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birds. In the Barn Owl (S. flammed) the upper part of the chest 

 is closed (as in most birds) by a tough membrane ; in the thoracic 

 cavity are two anterior and two posterior sacs on each side, and a 

 middle cavity, which may be called the sternal sac. The inferior 

 boundary of these sacs is a transverse duplicature at the base of the 

 heart. In the upper part of the abdomen are two large sacs sur- 

 rounding both lobes of the liver, and a triangular sac between them. 

 On each side is a long anterior sac, the left extending from the lung 

 above, having the stomach and gullet on the inner side and the ribs 

 on the outer, bounded below by a membranous expansion extending 

 from the last ribs to the abdominal muscles. On the right side 

 nearly the same arrangement prevails. Posterior to the last de- 

 scribed are two large sacs covering the kidneys, and extending to 

 the coccyx. 



In the abdomen the apertures in the lungs, by which air escapes 

 into these cavities, are seated below the membranous diaphragm on 

 each side ; they are best seen by inflating the trachea when the bird 

 is under water ; but in some birds, the geese for example, the aper- 

 tures are so large, that they are readily seen without inflation. In 

 the Long-eared Owl (S. otus), the Tawny Owl (S. aluco), Marsh 

 Harrier (F. ceraginosus) , and Common Kite (F. milvus), there is 

 nearly the same arrangement. In the Gulls (Laridee), which, 

 judging from those dissected, have no air in the bones, these sacs 

 are very large, and the bodies of these birds may be blown out to a 

 great size. In the Pelican (P. onocrotalus), they are comparatively 

 larger than in any bird I have examined. 



These cells are all readily distended by inflation through the 

 trachea, and when one of them is punctured the others become lax. 

 If the inflation is made through the femur in one of the Falconidae, 

 the air escapes by the trachea, but I have not succeeded in inflating 

 the abdominal cells through the humeral aperture ; numerous ex- 

 periments, however, will be required before one can speak positively 

 upon this subject. 



The aperture by which the air is admitted into the humerus is 

 seated upon the upper and inner part of the head of this bone. It 

 is of a rounded or oval form, sometimes consisting of a single open- 

 ing, and in other instances, especially in the smaller birds, of several 

 small perforations in a thin layer of bone. In the Black Swan (C 

 niffer) there is a curious net-work at the entrance, consisting of 

 eighty or a hundred openings. In the Golden Eagle (A. chrysa'eta) 

 the external aperture is large, with numerous small perforations. In 

 the Snowy Owl (S. nyctea) it is of a circular form, with a few small 

 openings through a thin layer of bone. In the thigh bone this aper- 

 ture (when present) varies in shape : in some of the Owls two small 

 openings exist, with a ridge between them ; in the Golden Eagle 

 and in most of the Falconida? I have examined, it is of large size. 



I have examined the humeral aperture in many young birds, 

 Hawks, Owls and Magpies, when full-fledged, and have found it 

 closed by a thick covering, the spot being indicated by the bloody 

 appearance of the membrane. In these young birds, which in the 



