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 pleure). 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 mediastinale medium); the parts before and behind 
this are termed respectively the anterior and posterior mediastinal spaces (Cavum 
mediastinale craniale, caudale). 
THE PLEURA 
The pleurez are two serous membranes, right and left, which enclose on each 
side a pleural cavity (Cavum pleura). They line the walls of the thorax, form the 
lateral lamin of the mediastinum, and are reflected from the latter upon the lungs. 
We therefore distinguish parietal, mediastinal, and pulmonary or visceral parts of 
the pleure. 
The parietal pleura (Pleura parietalis) is attached to the thoracic wall by the 
endothoracie 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, especially 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 ven cave). 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 ven cave, 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 cesophagus, and 
1The student should bear in mind that these terms are employed simply as a matter q 
convenience in description; all the parts of each pleural sac, though differently named, form a 
continuous whole. 
2 Here the mediastinal pleura does not extend continuously from the dorsal to the ventral | 
wall, but consists of dorsal and ventral parts. 
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