THE ORGANS OF THE INTERMEDIATE LAYER OR MESENCHYME. 569 



the lateral wall of the trunk, its terminal part is afterwards enclosed 

 in the mediastinum. 



After the closure of the pericardia! sac, the narrow, tubular 

 thoracic cavities (fig. 314 Mi) continue for a time to remain in 

 communication behind with the abdominal cavity. The fundaments 

 of the lungs (ly] meantime grow farther into them, and their tips 

 finally come in contact with the upper surface of the liver, which 

 also has now become larger. Then a closure is effected at these 

 places also. From the lateral and posterior walls of the trunk 

 project folds (the pillars of USKOW), which fuse with the septum 

 transversum, and thus form the dorsal part of the diaphragm. One 

 can therefore distinguish a ventral older part and a dorsal younger one. 



As GEGENBAUR points out, this explains the course of the phrenic nerve, 

 which runs in front of [ventral to] the heart and lungs and approaches the 

 diaphragm from in front. 



Occasionally the fusion of the dorsal and ventral fundaments is 

 interrupted on one side. The consequence of such arrested develop- 

 ment is a diaphragmatic hernia i.e., a permanent connection between 

 abdominal and thoracic cavities by means of a hernial orifice, through 

 which loops of the intestine can pass into the thoracic chamber. 



When the four large serous spaces of the body have been com- 

 pletely shut off from one another, the individual organs must still 

 undergo extensive alterations of position, in order to attain their 

 ultimate condition. The pericardial sac at first takes up the whole 

 ventral side of the breast, and over a large area is connected with 

 the anterior wall of the thorax and with the upper wall of the 

 diaphragm. Moreover, the latter is united with the liver along its 

 whole under surface. The lungs lie hidden in narrow tubes at the 

 dorsal side of the embryo. 



There are two factors that come into the account in this con- 

 nection (fig. 315). With the increase in the extent of the lungs (/(/), 

 the thoracic cavities (pl.p) extend farther ventrally, and thereby 

 detach the wall of the pericardial sac (jpc), or the pericardium, on 

 the one hand from the lateral and anterior walls of the thorax, and 

 on the other from the surface of the diaphragm. Thus the heart (ht\ 

 with its pericardial sac, is displaced step by step toward the median 

 plane, where, together with the large blood-vessels (ao), the oeso- 

 phagus (al), and the bronchial tubes, it helps to form a partition 

 the mediastinum -between the greatly enlarged thoracic cavities. 

 In front the pericardial sac then remains in contact with the wall of 



