SKELETON, 119 
markable for the great development of the fifth digit (elongation of the 
phalanges) as a support for the wing; the other digits are more normal. 
Fic. 128.—Hind leg of snapping turtle (Chelydra) showing intratarsal joint at 7 h, 
humerus, 7, radius; #4, ulna; /-V, digits. 
The wings of birds (fig. 55) are even more modified. Until the 
carpus is reached the structure is approximately normal, but the carpal 
bones are greatly reduced by fusion, while the metacarpals and digits, 
extensively modified, number only 
three. Developmentshows thatthe 
first digit is entirely lost and that the 
fifth metacarpal, which is present in 
the embryo, fuses early with the 
fourth, so that the digital formula 
is JJ, III, IV. There is also an ex- 
tensive fusion of the bones of the 
tarsus and pes. The ankle-joint is 
markedly intratarsal, the basal row 
of tarsal bones fusing with the tibia 
(the fibula is reduced) to form a 
‘tibiotarsus,’ while the tarsales 
have united in the same way with 
the fused metatarsals, forming a 
‘tarso-metatarsus’ (fig. 129). 
The toes are rarely more than four 
in number, the first apparently lack- 
ing, and as a rule the number of 
phalanges increases from two in Pe erg am al ap 
digit II to five in digit V. Many 1, tarsometatarsus; #, tibiotarsus; II-V, 
birds have the toes reduced to “8's 
three and in the true ostriches to two. 
In the mammals the limbs, especially the fore limbs, exhibit a con- 
siderable range of modification. Thus in the primates the skeleton is 
