Skeleton of Common Pigeon. 17 



5. Skeleton of Common Pigeon {Columba Livia). 



The relations held by the hind limbs and the pelvic bones to the 

 rest of the body serve at once to distinguish the Bird from all other 

 classes of animals. The peculiarly distinctive character of its 

 skeleton as compared with that of the living and extinct animals 

 which it most essentially resembles, viz. Reptiles, depends in the 

 second place upon the relative proportions and the different degrees 

 of mobility of the cervical, dorsal, sacral, and caudal vertebrae re- 

 spectively ; upon the great development of the sternum relatively 

 to the entire body, of the ilium anteroposteriorly relatively to the 

 vertebral column, and of the tibia relatively to the bones of the 

 whole hind limb ; and, thirdly, ujKtn the greater proportion of in- 

 organic elements, the greater tendency to anchylose with their 

 fellows, and the gi'cater pneumaticity which the individual bones 

 ordinarily possess. Peculiarities less striking, but not less distinc- 

 tively and universally avian, are furnished us in the hand and foot. 

 Parts of the carpus and tarsus are in either extremity fused with 

 three of the bones of the segment, placed distally to them into 

 a single bone, which directly supports all, or, in the foot, all but 

 one, of the terminal digits. In the foot the fifth or outer digit is 

 never present ; in the hand neither fourth nor fifth, and in it the 

 outer, the homologue of the middle finger, is never armed with 

 a claw. In very nearly all Birds, and in very few and in some cases 

 in no other animals, may the following peculiarities be noted : — 

 The articulations of the vertebrae when looked at from in front pre- 

 sent the procoelian appearance, the anterior articular surface being 

 concave from side to side, though convex, and therefore saddle- 

 shaped, from before backwards. The basitemporals form a second- 

 ary floor to the cranium, concealing the junction of the basisphenoid 

 to the basioccipital. 



The caudal vertebrae vaiy little in number, are not numerous, 

 are mobile, and are terminated by a single bone compounded of 

 several fused segments and of a ploughshare shape. The ossa ilii 

 and ischii coalesce posteriorly to the acetabulum to form a foramen 

 more or less homologous with the sacroischiatic notch of anthropo- 

 toray. Finally, in most birds most or all of the vertebrae, with the 

 exception of the atlas, and also sometimes of the caudal vertebrae, 



c 



