640 



CHORD ATA. 



permit freer motion, the clavicles are absent ; the feet touch but 

 the tips of the toes, enclosed in hoofs, to the ground (unguligrade). 

 Since the metacarpals and metatarsals are greatly elongate, the 

 wrist and ankle are raised high from the ground so that they are 

 frequently confounded with elbow and knee. With this exclu- 

 sively supporting character of the limbs there is the same tendency 

 to reduction and fusion of bones which was noticed in birds (p. 

 606). There is a constant increase in the development of radius 

 and tibia to the chief supports of the body, the fibula becoming 

 rudimentary, the ulna being developed sometimes throughout its 

 whole extent, sometimes only in its upper part, which serves for the 

 attachment of muscles (olecranon), and is more or less fused with 

 the radius. The same tendency to simplification prevails in the 

 feet, but is expressed differently in the odd-toed (perissodactyle) 

 and even-toed (artiodactyle) forms. In the Perissodactyla the 



\ 



FIG. 664. Fore feet of ungulates. (After Flower.) A-C, perissodactyle ; D-F, artio- 

 dactyle. A, tapir; R, rhinoceros; C, horse; D, pig; E, deer; F, camel, c, trique- 

 trum (ulnare); /, lunatum (intermedium); rn, capitatum; w 2 -w 5 , rudiments of 

 metacarpals II and V; p, pisiforme; R, radius; ,s, scaphoid (radiale); td, trapezoid; 

 tm, trapezium; U, ulna; w, hamatum; II- V, digits. 



axis of pressure passes through the middle toe (fig. 664, AC, 

 III), while the other toes disappear symmetrically around this. 

 Since the first toe is early lost, toe V is next to disappear (J?), and 

 then toes II and IV (C), so that at last there remain only the 

 skeleton and hoof of the middle toe (horse), the rudiments of toes 

 II and IV persisting as the small splint bones. 



In the Artiodactyla the axis of pressure falls between toes III 

 and IV (fig. 664, /)), which both unite in supporting the body 

 and are equally developed and frequently fuse, at least so far as 

 the metacarpals are concerned (E, F}. The figures D-F show 



