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The thorax is lined by two serous membranes, the right and 

 left pleuras, which consist of parietal and visceral portions, and forpi 

 distinct sacs. Each pleura lines one side of the thorax and half 

 of the diaphragm ; the pleura costalis is the portion lining the 

 ribs and intercostal spaces, the pleura diaphragrsiatica is that 

 covering the diaphragm ; in the median longitudinal plane it 

 forms with the opposite pleura the mediastinum, whence each is 

 reflected over one of the limgs, forming the pleura pulmonalis. 

 The mediastinum is therefore a longitudinal passage through 

 the thorax, bounded on each side by a pleural fold ; it is divided 

 into three portions — the anterior mediastinum lies in front of the 

 heart, the middle contains it, while the posterior Ues behind it. 

 The mediastina are occupied by the trachea, oesophagus, heart, 

 vessels, and nerves, and the anterior one in the foetus by the 

 thymus gland. 



In tracing the course of a pleura, if we commence on the 

 lateral aspect of the bodies of the dorsal vertebrae, we find that it 

 extends downwards over the inner surfaces of al] the ribs and 

 intercostal spaces, forming the pleura costalis ; posteriorly it passes 

 to the anterior convex surface of the diaphragm, constituting the 

 pleura diaphragmaticaj At the centre of the sternum it is 

 reflected upwards between the lungs, enclosing the pericardial 

 sac. Gaining the root of the lung, the parietal portion passes on 

 to it, forming the visceral portion or pleura palmonalis, clothes 

 the free surface of the lung, and again on the lung root it is 

 reflected up to the vertebra, returning to its place of origin. The 

 pleura is thick and loosely attached over the ribs, attenuated over 

 the diaphragm and pericardium, and extremely so on the lungs ; 

 its surface is smooth and glistening, emitting a vapoury fluid, 

 which lubricates the contacting surfaces and facilitates motion. 

 The posterior mediastinum is cribrated inferiorly, several open- 

 ings leading from one pleural sac to the other. This arrange- 

 ment is peculiar to solipedes, and explains the fact that in these 

 animals there cannot be pleural effusix)n confined to one side of 

 the chest. 



LUNGS. 



The lungs, the essential organs of respiration, are spongy 

 organs of a conical shape, situated in the thoracic cavity, right 



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