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. 



FIG. 128. Hind leg of snapping turtle (Chelydra) showing intratarsal joint at i. h, 

 humerus, r, radius; u, ulna; I-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. Development shows that the 

 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 II, 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' ( fi g . 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 

 digit II to five in digit V. Many 

 birds have the toes reduced to 

 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 



FIG. 129. Foot of parrot (Psittacus 

 amazonicus),/, femur; fb, fibula; p, patella; 

 tnt, tarsometatarsus; tt, tibiotarsus; I I-V, 

 digits. 



