214 ANATOMY OF VERTEBRATES. 



the bird is uuable to raise itself in flight. If the trachea be tied, 

 and an opening be made into the humerus, the bird will respire 

 by that opening for a short period, and may be killed by inhaling 

 noxious gases through it. If an air-bone of a living bird, simi- 

 larly perforated, be held in water, bubbles will rise from it, and a 

 motion of the contained air will be exhibited, synchronous with 

 the motions of inspiration and expiration. 



The proportion in which the skeleton is permeated by air 

 varies in different Birds. In the Alca impenjiis, the Penguins 

 (^Aptenoch/tes) and the Apteryx, air is not admitted into any of 

 the bones. The condition of the osseous system, therefore, which 

 all birds present at the early periods of existence, is here retained 

 through life. 



In the laro'e Struthious Birds, which are remarkable for the 

 rapidity of their course, the thigh-bones and bones of the pelvis, 

 the vertebral column, ribs, sternum and scapular arch, the 

 cranium and lower jaw, have all air admitted into their cavities or 

 cancellous structure. In the Ostrich the humeri and other 

 bones of the wings, the tibias and distal bones of the legs, retain 

 their marrow. INIost Birds of Flight have air admitted to the 

 humerus : the Woodcock and Snipe are exceptions. The Pigeon 

 tribe, with the exception of the Crown Pigeon, have no air in the 

 femur, which retains its marrow. In the Owls also the femur is 

 flUed wdth marrow ; but in the Diurnal Birds of Prey, as in 

 almost all other Birds of Flight, the femur is filled with air. In 

 the Pelican and Gannet the air enters all the bones with the 

 exception of the phalanges of the toes. In the Hornbill even 

 these are permeated by air. 



Hunter has given the following characters as distinguishing the 

 bones wdiich receive air. They may be known — ' first, by their 

 less specific gravity ; secondly, by their retaining little or no oil, 

 and, consequently, being more easily cleaned, and when cleaned, 

 appearing much whiter than common bones : thirdly, by having 

 no marrow, or even any bloody pulpy substance in their cells ; 

 fourthly, by not being in general so hard and firm as other bones ; 

 and, fifthly, by the passage that allows the air to enter the 

 bones.'' Tlie openings by which the air penetrates the bones, 

 may be readily distinguished in the recent bone, since they are 

 not filled up by blood-vessels or nerves, but have their external 

 edges rounded off. 



In the dor:ial vertebrae the air-orifices are small, numerous, and 

 irregular ; situated along the sides of the bodies, and the roots of 



' xciv p. 1 78. 



