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 (metaccele) 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 metaccele. 
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 
Fic. 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 coelom; 
L, liver; P, lungs in C in pleural ccelom; 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- 
