526 RESPIRATORY SYSTEM OF THE HORSE 



tinale), extends from the aorsal wall to the ventral and posterior walls, and divides 

 the cavity into two lateral chambers. Each of these chambers is lined by a serous 

 membrane called the pleura, and is termed a pleural cavity (Cavum pleurae) . The 

 mediastinum is, for the most part, not median in position, as might be inferred 

 from its name; this is chiefly due to the fact that the largest organ contained in it, 

 the heart, is placed more on the left side than on the right; consequently the right 

 pleural cavity and lung are larger than the left. Practically all of the organs in 

 the thorax are in the mediastinal space, with the exception of the lungs, the posterior 

 vena cava, and the right phrenic nerve. The part in which the heart and the peri- 

 cardium are situated, together with that dorsal to it, is usually called the middle 

 mediastinal space (Cavum mediastinal medium); the parts before and behind 

 this are termed respectively the anterior and posterior mediastinal spaces (Cavum 

 mediastinale craniale, caudale). 



THE PLEURE 



The pleurae are two serous membranes, right and left, which enclose on each 

 side a pleural cavity (Cavum pleurae) . They line the walls of the thorax, form the 

 lateral laminae of the mediastinum, and are reflected from the latter upon the lungs. 

 We therefore distinguish parietal, mediastinal, and pulmonary or visceral parts of 

 the pleurae. 



The parietal pleura (Pleura parietalis) is attached to the thoracic wall by the 

 endothoracic fascia. On the lateral thoracic wall it is adherent to the ribs and 

 intercostal muscles and is termed the costal pleura (Pleura costalis). Behind it is 

 closely attached to the diaphragm, forming the diaphragmatic pleura (Pleura 

 diaphragmatica) . 



The mediastinal pleura (Pleura mediastinalis) covers the organs in the medi- 

 astinal space and is in part in apposition with the opposite sac. Where this layer 

 is adherent to the pericardium, it is distinguished as pericardiac pleura (Pleura 

 pericardiaca).^ 



From the mediastinum each pleura is reflected upon the corresponding lung, 

 which it covers, constituting the pulmonary or visceral pleura (Pleura pulmonalis) . 

 The reflection occurs around and behind the hilus of the lung, and is in great part 

 direct, so that a portion of the mediastinal surface of each lung has no pleural 

 covering. Behind the hilus of the lung a considerable triangular area is not covered 

 by the pleura, the two lungs being attached to each other by a thin layer of con- 

 nective tissue in this situation.^ Posteriorly the reflection is not direct, so that 

 there is a fold formed by the reflection of the pleura from the mediastinum and the 

 diaphragm to the lung, behind the triangular area of adhesion just mentioned; it 

 is seen when the base of the lung is drawn outward. This is the pulmonary liga- 

 ment (Lig. pulmonale) ; it consists of two layers of pleura, between which there is 

 elastic tissue, especiaUy abundant in its posterior part. 



The right pleura forms a special sagittal fold about a nandbreaatn to the right 

 of the median plane, which encloses the posterior vena cava in its upper edge; it is 

 therefore called the fold of the vena cava (Plica venae cavae) . It gives off a small 

 accessory fold for the right phrenic nerve. The fold arises from the thoracic floor 

 and from the diaphragm below the foramen venae cavae, and intervenes between the 

 intermediate lobe and the body of the right lung. It is delicate and lace-like. 



The posterior mediastinum is very delicate ventral to the oesophagus, and 



1 The student should bear in mind that these terms are employed simply as a matter of 

 convenience in description; all the parts of each pleural sac, though differently named, form a 

 contiauous whole. 



^ Here the mediastinal pleura does not extend continuously from the dorsal to the ventral 

 wall, but consists of dorsal and ventral parts. 



