X28 



COMPARATIVE MORPHOLOGY OF VERTEBRATES 



The wings of birds (fig. 60) are even more modified. Until the 

 carpus is reached the structure is approximately normal, but the car- 



FiG. 139. — Hind leg of snapping turtle (Chelydra) showing intratarsal joint at i. f, 

 femur; fi, fibula; I, tibia; I-V, digits. 



pal 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 extensive 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 

 *tarsometatarsus' (fig. 140). 

 The toes are rarely more than 

 four in number, the first ap- 

 parently lacking, and, as a rule, 

 the number of phalanges increases 



Fig 140.— Foot of parrot {Psittacus from two in digit II to five in 

 awazomcMs),/, femur; /&, fibula; /», patella; ,. .^ ,t -nr i.* j i. iU 



<m, tarsometatarsus; ti, tibiotarsus; II-V; digit V. Many birds have the 



^'S^^^- toes reduced to three and in the 



true ostriches to two. 



In the mammals, the limbs, especially the fore limbs, exhibit a 



considerable range of modification. Thus in the primates the 



skeleton is nearly typical, but there is a marked power of rotation of 



the foot and especially of the hand by the motion of the lower end of 



