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and alfo fhew wherein thofe bones which receive 
air, differ from fuch as do not. 
The air-cells, which are found in the foft parts of 
birds, have no communication with the cavity of 
the common cellular membrane of the body /ome 
of them communicate immediately with one ano- 
ther ; and all of them may be faid to have a com- 
munication together, by means of the lungs, as 
a common centre. Some of thefe are bags placed 
in larger cavities, fuch as the abdomen ; others are fo 
lodged in the interfaces of parts, that they would, at 
firft, appear to be the common connecting mem- 
brane, as about the bread, axilla , &c. 
7 'hey are of very different fizes, juft as beft fuits 
the circumftances of the particular parts, where they 
are placed. 
The bones which receive air are of two kinds ; 
fome, as the Jlernum , ribs, and vertebrae, have 
their internal fubftance divided into innumerable 
cells ; whilft others, as the os humeri and the os fe- 
moris , are hollowed out into one large canal, fome-- 
times with a few bony columns running acrofs, at 
the extremities. Bones of this kind may be diftin- 
guifhed from thofe that do not receive air, by feveral 
marks; id, by their lefs fpecific gravity; adly, by 
being lefs vafcular than the others, and therefore 
whiter ; 3diy, by their containing little or no oil, and 
confequently being more eafily cleaned ; and appearing 
much whiter, when cleaned, than common bones; 
4thiy, by having no marrow, nor a bloody, pulpy 
lubflance, even in their cells; 5 th ly, by their not 
being, in general, fo hard and firm as other bones ; 
thofe of fome birds are fo foft, that they can be 
fqueezed 
