SKELETON 



127 



appendages, including the patella (knee-pan) in some reptiles, birds 

 and many mammals, in the tendon that passes over the knee-joint, the 

 fabellse in the angle of the knee of a few mammals, and the pisiforme 

 in the carpus of man and some other mammals. 



In the ancestral chiropterygium, as exemplified in the urodeles, 

 le basal joint was directed horizontally at right angles to the axis of 

 le body, but higher in the scale it approaches the sagittal plane and 

 such a way that the angles of the fore and hind limbs open in oppo- 

 site directions. Besides there is frequently a torsion of the bones of 

 the forearm (fig. 138) or shank on each other. The lower amphi- 

 bians have nearly typical legs (fig. 137), although, as in Siren and 

 Amphiuma, they may be greatly reduced, 

 while in some stegocephals and the gymno- 

 phiones they are entirely lacking. In the 

 anura the radius and ulna or tibia and fibula 

 are frequently fused and the tarsals elongated. 



I 





137- Fig. 138. 



Fig. 137. — Tarsus of Geolrilon, after Wiedersheitn, showing the arrangement of the 

 metapodial elements, c, centrale; /, fibulare; P, fibula, », intermedium; /, tibiale; 7, 

 tibia; 1-5, tarsales. 



Fig. 138. — Torsion in developing human arm, aiier Braus. m, r, ulna and radius; 

 dotted line, course of radial nerve. 



The mpstmarkedfeatur^f the repliliftftlimb is the occurrence of 

 an_intratarsal_ioijit^ the motion of the foot upon the leg being 

 largely between the two rows of tarsal bones, instead of between tar- 

 sus and the bones of the shank (fig. 139), a condition duplicated in 

 a way in the birds.V There is also a greater range of form than in the 

 amphibia. Limbs are lacking in snakes and some lizards; on the 

 ?her hand there is a great increase in the number of phalanges, 

 • rrelated with a shortening of the proximal bones, in the plesiosaurs, 

 •vhich reaches its extreme in the ichthyosaurs where there may be a 

 hundred phalanges in a digit. The wings of the pterodactyls are 

 ' markable for the great development of the fifth digit (elongation of 

 :iie phalanges) as a support for the wing; the other digits are more 

 normal. 



