1^0 THE PmLOSOPHY 



es for tKe tranfmiffion of air to the other parts of the body*^. 

 To each of thefe perforations a diftincl membranous bag is 

 joined. Thefe bags are extremely thin and tranfparent. 

 They extend through the whole of the abdomen, are attach- 

 ed to the back and fides of that cavity, and each of tliem re- 

 ceives air from their refpetSlive 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 London, remarks, 

 that the bones of birds which receive air are of two kinds : 



* Some, as the fternuqi, ribs, and vertebrae, have their Inter- 



< nal fubflance divided into innumerable cells, whilft others, 

 « as the OS humeri and the os femoris, are hollowed out into 



* one large canal, with fometimes a few bony columns run- 



* ning acrofs at the extremities. Bones of this kind may be 



* diftinguifhed from thofe that do not receive air by certain 

 ^ marks : 1 . By their lefs fpecific gravity : 2. By being lefs 

 « vafcular, and therefore whiter : 3. By their containing lit- 



* tie or no oil, and confequently being more eafily cleaned, 



* and, when cleaned, appearing much whiter than common 



< bones : 4. By having no marrow, or even any bloody pul- 



< py fubftanee 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 v/e may readily difcover 



< holes, or openings, not filled with any fuch foft fubftanee as 



* blood-veiTels or nerves ; and it happens that feveral of 



* thefe holes are placed together, near the end of the bone 



< which is next to the trunk of the bird ; and are diftinguilh- 



* able by having their external edges rounded off ; which is 



< not the cafe with the holes through which either nerves or 



< blood-vefTels pafs into the fubftanee of the bonef . 



•This fadl feems to have been firft mentioned by the celebrated Do^or 

 Harvey. See Harvey de Generat. Animal. Exercit, 3, 

 ■J Hunter's Obfwvations on cfrtain parts of the Animal Oeconomy, page 79, 



