240 



BODY-CAVITY 



cavity, the posterior part the peritoneal cavity. The pleural cavities are 

 at first only short and narrow clefts, bounded behind and on either side 

 by,: ; f certain folds which are concerned in the ultimate separation of the 

 several 'portions of the coelom from one another, and in the formation of the 

 diaphragm. 



The closure of the pleuro-pericardial opening is chiefly effected by lateral 

 folds (lateral mesocardia] related to the Cuverian veins (fig. 293). The veins pass 



cardinal vein 

 Wolffianfold J ~1 

 dorsal pilla ' *" 



ventral pillar | 

 trachea 



L hepatic vein 



cardinal vein 



Wolffianfold 

 dorsal pillar 

 cesophagus 



ventral pillar 

 r. hepatic vein 



sinus venosus 

 right auricle 



Iruncus arteriosus 



canalis auricularis 

 FIG. 295. SAME MODEL AS SHOWN IN PIG. 294, IN TKANSVEKSE SECTION. 



The figure shows the cranial half of the model. The broad band containing the hepatic veins is the 

 septum transversum thickened by the liver-trabeculee. The pleuro- peritoneal membrane is seen on 

 each side stretching between its dorsal and ventral pillars. 



in a dorso-ventral direction in the lateral body-walls into the septum transversum, 

 in which they then take a transverse direction. With the expansion of the body- 

 wall the dorso-ventral portion of each vein comes to lie in a fold projecting into 

 the coelom. Owing to the backward displacement of the septum transversum 

 and its related parts, the veins come to take an increasingly antero-posterior direc- 

 tion. The lateral folds, and the anterior edge of the septum transversum which 

 is continuous with them, are necessarily brought from the transverse into a coronal 

 plane. The terminal portions of the Cuverian veins now run parallel to one 



