756 COMPARATIVE ANATOMY 



A partition, the transverse septum (Fig. 348), separates the two 

 cavities. In vertebrates lower than Anura the pericardial cavity lies 

 cephalad to the pleuroperitoneal cavity, but beginning with the Anura, 

 the pericardial cavity comes to lie ventral to even the cephalic end of 

 the pleuroperitoneal cavity, because the heart and the pericardial cavity 

 descend and carry the transverse septum with them. This descent 

 causes the wall of the pericardial cavity, together with the transverse 

 septum, to form a sac the pericardial sac around the heart. 



The part of the pleurocardial cavity dorsal to the heart later be- 

 comes the pleural cavities. 



In birds and mammals the pleuroperitoneal cavity divides into an- 

 terior and posterior regions by a partition which descends from the dor- 

 sal body-wall to unite with the transverse septum. This partition is 

 known as the oblique septum in birds and the diaphragm in mammals 

 (Fig. 439). In mammals this diaphragm contains a great amount of 

 striated muscle. 



In birds and mammals the coelom has become divided into four 

 ;ompartments : one pericardial, two pleural and one peritoneal cavity. 



While a dorsal mesentery supports the digestive tract in all verte- 

 brates, the ventral mesentery is absent in the adult, except in the regions 

 of the liver and bladder. 



In mammals, the mesentery of the stomach is prolonged posteriorly 

 to become the greater omentum. An ileo-colic valve and a single caecum 

 are usually found where small and large intestine, meet in mammals, 

 although there are a few instances where there are two caeca. In some 

 edentates, in bats, in some carnivorous animals, and in many whales, 

 neither valve nor caecum are found. 



In some rodents and marsupials the caecum grows as long or longer 

 than the animal's body (it is of great value in digestion here), while in 

 man, it degenerates into the vermiform appendix, the lumen of which 

 tends to close with increasing age. 



In mammals the intestine and colon are straight tubes at first but 

 grow into folds later. 



In monotremes the rectum terminates in a cloaca as it does in the 

 Sauropsida. This condition also occurs in the young of all mammals, 

 but, in all these, the urogenital and digestive openings become separated 

 later, and a perineal fold develops between the openings. 



