102 THE ANATOMY OF THE HORSE. 



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 it, 

 together with the corresponding layer of the opposite side, 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 

 close sac termed the pleural cavity. This disposition of the pleura will be 

 more readily understood by reference to the accompanying diagrams (page , 

 101), the first of which represents the arrangement of the membrane at the 

 root of the lung, the second in front of, and the third behind, that point. 



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, the 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 inflamma- 

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

 dry, these movements, which normally give rise to no sensation, are 

 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 bloodvessels, nerves, and lymphatics. 



The Mediastinum. This, as has already been said, is a septum 

 formed towards the mesial plane of the chest by the approximation of 

 two layers of pleura, one from each sac. At some points the right and 

 left layers are in close contact, as, for example, in front of the heart in 

 a lean subject ; but at other points the layers are pushed apart by organs 

 included between them. The largest of these organs is the heart, 

 opposite which the right and left layers of the mediastinum are distant 

 four or five inches from one another. In the foetus of the horse, and 



