THE SKELETON IN BIRDS. THE ENDOSKELETON. 331 



Except that teeth are partly dermal in origin, a dermal 

 exoskeleton is quite unrepresented in birds. 



ENDOSKELETON. 



Perhaps the most striking feature of the endoskeleton of 

 birds is its pneumaticity. In the embryo all the bones con- 

 tain marrow, but as growth proceeds this becomes replaced by 

 air to a variable extent in different forms. In all birds some 

 part of the skeleton is pneumatic. Many small birds and 

 Apteryx and Penguins among larger ones have air only in the 

 skull ; in Pigeons air is present in all the bones except the 

 caudal vertebrae, the leg bones, and those of the antibrachium 

 and manus ; in Hornbills every bone contains air. 



FIG. 62. THIRD CERVICAL VERTEBRA OF AN OSTRICH (Struthio camelus). 

 x 1. A anterior, B posterior, C dorsal view (A and B after MIVART). 



1. neural spine. 



2. neural canal. 



3. prezygapophysis. 



4. postzygapophysis. 



5. posterior articular surface of 



centrum. 



6. anterior articular surface of 



centrum. 



7. vertebrarterial canal. 



8. hypapophysis. 



