196 LABORATORY MANUAL FOR VERTEBRATE ANATOMY 



posterior wall of the septum, which thus becomes the peritoneal covering or serosa of the liver. 

 Later, the region where the liver bulges from the septum narrows down on dorsal and lateral 

 sides, leaving anterior and ventral connections between the liver and the septum (Fig. 48). 

 The anterior connection is named the coronary ligament and is, as the name indicates, a circular 

 ligament, by means of which the liver is permanently in all vertebrates suspended from the 

 transverse septum or its derivative. The ventral partition formed by the constriction of the 

 liver from the septum is the falciform ligament of the liver, which is also by virtue of its loca- 

 tion a part of the ventral mesentery (Fig. 44$, p. 158). We thus see that the transverse septum 

 has the following parts: the posterior wall of the pericardial cavity, the anterior wall of the 

 pleuroperitoneal cavity, the falciform and coronary ligaments of the liver, the serosa of the 

 liver, and the mesoderm tissue of the liver. 



In Anura and reptiles the coelom is divided in the same way as in fishes and urodeles into 

 two compartments, the pericardial and pleuroperitoneal cavities. In these groups, however, 

 the pericardial cavity is no longer situated anterior to the pleuroperitoneal cavity but has 

 descended posteriorly until it lies ventral to the anterior part of the latter cavity. This descent 

 occurs during embryonic development. In this descent the pericardial cavity necessarily 

 carries with it the transverse septum, since this septum formed the posterior wall of the cavity. 

 Furthermore, since the parietal pericardium or wall of the pericardial cavity in the lower 

 vertebrates lined the inside of the body wall, it is necessary that this membrane become split, 

 at least in part, from the body wall before the descent can take place. This happens mainly 

 by the invasion of the pleuroperitoneal cavity forward. As a result of these processes, the 

 parietal pericardium is separated from the body wall, at least in part, and forms a delicate 

 sac about the heart, the pericardial sac. The separation of the pericardial sac from the body 

 wall is complete in the Anura but in the turtle is incomplete ventrally so that the ventral wall 

 of the pericardial cavity is still in contact ventrally with the body wall (see Fig. 456", p. 160). 



In the groups under consideration the relations of the transverse septum remain the same 

 as before, but its position is now oblique, indeed almost frontal, while formerly it was trans- 

 verse (Fig. 45Q. It still presents anterior and posterior walls (which are now nearly ventral 

 and dorsal in position) and still incloses the great veins and the posterior end of the heart 

 between its walls. Its anterior (ventral) face forms part of the pericardial sac, while its 

 posterior (dorsal) face forms part of the lining of the pleuroperitoneal cavity as previously. To 

 this latter portion the liver is attached as before by the coronary and falciform ligaments. 



The posterior descent of the pericardial cavity brings a portion of the pleuroperitoneal 

 cavity dorsal to the pericardia! cavity. In this portion of the pleuroperitoneal cavity which 

 lies above the heart, the lungs grow out. 



In birds the relations of pericardial cavity and sac and of the transverse septum are the 

 same as just described. The pericardial sac is, however, only partially separated from the 

 body wall. The remainder of the coelom, corresponding to the pleuroperitoneal cavity of 

 fishes, amphibians, and reptiles is in birds further subdivided. This is accomplished by means 

 of a pair of mesenterial folds which grow ventrally from the sides of the esophagus and fuse with 

 the transverse septum (Fig. 45!) and E, p. 160). The pleuroperitoneal cavity is thus divided 

 into anterior and posterior compartments. The anterior compartment consists of the two 

 pleural cavities, one on each side of the pericardial cavity and each containing a lung. The 

 posterior compartment is named the peritoneal cavity. The lining of the pleural cavities is 

 known as the pleura; that of the peritoneal cavity as the peritoneum. The partition between 

 pleural and peritoneal cavities is called the oblique septum. Its median portion is composed 

 of the transverse septum; its lateral portions, of the two folds already mentioned which are 

 known as the pleuroperitoneal membranes. 



In mammals the coelom is divided as in birds into four compartments: the pericardial, 

 the two oleural, and the peritoneal cavities. The pericardial cavity has descended posteriorly 



