CCELOM. 



123 



transverse partition, the septum transversum, attached to the anterior 

 wall of the liver, which cuts off an anterior pericardial cavity, con- 

 taining the heart, from the posterior part (metacoele) of the body cavity. 

 In many lower vertebrates the septum is not complete, but one or more 

 openings (pericardio-peritoneal canals) connect the pericardium 

 with the metacoele. 



In the mammals a second partition, the diaphragm (p. 135), cuts 

 off another pair of (pleural) cavities from the metaccele. Traces of 

 similar structures occur as low as the amphibia; their homology with 

 the mammalian diaphragm is not always certain, but in some cases the 



FIG. 132. Diagram showing the relations of the ccelomic cavities (black) in A, fishes, 

 B, amphibians and sauropsida; and C, in mammals; H, heart in pericardial calom; 

 L, liver; P, lungs in C in pleural coelom; S, septum transversum; D, diaphragm. 



parts concerned have the same nerve supply. The development of the 

 diaphragm is very complicated and can be stated here only in outline. 

 It involves in part the septum transversum, in part is a new formation. 

 At first a part of the metaccele extends forward, dorsal to the pericardial 

 cavity and alimentary canal, and into this the lungs protrude as they 

 are developed. Then a pair of muscular folds arise from the dorsal 

 surface of the metaccele, posterior to the lungs; these grow downward 

 until they meet the septum adjacent to the attachment of the liver, 

 cutting off a pair of pleural cavities containing the lungs, from the 

 rest of the metaccele, now known as the peritoneal cavity. With 

 increase of the lungs in size the pleural cavities increase, insinuating 

 themselves laterally beween the pericardium and the body wall, and 

 eventually reaching the ventral side, where the two are separated by 

 their two walls, the ventral mediastinum. From the original folds 

 the dorsal muscles of the diaphragm are derived; the ventral come 

 from the rectus muscles of the ventral abdominal wall. The dia- 



