530 EESPIBATORY SYSTEM OF THE HORSE 



usually appears fenestrated; when these apertures are present, vae two pleural 



cavities communicate with each other.^ 



The pleural sacs contain a clear serous fluid, the liquor pleurae ; in health there 



is only a sufficient amount to moisten the surface, but it 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 fihn 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 tendinous center of the diaphragm. A strong layer 

 descends from it in the mediastinum and blends with the fibrous part of the peri- 

 cardium. The subserous tissue of the pulmonary pleura is continuous with the 

 interstitial tissue of the limg. 



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 rnediastinal 

 pleura; it extends along the longus colli and the bodies of the thoracic vertebra to the vertebral 

 end of the last intercostal space, where it joins 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 hnes 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 cav- 

 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 httle 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 line. The nar- 

 row angular recess of the plem-al cavity here is termed the phrenico-costal sinus (Sinus phren- 

 icocostalis) . 



The apex of each pleural sac (Cupula pleursB) 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 anterior vena cava and may extend about an inch (ca. 2.5 cm.) 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 

 cavity 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- 



' The 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 clinical observation that in the horse fluid exu- 

 date resulting from unilateral pleurisy is usually present in both pleural sacs in like amount. 



