THORAX AND ABDOMEN OF THE HORSE 15 



which the pleura passes from the ribs on to the diaphragm, should be 

 examined with care, since it is of clinical importance. The line runs 

 over the union of the seventh and eighth ribs and their cartilages, 

 crosses the sternal end of the ninth rib close to the costo- 

 cartilaginous articulation, and then sweeps over the ribs, with a gentle 

 curve, the convexity of which looks downwards, in such a manner as to 

 recede gradually from the sternal end of the successive ribs until the 

 second last of these is crossed scarcely ventral to the middle of its length. 

 About the middle of the cranial border of the last rib the line turns 

 towards the middle line, and, with a slight forward inclination, ends 

 opposite the middle of the width of the last intercostal space. 



On the right side of the thorax the arrangement of the pleura is 

 complicated by the presence of a fold, the fold of the vena cava (plica 

 venae cavse), that leaves the diaphragm and passes dorsalwards to' 

 surround the caudal vena cava as this travels from its point of perfora- 

 tion of the diaphragm to the heart. The right phrenic nerve is 

 enclosed in a secondary fold that springs from the right face of the 

 main fold. The plica venae cavse is delicate and lace-like in the adult 

 horse, and occupies a deep fissure between the intermediate lobe and 

 the main mass of the right \nn<y. 



The mediastinal septum (septum mediastinale). The septum 

 between the two pleural cavities is formed, as has been seen, by the 

 apposition of the two pleural membranes at or about the median plane. 

 The space between the two membranes is known as the tnediastinum ^ 

 or mediastinal cavity, and is bounded laterally by the mediastinal 

 pleurse, ventrally by the sternum, dorsally by the vertebral column, and 

 caudally by the diaphragm. Since the unpaired, more or less median 

 structures of the chest occupy a position therein, the greater part of the 

 dissection of the thorax takes place within the mediastinal cavity. 



For convenience in description, the cavity is divided convention- 

 ally into three parts : (1) A precardial mediastinum, cranial to the 

 heart ; (2) a cardial mediastinum, containing the heart and other 

 structures occupying the same transverse zone of the thoracic cavity ; 

 and (3) a postcardial mediastinum, of triangular outline and cir- 

 cumscribed by the heart, the diaphragm and the vertebral column. 

 The precardial and cardial mediastina occupy approximately the 

 median plane ; but the postcardial mediastinum is pushed over to 

 the left by the intermediate lobe of the right lung. The right 

 pleural sac, consequently, is more extensive than the left. 



It should be noted that, ventral to the oesophagus, the postcardial 



^ Mediastinum [L.], a common servant, a median partition or septum 

 {mediastinus [L.], being in the middle). 



