMMALIA. 



Snch IB the thoracic cavity. Like tho alxlomen, it is provided with a 

 serous liniiig, which remains to bo examined. 



THE PLEURJE. Tho serous lining of tho thorax comprises two <V 

 membranes, designated as tho pleura?, constituting two sacs jiLicnl ono 

 against tho other in the median plane, and forming a septum named tin* 

 iiH'ilinstinnm, which divides the thoracic cavity into two lad nil e.-mpart- 

 mnits. Each pleura, therefore, covers ono of tho external or costal walls 

 of the thorax, and the corresponding moiety of tho diaphragm ; it is 

 afterwards reflected in the vertical and antero-posterior plane of tho cavity, 

 to concur in the formation of the mediastinum, whence it is carried 

 over the lung. This arrangement exhibits the pleura in four portions : 

 a costal, diaphragmatic, mediastinal, together representing tho parietal hnji r 

 of the membrane, and a pulmonary or visceral portion. 



The costal pleura is applied to the inner face of the ribs and the internal 

 intercostal muscles. Strengthened on its adherent face, at each intercostal 

 space, by a lamina of yellow elastic tissue, this membrane responds, by 

 its free face, to tho external plane of the lung, with which it does not, in 

 a normal condition, contract any adhesions.. It is continued, posteriorly, 

 with the diaphragmatic layer ; in front, above, and below, with the medias- 

 tinal pleura. 



The diaphragmatic pleura adheres somewhat loosely to the fleshy portion 

 of the muscle, but the union is more intimate on the aponeurotic portion. 

 This layer is contiguous, by its free face, with the base of the lung ; it is 

 confounded with the mediastinum by the internal part of its periphery. 



The mediastinal pleura is placed, by its adherent face, against that of the 

 opposite side, and in this way produces the middle septum which divides 

 the thoracic cavity into two portions. Several organs are comprised between 

 the two layers of this partition, but most important of all is the heart. In 

 Veterinary Anatomy, that part of the septum in front of this organ is named 

 the anterior mediastinum; the appellation of posterior mediastinum bein^ 

 reserved for tho portion situated behind it. These terms have not the same 

 signification as in human anatomy, though they are retained here to prevent 

 misunderstanding. 



Tho anterior mediastinum, thicker than the posterior, but much less 

 extensive, contains, superiorly, the trachea, oesophagus, the anterior aorta ami 

 its divisions, tho anterior vena cava, thoracic duct, the cardiac, pncunio- 

 gastric, recurrent, and diaphragmatic nerves; it also includes tho thynnis 

 gland in the foetus and young animal. The posterior mtilinxfiintm is 

 incomparably narrower below than above, in consequence of the oblique 

 position of the diaphragm. Its inferior part, always deviated to tho left, is 

 extremely thin, and perforated by small openings, which give it the 

 appearance of fine lace-work. Traversed altogether superiorly by tho pos- 

 terior aorta, the vena azygos, and tho thoracic duct, this mediastinum gives 

 passage, a little lower between its layers, to tho oesophagus, the oesophageal 

 branches of tho pneumogastric nerves, and to tho left diaphragmatic nerve. 

 It is these layers of tin's mediastinum which pass to tho lung to constitute 

 tho pulmonary pleura, in becoming reflected above and below, in a hori- 

 zontal lino extending from tho root of the pulmonary lobo to the anterior 

 face of tho diaphragm. 



The pulmonary or rim-crnl //< />/, a continuation, as has been said, of the 

 mediaHtimil pleura, is in contact, by its free face, with the parietal layer of 

 the membrane. Its deep face adheres intimately, in Solipcds, to the proper 

 tissue of the lungs. 



