34 



A HISTORY OF BIRDS 



sents a very striking resemblance to that of the Dinosaurian 

 reptiles ; and the shoulder-girdle no less emphatically bespeaks 

 a reptilian descent. The hind-limb again is built on a completely 

 reptilian model, and this is seen in the peculiar character of the 

 ankle-joint, and the nature of the tarso-metatarsal segment. 

 Both in reptiles and birds the joint between the foot and the 

 shank of the leg is formed by a hinge passing between the two 

 rows of ankle-bones, and not, as in the mammals, by a hinge 

 between the shank and the first or uppermost row. In the adult 

 bird, however, no separate ankle-bones are traceable. To find 

 these the embryo, or at least the nestling, must be examined. 

 In the former several distinct tarsals can be made out, but in 



the nestling these are represented 

 (a) by a mallet-shaped bone, and 

 (d) by a flat disc. The first, or 

 astragalus, fits on to the end of the 

 shank, up the front of the shaft of 

 which it sends a short spur ; in a 

 few weeks after hatching this as- 

 tragalus fuses completely with the 

 end of the shank, when all trace of 

 its former existence is lost. The 

 second is applied to the surfaces of 

 the three metapodial or foot-bones, 

 and this similarly fusing therewith 

 disappears rapidly as an indepen- 

 dent element. This plate, as may 

 be seen in the embryo, is made up of 

 distinct elements, but these rapidly 

 coalesce to form the plate, which 

 in turn disappears to form the top 

 of the " cannon-bone ". 



This " cannon-bone " or tarso- 

 metatarsus — the " tarsus " or scale- 

 covered portion of the leg, in works on Ornithology — in the 

 embryo, or nestling bird, is seen to be made up of three sepa- 

 rate shafts or rods, answering to the similar bones extending 

 from the ankle to the base of the toes in the human foot, for 

 example, and consequently corresponding in number to the 

 number of the toes. The dividing lines in the nestling bird 



III. io. — The End of the Shank, 

 AND Middle Bones of the 

 Foot (Tarsometatarsus) of 

 A Young Bird (B, D) Com- 

 pared WITH THE SAME BONES 



IN AN Adult Reptile (Dino- 

 saur) (A, C) 

 A. = Astragalus. 



