THE COELOM, DIGESTIVE, AND RESPIRATORY SYSTEMS 181 



From the points where the pericardia! sac meets the lateral body wall a mem- 

 branous partition extends obliquely posteriorly on each side. This partition is 

 called the oblique septum. It contains a large air sac. It stretches across from 

 the lateral body wall to that part of the pericardial sac which is derived from the 

 transverse septum, and thus divides the pleuroperitoneal cavity into anterior and 

 posterior portions. That part of the original pleuroperitoneal cavity left anterior 

 to the oblique septum consists of the two pleural cavities, one on each side of 

 the pericardial cavity. That part of the pleuroperitoneal cavity posterior to the 

 oblique septum is the peritoneal cavity, already mentioned. The oblique septum 

 is produced by a pair of mesenterial folds which arise one on each side of the 

 esophagus and grow ventrally, eventually fusing with the transverse septum 

 and with each other (Fig. $$D and E, p 160). 



Inside of the oblique septum inclosed between its anterior and posterior walls 

 is a large air sac, the posterior intermediate air sac. Immediately anterior to 

 this, lying to each side of the heart, is the small anterior intermediate air sac. 



In the peritoneal cavity cut through the falciform ligament and ligament of 

 the gizzard at their line of attachment to the ventral body wall. On either side 

 of the viscera and slightly dorsal to them find the large abdominal air sac. 



From the foregoing account it is seen that the coelom of birds is divided 

 into four compartments, the pericardial cavity, the two pleural cavities, and the 

 peritoneal cavity. 



5. The peritoneal cavity and its contents. This cavity has already been 

 mentioned. As in other vertebrates it is lined by the parietal peritoneum 

 which is deflected at certain points to form mesenteries and which continues over 

 the surface of the viscera as the visceral peritoneum. 



The viscera of the peritoneal cavity may now be studied in more detail. At 

 the anterior end is the large liver, consisting of right and left lobes, the former 

 the larger. The pericardial sac rests between the two lobes of the liver. The 

 liver is attached to the pericardial sac (that portion of it derived from the trans- 

 verse septum) by the coronary ligament. The falciform ligament of the liver 

 was already noted and severed. To the left and slightly covered by the left lobe 

 of the liver is the gizzard. On raising the left lobe of the liver the gastrohepatic 

 ligament will be noted passing between the gizzard and the liver. The mesogaster 

 connects the gizzard with the dorsal body wall. The ventral ligament of the 

 gizzard was already noted and cut. On breaking through the gastrohepatic 

 ligament the soft proventriculus will be found extending anteriorly from the 

 gizzard dorsal to the liver. Proventriculus and gizzard together correspond to 

 the stomach of other vertebrates; they are specialized regions of the stomach 

 correlated with the absence of teeth. From the stomach, at the place where 

 proventriculus and gizzard join, the small intestine arises. The first portion of 

 this, the duodenum, makes a long U-shaped loop posteriorly. The beginning of 

 the duodenum is attached to the right lobe of the liver by the hepatoduodenal 



