53) RESPIRATORY SYSTEM CF THE HORSE 
usually appears fenestrated; when these apertures are present, tne two pleural 
‘avities communicate with each other.' 
The pleural sacs contain a clear serous fluid, the liquor pleure; in health there 
is only a sufficient amount to moisten the surface, but 1t accumulates rapidly after 
death. 
It should be borne in mind that the pleural cavity is normally a capillary space between the 
parietal and visceral parts of the pleura, and contains a film of serous fluid. In illustrations it is 
necessary, for the sake of clearness, to exaggerate the space. 
The pleura resembles the peritoneum in structure and appearance. It is 
attached to the structures which it covers by subserous tissue, which is elastic and 
in some situations contains fat. In the case of the parietal pleura the subserous 
tissue is termed the endothoracic fascia. This lines the thoracic walls, but is 
practically absent over the tendimous center of the diaphragm. A strong layer 
descends from it in the mediastinum and blends with the fibrous part of the peri- 
eardium. The subserous tissue of the pulmonary pleura is continuous with the 
interlobular tissue of the lung. 
The pleura receives an abundant blood supply which is derived chiefly from 
the intercostal, internal thoracic, and bronchial arteries. Lymph vessels are very 
numerous in the pleura and subserous tissue; they go chiefly to the intercostal and 
mediastinal glands. 
The parietal pleura is reflected along three lines which are known as the lines of pleural re- 
flection; these may be termed vertebral, sternal, and diaphragmatic. The vertebral line of 
pleural reflection is that along which the costal pleura turns ventrally to form the mediastinal 
pleura; it extends along the longus colli and the bodies of the thoracic vertebre to the vertebral 
end of the last intercostal space, where it joims the line of diaphragmatic reflection. The 
sternal line of pleural reflection is that along which the costal pleura is reflected dorsally 
to become the mediastinal pleura. Anteriorly the two lines are close together along the middle of 
the floor of the thorax, but further back they diverge to each side of the sternal attachment of the 
pericardium. The reflection is at an acute angle and the narrow angular recess of the pleural cay- 
ity here is termed the costo-mediastinal sinus (Sinus costomediastinalis). The diaphragmatic 
line of pleural reflection is that along which the costal pleura passes from the lateral wall 
to the diaphragm. This line is important clinically, since it is, from the standpoint of physical 
diagnosis, the demarcation between the thoracic and abdominal cavities. It extends along the 
eighth and ninth costal cartilages, crosses the sternal end of the ninth rib, and passes backward and 
upward in a gentle curve and at a gradually increasing distance from the sternal ends of the ribs, 
so that its most posterior part is about the middle of the anterior border of the last rib; this is the 
posterior limit of the pleural cavity. Here it turns medially and a little forward and ends at the 
vertebral end of the last intercostal space. This reflection is also at an acute angle and the costal 
and diaphragmatic pleura are in contact over an area of variable width along this Ime. The nar- 
row angular recess of the pleural cavity here is termed the phrenico-costal sinus (Sinus phren- 
icocostalis). 
The apex of each pleural sac (Cupula pleurz) lies at the anterior aperture of the thorax. 
On the right side it forms two culs-de-sac; one of these may extend forward more than an inch 
(ca. 3 cm.) beyond the first rib in contact with the deep face of the scalenus; the other is ventral 
to the ee ior vena cava and may extend about an inch (ca. 2.5 em.) beyond the first rib. The 
apex of the left pleura usually does not extend beyond the plane of the first rib. 
THE LUNGS 
The lungs, right and left (Pulmo dexter, sinister), occupy much the greater 
part of the thoracic cavity. They are accurately adapted to the walls of the 
savity and the other organs contained therein. The two lungs are not alike in 
form or size, the right one being considerably larger than the left; the differ- | 
ence is chiefly in width, in conformity with the projection of the heart to the 
left. The lung is soft, spongy, and highly elastic. It crepitates when pressed 
between the finger and thumb, and floats in water. When the thoracic cavity of 
the unpreserved subject is opened, the lung collapses immediately to about one- | 
1The apertures do not exist in the foetus, and are sometimes absent in the adult subject. 
Some of them are doubtless produced in dissection by the necessary disturbance of the parts. The 
character of the pleura here probably explains the clinies al observation that in the horse fluid exu- 
date resulting from unilateral pleurisy is usually both pleural sacs in like amount. 
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