274 COMPARATIVE ZOOLOGY OF CHORDATES 



omental cavity. This pocket communicates with the general 

 perivisceral cavity by an opening the front edge of which is 

 formed by the hind border of the accessory mesentery and 

 pulmo-hepatic ligament. Along this edge runs the inferior 

 vena cava. The hind edge of the opening is formed from the 

 dorsal mesentery and lesser omentum and along the latter run 

 the bile-duct, the portal vein and the hepatic artery. The 

 opening is the primitive foramen of Winslow. 



In the birds, the conditions start similarly with regard to 

 the accessory mesenteries and the pulmo-hepatic ligaments, 

 but the latter in addition are connected to the side wall of the 

 perivisceral cavity. In this manner the oblique septa are 

 formed, which separate a pair of dorso-lateral pleural cavities 

 (into which the lungs project) from the perivisceral cavity. 

 The latter is further obstructed by the post-hepatic septum 

 which connects the gizzard to the floor of the cavity. 



The mammals are characterised by the presence of the 

 diaphragm. This is formed partly from the transverse 

 septum which separates the pericardial cavity from the rest, 

 and partly from ccelomic epithelium in connexion with the 

 mesentery and the folds in which the kidneys hang down from 

 the roof of the perivisceral cavity. By this means the pleural 

 cavities (already separated from the pericardial by the trans- 

 verse septum) are separated from the remainder of the peri- 

 visceral cavity. But it is important to note that the pleural 

 cavities of the mammal are formed in an altogether different 

 manner from those of birds. 



Anterior to the diaphragm in mammals therefore there are 

 three ccelomic spaces : the pericardium and the two pleural 

 cavities. The diaphragm contains striped myotomic muscles 

 innervated by the phrenic nerves. Originally the heart and 

 the transverse septum were far forward in the body in the region 

 of the neck, from the spinal nerves of which the phrenic nerve 

 arises. Later in development the heart and transverse septum 

 become shifted backwards, with the result that the phrenic 

 nerves have long courses to run from their origin in the neck 

 to the diaphragm. The diaphragm is pierced by the gut, 

 aorta, and the inferior vena cava. 



