THE HIND-LIMB IN BIRDS. 



253 



The extremity of the cnemial process in Struthio and JRhea 

 is ossified as an epiphysis ; and, in young birds, the whole of 

 the distal articular end of the bone is separated from the rest 

 by a suture, and also appears to be an epiphysis. But it is, 

 in fact, as Prof. Gegenbaur has proved, the proximal division 

 of the tarsus (apparently representing only the astragalus of 

 the other Vertebrata), which exists in the embryo as a separate 

 cartilage, and, as it ossifies, anchyloses with the tibia. The 

 so-called tibia of a bird is therefore, properly speaking, a tibio- 

 tarsus (see p. 226, Fig. 87, A, and Fig. 88). 



In all birds, even in Archceopteryx, tlie fifth digit of the 

 pes remains undeveloped; and the second, third, and fourth 

 metatarsals are anchylosed together, and, by their proximal 

 ends, with a bone, which is a distinct cartilage in the foetus, 

 and represents the distal division of the tarsus. Thus a tarso- 

 metatarsus is formed. The distal ends of the metatarsals re- 

 main separate, and offer convex articular surfaces to the proxi- 

 mal phalanges of the digits. 



Fio. 88 —The distal end of the 

 left tibia (T*.) vrtth the 

 astragalus (^s.) detached, 

 of a young Fowl. Viewed 

 from in front, and from the 

 onter side. 



Fig. 89. — The right tarso-metatarsus 

 of a Fowl, consisting of three 

 digits, ii., iii., iv., anchylosed with 

 one another, and with the osseous 

 core of the spur. A., front aspect ; 

 B., inner aspect. 



In the Penguins, large apertures lie between the several 

 metatarsals of the adult tarso-metatarsus ; and, in other birds, 

 more or less considerable passages persist between the middle 

 and the lateral metatarsals proximaUy, and the middle and the 



