LUNGS. 337 



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

 left pleurse, which consist of parietal and visceral portions, and 

 form 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 i^leura diaphragmatica 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 lungs, 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 lies 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 vertebra, we find that it 

 extends downwards over the inner surfaces of all the ribs and 

 intercostal spaces, forming the pleura costalis ; posteriorly it 

 passes to the anterior convex surface of the diaphragm, constitut- 

 ing the pleura diaphragmatica. 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 pulmonalis, 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 openings leading from one pleural sac to the other. This 

 arrangement is peculiar to solip)edes, and explains the fact that 

 in these animals there cannot be pleural effusion 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 



z 



