i8o LABORATORY MANUAL FOR VERTEBRATE ANATOMY 



the other raising the limb. This arrangement eliminates all powerful muscles 

 from the back and enables all of the wing muscles to take their origin from the 

 firm and strong sternum. 



4. The divisions of the coelom and the posterior air sacs. — Cut through the 

 ventral abdominal wall to the right of the median line. Beneath the skin are 

 the thin layers of abdominal muscles corresponding to those of mammals, and 

 internal to this the parietal peritoneum generally impregnated with streaks 

 of fat. Cut through this and extend the incision anteriorly cutting through 

 the sternum slightly to the right of the keel, keeping the scissors in contact with 

 the bone so as to avoid injuring internal parts. Spread apart the cut edges and 

 look within. 



The small cavity posterior to the sternum is the peritoneal cavity. Note 

 in it the liver dorsal to the posterior end of the sternum, the closely coiled intes- 

 tine, and to the left the large firm gizzard. From the gizzard a mesentery extends 

 to the ventral body wall to the left of the median line. This may be designated 

 the ventral ligament of the gizzard. 1 It is continuous anteriorly with the falci- 

 form ligament of the liver which extends from the median ventral region of the 

 liver to the midventral line of the body wall and inner surface of the sternum. 

 The falciform ligament and ventral ligament of the gizzard together constitute 

 a partition which divides the peritoneal cavity into a large right portion and 

 smaller left portion. This division is not found in other vertebrates. In the 

 partition courses a small vein 2 extending from the mesenteries in question to the 

 liver. 



Deflect the pectoralis major muscle on the left side of the sternum and make 

 a cut through the left side of the sternum slightly to the left of the keel. Remove 

 and discard the median piece of sternum containing the keel. 



Immediately dorsal to the sternum is situated the delicate pericardial sac 

 containing the heart. The ventral wall of the pericardial sac will probably have 

 been opened in cutting through the sternum. The heart, as in the turtle, has 

 descended posteriorly, and a pericardial sac has been formed of the anterior 

 face of the transverse septum and the parietal pericardium as described in con- 

 nection with the turtle. The space between the pericardial sac and the heart 

 is, as before, the pericardial cavity, a portion of the coelom. The pericardial 

 sac is in contact on its ventral surface with the inner surface of the sternum, and 

 anteriorly and laterally is also in contact with the inner surface of the body wall. 

 Hence, only the posterior part of the pericardial sac is freed from the bod)- wall. 



1 This is commonly called the greater omentum in texts and manuals, but since it is not at all 

 homologous with the structure so named in mammals, it is desirable that the name be dropped. The 

 ligament of the gizzard is a mesentery peculiar to birds, and arises as a secondary outgrowth from the 

 serosa of the gizzard to the ventral body wall. It is probably due to the need for additional support for 

 the heavy gizzard. 



2 This vein is named in manuals the ventral abdominal vein but it docs not appear to be homologous 

 with the vein of that name in other vertebrates. 



