A HISTORY OF VERTEBRATES: BIRDS AND MAMMALS 



473 



the skeleton of mammals. The bones are very strong because most of the 

 bone substance is located at the periphery of the bone where it gives 

 better structural support. A bird bone may be compared to a metal tube, 

 which is more resistant to certain types of stress than a metal rod of 

 equal weight. The rod would be much narrower and could be bent more 

 easily than the tube. Many bird bones are further strengthened by in- 

 ternal struts of bone arranged in a manner similar to the struts inside 

 the wing of an airplane. 



The skull is notable for the large size of the cranial region, the large 

 orbits and the toothless beak. The neck region is very long, and the 

 cervical vertebrae are articulated in such a way that the head and neck 

 are very mobile. Since the bird's bill is used for feeding, preening, nest 

 building, defense and the like, freedom of movement of the head is very 

 important. The trunk region, in contrast, is shortened and the trunk 

 vertebrae are firmly united to form a strong fulcrum for the action of 

 the wings and a strong point of attachment for the pelvic girdle and 

 hind legs. The hind legs bear the entire weight of the body when the 

 bird is on the ground. In the pigeon, thirteen of the more posterior 

 vertebrae (some of the trunk, all of the sacral and some of the caudal ver- 

 tebrae) are fused together to form a synsacrum with which the pelvic 

 girdle is fused. Several free caudal vertebrae, which permit movement 



CavpometaczLrpus — 



Cranium 

 i-Oi'bit 



PKala.n§cs 

 Digib B 

 Digit C 



Cei'vica.1 

 vertebra 



Pygosbyle^. 



External 

 ares 



Beak 



Free 

 caudal 

 verte.br a 



IschiuHT 

 Pubis 

 Famui 



Phalanges 



Figure 24.4. Skeleton of a pigeon. The distal part of the right wing has been 

 tted. (Modified after Heilmann.) 



