EXDObKELETONv AXIMALS. 43 



femoris, are hollowed out into one large canal, sometimes with a 

 few bony columns running across at the extremities. Bones of 

 this kind may he distinguished from those that do not receive air 

 by several marks : 1st, by their less specific gravity ; 2ndly, by 

 being less vascular than the others, and therefore whiter ; 3rdly, 

 by their containing little or no oil, and consequently being more 

 easily cleaned, and appearing much whiter when cleaned than com- 

 mon bones ; 4thly, by having no marrow, nor a bloody pulpy sub- 

 stance, even in their cells ; othly, by their not being, in general, 

 so hard and firm as other bones ; those of some birds are so soft, 

 that they can be squeezed together with the finger and thumb ; 

 however, the bones of the extremities have very solid sides : 6thly, 

 the passage by which the air gets into the bones can be easily per- 

 ceived, even in cleaned bones. Generally there are several holes 

 placed together near the end of the bone which is next the trunk 

 of the bird ; and distinguishable by having their external edges 

 rounded off, which is not the case with those holes through which 

 either nerves or blood-vessels pass into the substance of the bone." 

 An Account of certain Receptacles of Air in Birds, by John 

 Hunter, Phil. Trans. 1 774, Ixiv. p. 205. 



" In the common fowl no air appears to enter any bone except 

 the os humeri." Hunter, On certain Receptacles of Air in Birds, 

 ut sujjra, p. 210. 



Hunterian. 



A. 194. A section of the upper extremity of the femur of an 

 Ostrich, showing its cellular structure for containing air. 

 A bristle is passed through the aperture by which the 

 air enters. 0. C. 215. Hunterian. 



A. 195. A section of the upper extremity of the femur of a young 

 Ostrich, showing its cellular structure, and the aperture by 

 which the air enters the cavity of the bone. 0. C. 216. 



Hunterian. 



A. 196. A section of the lower extremity of the femur of a young 

 Ostrich, showing its cellular structure for containing air. 

 0. C. 217. ffunterian. 



A. 197. Cranium of a Koala (Phascolan-tus ciuereus). A large 

 alisplienoid bulla ( = thin-walled projecting bony chamber 



