536 



CHORDATA 



In the hind limbs occur conditions similar to those in the ungulates. 

 The weight of the body makes it necessary that the simplification found 

 in the wing should be repeated in the lower leg and foot, and that the 

 numerous bones usually occurring in these regions be replaced by one 



B 



C 



FIG. 587. A, leg of Buteo vulgaris. a, femur; b, tibio tarsus ; b', remains of fibula; 

 c, tarsp-metatarsus, c', same, front view; d-d 3 , toes. B, lower leg of bird embryo; 

 i of lizard. /, femur; /, tibia; p, fibula; ts, tarsales of first row (talus); //, tarsales'of 

 second row; between these intertarsal joint; I-V, digits (from Gegenbaur). 



to support the pressure (fig. 587). Therefore the fibula, well developed in 

 the embryo (B), becomes reduced to an inconspicuous rudiment; the 

 metatarsals, distinct in. the embryo (B), fuse to a single tarso-metatarsus 

 (A , r ) , which has below as many articular surfaces as there are toes (since 

 the fifth toe only appears in the embryo, at most four, in some three or 

 even two, d-d'"). At the same time the tarsals disappear by fusion with 

 adjacent parts. Even in reptiles (Q a part of the tarsals unite with the 

 bones of the shank, and the remainder with the metatarsals; in the birds 

 the union is completed, the proximal series fusing with the lower end of the 

 tibia to form a tibio-tarsus, the distal with the metacarpus to form the 

 tarso-mctatarsus, in this way producing the intertarsal joint so characteristic 

 of birds. 



