104 • THE ANATOMY OF THE HORSE. 



the cavity lodges the main arterial and venous trunks, the thoracic duct, 

 the trachea, the oesophagus, and many important nerves, all of which 

 will be examined in due course. 



The Pleurae. Each half of the thorax possesses a serous membrane 

 termed the pleura. Like other serous membranes, the pleura is arranged 

 in the form of a closed sac, and consists of a visceral and a parietal 

 portion. The visceral pleura is that which invests the lung, and it is 

 therefore termed the pulmonic pleura. Around the root of the lung it- 

 is continuous with the parietal portion. The 'parietal pleura lines the 

 walls of the chest on the side to which it belongs. It covers the inner 

 surface of the ribs and intercostal muscles, forming the costal pleura ; it 

 is spread over the anterior surface of the diaphragm, constituting the 

 diaphragmatic pleura ; and towards the middle line of the cavity, 

 together with the corresponding layer of the opposite side, it forms a 

 vertical septum termed the mediastinum. This is the mediastinal 

 pleura. Behind the root of the lung a double fold of pleura,, termed 

 the ligamentum latum pulmonis, is prolonged along the mediastinum to 

 the diaphragm. On the right side of the chest the pleura forms a 

 special fold that includes between its two layers the posterior vena cava 

 and the right phrenic nerve. All these differently named divisions of 

 the pleura are continuous the one with the other; and they unite to 

 form a closed sac termed the pleural cavity. This disposition of the 

 pleura will be more readily understood by reference to the accompanying 

 diagrams (page 103), the first of which represents the arrangement of 

 the membrane at the level of the heart, the second in front of, and the 

 third behind, that organ. 



These diagrams, it is to be observed, however, are not true to nature ; 

 for, whereas they show a distinct pleural cavity, in the living healthy 

 animal that cavity has only a potential existence, the pulmonic being 

 everywhere in contact with the visceral pleura. But when air is 

 admitted to the cavity, or when inflammatory or other effusions are 

 poured out from the surface of the membrane, t]ie parietal and the 

 visceral pleura become separated, and the cavity comes to have an 

 actual existence. The free surface of the healthy pleura is exquisitely 

 smooth, and is lubricated by a sparing amount of serous fluid, which 

 gives it a glistening aspect. Its function is to facilitate the movements 

 of the lung on the walls of the chest during respiration. When, in 

 inflammation of the membrane (pleurisy), it loses its smoothness and 

 becomes dry, these movements, which normally give rise to no sensa- 

 tion may be attended with the most acute pain. In structure, the pleura, 

 like other serous membranes, comprises a single layer of endothelial 

 cells forming the free surface of the membrane, and a sub-endothelial 

 layer of fibrous connective tissue supporting the blood vessels, nerves, 

 and lymphatics. 



