THE COELOM, DIGESTIVE, AND RESPIRATORY SYSTEMS 



195 



make some concluding general remarks concerning them. Only a simplified account can be 

 given here. 



The coelom of fishes and urodeles is divided into two compartments, a small anterior 

 pericardial cavity and a much larger posterior pleuroperitoneal cavity. The former is situated 

 entirely anterior to the latter and far anterior in the body, at the level of the pharynx. The 

 partition between the two coelomic cavities in these groups is the transverse septum. This 

 septum is formed, as previously explained, for the purpose of conveying the great veins from 

 the dorsal body wall into the posterior end of the heart. It arises first as a column of mesen- 

 chyme on each side, conveying a vein into the heart; later these columns enlarge and fuse to 

 form a partition. The transverse septum is a rather thick membrane with anterior and 

 posterior walls, which inclose between them the great veins and the posterior end of the heart. 

 The septum in fishes and urodeles lies as the name implies in a plane transverse to the body 

 axis (Fig. 45.I and B, p. 160). 



The lining of the pericardial cavity is named the parietal pericardium. In fishes and 

 urodeles it also constitutes the lining of the body wall in the region under consideration. The 



intestine 



peritonea] 



cavity 



cloaca 



Fig. 48. — Diagrams to show the separation of the liver from the transverse septum and formation 

 of the coronary ligament. A, liver inclosed in the transverse septum. B, beginning of constriction of 

 the liver from the septum at x. C, completion of the constriction, leaving the liver suspended from 

 the septum by the coronary ligament. The falciform ligament is also formed at the same time by the 

 same constriction. (S and C after McMurrich's Development of the Human Body, copyright by P. Blakis- 

 ton's Son and Company.) 



parietal pericardium is continuous with the anterior face of the transverse septum, which, in 

 fact, forms the posterior part of the parietal pericardium and the posterior wall of the pericardial 

 cavity. The lining of the pleuroperitoneal cavity is the pleuroperitoneum. This is continuous 

 anteriorly with the posterior face of the transverse septum, which constitutes the anterior part 

 of the pleuroperitoneum and forms the anterior wall of the pleuroperitoneal cavity (see Fig. 



454 and B). 



The relations of the transverse septum to the liver are somewhat complicated and require 

 explanation. The liver is a diverticulum from the small intestine. It happens that at the 

 point where the liver diverticulum grows out ventrally from the intestine, the transverse 

 septum is situated. Consequently, the liver is compelled to grow out into the septum. It 

 lies at first within the mesenchyme of the septum, between the two walls of the septum. Owing 

 to the fact as already explained that the great veins enter the heart by way of the septum, the 

 liver also acquires important relations to these veins, as will be discussed in the section on the 

 circulatory system. The liver rapidly increases in size so that it can no longer be contained 

 within the limits of the septum. It consequently bulges posteriorly, carrying with it the 



