AVES. 119 



the fibula, which is small. The tarsus is early fused with the metatarsus, the 

 latter consisting generally of one long bone. The digits vary in number ; the 

 domesticated birds usually have three anteriorly and the hallux postero-inter- 

 nally ; the outermost has five phalanges, the next four, the next three, while the 

 hallux has only two. The spur, a horny projection from the metatarsus, is 

 sometimes counted as a digit. 



The bone tissue of birds is exceedingly compact and hard, and white in 

 colour ; and some of the bones are pneumatic, or contain air instead of marrow ; 

 notably these are the bones of the skull, the sternum, and the proximal bones 

 of the limbs. There is great variety in the amount of pnemnaticity possessed 

 by the skeleton in different species, but it is not necessarily guided by the 

 flying power of the animaL 



