OF NATURAL HISTORY. in 



almoft every part of their bodies. The lungs of birds are fo firmly- 

 attached to the diaphragm, the ribs, the fides, and the vertebrae, that 

 they can admit of very little dilatation or contradion. Inftead of 

 being impervious, the fubftance of the lungs, as well as of the dia- 

 phragm, to which they adhere, is perforated with many holes or 

 pafTages for the tranfmilfion of air to the other parts of the body *. 

 To each of thefe perforations a diftindt membranous bag is joined. 

 Thefe bags are extremely thin and tranfparent. They extend 

 through the whole of the abdomen, are attached to the back and 

 fides of that cavity, and each of them receives air from their refpec- 

 tive openings into the lungs. The cells in birds which receive air 

 from the lungs are found not only in the foft parts, but in the bones. 

 That ingenious and accurate anatomift, Mr John Hunter of Lon- 

 don, remarks, that the bones of birds which receive air are of two 

 kinds: ' Some, as the fternum, ribs, and vertebrae, have their inter- 

 nal fubftance divided into innumerable cells, whilft others, as the 

 OS humeri and the os femoris, are hollowed out into one large ca- 

 nal, with fometimes a few bony columns running acrofs at the ex- 

 tremities. Bones of this kind may be diftinguilhed from thofe 

 that do not receive air by certain marks: i. By their lefs fpecific 

 gravity : 2. By being lefs vafcular, and therefore whiter : 3. By 

 their containing little or no oil, and confequently being more eafi- 

 ly cleaned ; and, when cleaned, appearing much whiter than com- 

 mon bones : 4. By having no marrow, or even any bloody pulpy 

 fubftance in their cells : 5. By not being, in general, fo hard and 

 firm as other bones ; and, 6. By the pafTage that allows the air to 

 enter the bones, which can eafily be perceived. In the recent 

 bone we may readily difcover holes, or openings, not filled with 

 any fuch foft fubftance as blood-veflels or nerves ; and it happens 



•that 



* This faft feems to have been firfl: mentioned by the celebrated Doftor Harvey. 

 See Harvey de Generat. Animal. Exercit. 3. 



