296 



THE ANATOMY OF VEETEBRATED ANIMALS. 



Fig. 88. 



-T6. 



whieli exists in the embrjo as a separate cartilage, and, as 

 it ossifies, ankyloses with the tibia. The so-called tibia of 

 a bird is therefore, i3roperly speaking, a tibio-tarsus (see 

 p. 263, Fig. 77, A, and Fig. 88). 



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

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

 metatarsals are ankylosed together, 

 and, by their proximal ends, with a 

 bone, which is a distinct cartilage in 

 the fcetus, and represents the distal 

 division of the tarsus. Thus a tarso- 

 metatarsus is formed. The distal ends 

 of the metatarsals remain separate, and 

 offer convex articular, surfaces to the 

 proximal phalanges of the digits. 



In the Penguins, large apertiires lie 

 between the several metatarsals of the 

 adult tarso-metatmsiis ; and, in other 

 birds, more or less considerable pas- 

 sages persist between the middle and 

 the lateral metatarsals proximaUy, and 

 Fowl. Viewed from the middle and the outer, distally. In 

 in front, and from ^^ogt birds, the middle metatarsal does 

 the outer side. , . n , . i , 



not remain parallel with the others, but 



its proximal end incUues a little backwards, and its distal 

 end a little forwards. Hence the two apertures on each side 

 of its proximal end may lie at the bottom of a fossa, or run 

 into one, in front, while they remain distinct behind. 



Again, in most birds, the posterior face of the proximal 

 end of the middle metatarsal, and the adjacent sm-face of 

 the tarsal bone, grow out into a process, which is commonly, 

 but improperly, termed " calcaneal." The inferior surface 

 of this hypo-tdmtis is sometimes simply flattened, sometimes 

 traversed by grooves or canals, for the flexor tendons of the 

 digits. 



When a hallux exists, its metatarsal bone is usually 

 incomplete above, and is united by ligament to the inner, 

 or the posterior, surface of the tai-so-metatarsus. In the 



As 



Fig. 88.— The distal 

 end of the left tibia 

 (Tb.) with the as- 

 tragalus (As.) de- 

 tached, of a young 



