34 DISSECTION OF THE DOG 



the result that an extensive mediastinal space is formed. Followed over the 

 heart — or more correctly, the pericardium — it ^vill be discovered that the 

 pleura is reflected over the root of the lung to the lung itself ; that is, the 

 parietal pleura here becomes continuous ^vith the visceral or 'pulmonary pleura 

 (pleura pulmonahs). Caudal to the root of the lung the reflection of medi- 

 astinal pleura does not cease but is continued as far as the diaphragm in the 

 form of the pulmo7iary ligament (ligamentum pulmonale). The ligament runs 

 obHquely from the root of the lung to the dorsal border of this organ, along 

 which it is then continued to its termination. 



Now trace the costal pleura in a dorsal direction. On reaching the vertebral 

 column the membrane is reflected ventralwards as the mediastinal septum (medi- 

 astinal pleura), in which the oesophagus and aorta are conspicuous objects. 

 Again the mediastinal pleura is continued onto the surface of the lung by way 

 of the root of the lung and the pulmonary ligament. 



Now investigate the disposition of the pleura at the apex of the chest. 

 Here it -will be found that each sac ends blindly in the cupula pleurce 

 extending for a short distance beyond the first rib, and supported, in part, by 

 the origin of the sterno-thyroid muscle. Finally the costal pleura should be 

 followed to the diaphragm — where it forms the diaphragmatic pleura (pleura 

 diaphragmatica) — and from this once again to the mediastinal septum. 



On the right side of the thorax the arrangement of the pleura is compUcated 

 by the presence of a fold which leaves the diaphragm and passes dorsalwards 

 to surround the caudal vena cava and the right phrenic nerve. 



Septtjm mediastinalb. — The mediastinal septum between the two pleural 

 cavities is formed, as has been seen, by the apposition of the two pleural mem- 

 branes at or about the median plane. The space between the two membranes 

 is known as the mediastinwn or mediastinal cavity, and is bounded laterally by 

 the mediastinal pleurae, ventraUy by the sternum, and dorsally by the vertebral 

 column. Since the unpaired, more or less median structures of the chest 

 occuj)y a position therein, the greater part of the dissection of the thorax 

 takes place within the mediastinal cavity. 



For convenience of description the cavity is divided into three parts ; (1) 

 A precardial mediastinum, cranial to the heart ; (2) a cardial m,ediastinum, 

 containing the heart and other structures occupying the same transverse zone 

 of the chest ; and (3) a postcardial mediastinum, of triangular outline and 

 circumscribed by the heart, the diaphragm, and the vertebral coljimn. The 

 precardial and cardial mediastina are median ; but the postcardial mediastinum 

 is pushed over to the left by the accessory lobe of the right lung. 



The precardial mediastinum contains the large blood-vessels connecting 

 the heart with the head, neck, and thoracic Umbs, the terminal part of the 

 thoracic duct, the vagi, recurrent, phrenic, sympathetic, and cardiac nerves, 

 the oesophagus and trachea, and the thymus and lymph-glands. 



The cardial mediastinum contains the heart and its enveloping pericardium, 



