MULLER] THE AIR-SACS OF THE PIGEON 385 



Irregularities in the structure of these sacs are seldom met with. 

 Once I found near the ostium a few septa that did not, indeed, 

 project far into the sac, but still divided its basal part into several 

 chambers open towards its principal space. The ostium was situated 

 in the largest of these. In another case I found in the left sac a 

 rather stout transverse membrane pierced in the middle by a round 

 hole. This approaches the structure of these sacs in other birds. 

 In Steatornis and Platalea, for instance, there are two such mem- 

 branes dividing the cavity of the sac into three parts. 



The Sacci Intermedii Posteriores 



According to Bertelli these sacs appear in the developing chick 

 on the sixth day of incubation. They (figs. 11 and 12, SIP) arise 

 in the right and left ostium intermedium posterius, and are, in the 

 pigeon as in other birds, asymmetrical, the left sac reaching farther 

 back and being larger than the right. But even the left sac is not 

 so large as either of the sacci intermedii anteriores. The left saccus 

 intermedins posterior extends beyond the last rib into the abdominal 

 cavity ; its abdominal portion occupies a narrow slit-like space lying 

 against and partly covering the stomach. When the stomach is 

 pressed against the external abdominal wall this part of the air-sac 

 is undoubtedly compressed and the air driven out of it. The shorter 

 right sac extends but little or not at all beyond the last rib* In a 

 collapsed condition both sacs give the impression that they termi- 

 nate with an apex attached to the last rib. Owen (1866, p. 212) 

 and Baer (1900, p. 438) describe these sacs as pyramids with bases 

 resting upon the posterior wall of the sac in front and with an apex 

 extending to the pelvic cavity. I have never seen an apex of this 

 sort in good preparations of the air-sacs of the pigeon. The posi- 

 tion of these sacs and the way they are attached to the surrounding 

 structures make it appear not improbable that they might assume 

 such a shape when partly evacuated. The sacci intermedii pos- 

 teriores are in contact with a small portion of the lungs and the wall 

 of the thoraco-abdominal cavity, cover a part of the liver, the abdom- 

 inal sac, and the intestine, and, on the left side, a part of the stomach. 

 They are not connected with any other sac nor with any pneumatic 

 bones. When the body cavity is opened, they do not completely col- 

 lapse, because their walls are for the most part attached to the 

 surrounding structures. In most other birds they have the same 

 shape as in the pigeon. Sometimes, as in Anas and Phccnicopterus, 

 they are considerably larger, reaching in the latter species as far as 

 the anus. 



