464 THE RESPIRATORY APPARATUS IN MAMMALIA. 



Such is the thoracic cavity. Like the abdomen, it is provided with a 

 serous lining, which remains to be examined. 



THE PLEURAE. The serous lining of the thorax comprises two distinct 

 membranes, designated as the pleurae., constituting two sacs placed one 

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

 mediastinum, which divides the thoracic cavity into two lateral compart- 

 ments. Each pleura, therefore, covers one of the external or costal walls 

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

 afterwards reflected in the vertical and antero-posterior plane of the 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 the parietal layer 

 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 the 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 apoueurotic 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 being 

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

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

 misunderstanding. 



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

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

 its divisions, the anterior vena cava, thoracic duct, the cardiac, pneumo- 

 gastric, recurrent, and diaphragmatic nerves ; it also includes the thymus 

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

 incomparably narrower below than above, in consequence of the oblique 

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

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

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

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

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

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

 It is these layers of this mediastinum which pass to the lung to constitute 

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

 zontal line extending from the root of the pulmonary lobe to the anterior 

 face of the diaphragm. 



The pulmonary or visceral pleura, a continuation, as has been said, of the 

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

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

 tissue of the lungs. 



