i88 LABORATORY MANUAL FOR VERTEBRATE ANATOMY 



mediastinal septum. It consists of the two medial walls of the right and left 

 pleural sacs in contact with each other. At the level of the heart the two walls 

 separate so that the heart and its pericardial sac are inclosed between them. 

 This space between the two walls of the mediastinal septum is called the medias- 

 tinum. The posterior wall of the pleural sac is formed by a muscular dome- 

 shaped partition, the diaphragm. The pleural sac is lined by a smooth moist 

 membrane, the pleura. The pleura is divided into parietal and visceral parts. 

 The parietal pleura lines the inside of the pleural cavity, covers the anterior face 

 of the diaphragm, and together with the medial wall of the other pleural sac forms 

 the mediastinal septum. The visceral pleura is that part of the pleura which 

 passes over the surface of the lung to which it is indistinguishably fused. 

 Examine the left lung. It is a soft spongy organ divided into three lobes, a 

 smaller anterior, and larger middle, and posterior lobes. The anterior lobe is 

 quite small in the rabbit. The large posterior lobe fits very neatly on the convex 

 surface of the diaphragm. Cut into the lung; it appears solid but is really com- 

 posed of innumerable minute air-cells. 



Now carefully cut through the mediastinal septum ventral to the heart and 

 look into the right pleural cavity. The diaphragm may be slit along its left side 

 so as to facilitate the spreading apart of the thoracic walls. The right pleural 

 cavity is similar to the left cavity. It contains the right lung. The right lung 

 is somewhat larger than the left lung. It is divided into anterior, middle, and 

 posterior lobes. The large posterior lobe is subdivided into two lobules, a medial 

 and a lateral. The medial lobule projects into a pocket formed by a special 

 dorsally directed fold of the mediastinal septum. This fold, the caval fold, has 

 the function of supporting a large vein, the postcaval vein, which ascends from 

 the liver to the heart and will be found inclosed in the free dorsal margin of the 

 caval fold. 



Examine the heart and the pericardial sac. The pericardial sac or parietal 

 pericardium is a sac of thin tissue inclosing the heart but not attached to it except 

 at the anterior end where the great vessels enter and leave the heart. The heart 

 is freely movable inside of the pericardial sac. The narrow space between the 

 pericardial sac and the heart is the pericardial cavity, a portion of the coelom. 

 Cut through the pericardial sac so as to expose the heart. The surface of the 

 heart is invested by a thin membrane, the visceral pericardium, inseparably 

 adherent to the heart wall. The visceral pericardium is continuous with the 

 pericardial sac at the anterior end where the blood vessels enter and leave the 

 heart. As the heart with its pericardial sac is situated in the mediastinum, it is 

 evident that there are three coelomic layers surrounding the heart: the visceral 

 pericardium closely adherent to the heart wall, the parietal pericardium or 

 pericardial sac separated from the heart by the pericardial cavity, and the parietal 

 pleura of the mediastinal septum, which is closely fused to the pericardial sac 

 (Fig. 4 o, p. 197). 



