MULLER] THE AIR-SACS OF THE FIGEOX 387 



right and left abdominal sacs is the reverse of that of the sacci 

 intermedii posteriores. In the female, where only the left-hand one 

 of the two oviducts is developed, the difference in size of the abdom- 

 inal sacs is still greater than in the male. 



The abdominal sacs occupy the space limited by the external bor- 

 ders of the posterior parts of the lungs, the sacci intermedii pos- 

 teriores, the peritoneum, the ilium, the ischium, the pubis, the vena 

 cava, the iliac arteries, the urogenital organs, the organs of the 

 digestive tract, and the roof of the abdominal cavity. In life these 

 sacs_ are usually only partially distended, more extensive, bladder- 

 like air-spaces being found only at the sides of the cloaca. Every- 

 where else these sacs and their diverticula are in life narrow or 

 altogether closed folds, containing hardly any air, or no air at all. 

 In injected specimens these parts of the abdominal sacs are often 

 distended beyond their normal size. In the illustrations accompany- 

 ing this paper these sacs appear larger than they are, the narrow, 

 curved, fold-like diverticula, of which they consist, naturally looking 

 more like thick solid bodies than thin sheets. 



The dorsal portion (roof) of the outer abdominal wall is in imme- 

 diate contact with the abdominal sacs in only one place on each side. 

 On examining the back of a skinned pigeon, one finds that, in the 

 angular space between the pars renalis of the iliac bone and the 

 musculus levator coccygis on either side, a portion of the wall of the 

 abdominal sacs (fig. 6, SAD, SAS) is visible. At these points the 

 sacs are covered by the skin alone, both muscles and bones being ab- 

 sent. In injected specimens the sacs bulge out considerably in these 

 two places, and often form nearly hemispherical protrusions. These 

 are, of course, mere artifacts produced by the abnormal injection- 

 pressure. I might call the attention of physiologists to these por- 

 tions of their walls, because with some precaution a canula can be 

 introduced here into the system of the air-sacs without injuring the 

 muscles or any other important parts. 



The abdominal sacs are very differently arranged in different 

 species of birds, and are, as Baer (1896, p. 40) has demonstrated in 

 the cassowary, entirely absent in Ratites. 



The Diverticula of the Abdominal Sacs 



The abdominal sacs form several diverticula. These lie partly 

 within, partly without the abdominal cavity. Many of the intra- 

 abdominal ones, as, for example, the "duodenal cell" of Owen 

 (1866, p. 212), are, as stated above, variable folds of the principal 

 sac that have insinuated themselves between the viscera. There are, 



