ADAPTATIONS OF THE SKELETON 



59 



The salient features of the skeleton of the bird's leg 

 are the elongation of the different regions, the more or less 

 vertical position of the instep region, and the reduction of 

 the number of separate bones. The thigh-bone has a 

 conspicuously defined round 

 head, working deeply and freely 

 in the acetabulum ; the fibula 

 is reduced to an incomplete 

 splint, which may be partially 

 fused to the tibia ; the upper 

 or proximal row of ankle-bones 

 or tarsals has coalesced in 

 embryonic life with the lower 

 end of the tibia, forming a 

 tibio-tarsus, which is peculiar 

 to birds ; the lower or distal 

 row has coalesced with the 

 upper end of three (in the 

 ostrich, two) fused metatarsals, 

 forming a tarso - metatarsus, 

 which is also peculiar to birds. 

 Thus the adult shows no free 

 tarsal bones, but they are 

 clearly seen in the unhatched 

 bird ; and in a true, not merely 

 courtesy, chicken on the table 

 it is easy to separate an im- 

 perfectly ossified piece from 

 the lower end of the tibio- 

 tarsus and the upper end of 

 the tarso-metatarsus. These 

 separable pieces correspond to 

 the two rows of tarsal bones. 

 When a bird moves its ankle- 

 joint it is really moving the 



upper tarsals against the lower tarsals, the interest of which 

 is that it corresponds to the reptilian inter-tarsal movement. 

 This inter-tarsal ankle-joint of reptiles and birds is in 



Fig. II. — Fowl's hind-leg. From 

 a specimen. F., femur ; p., pa- 

 tella or knee-pan ; c.N., cnemial 

 crest on the tibio-tarsus (t.t.) ; 

 Fi.S the head of the fibula ; 

 FT. 2, lower end of the tapering 

 incomplete fibula ; A.j., the 

 ankle joint ; t.mt., tarsometa- 

 tarsus ; mt.i., the first meta- 

 tarsal, quite free ; i.-iv., the 

 toes. 



