318 THE HIND FOOT OR PES. [chap. 



All the animals of the order Ungulata are digitigrade, 

 the heel being raised from the ground, and the metatarsal 

 segment usually much elongated. There is never any trace 

 of a hallux. As in the corresponding segment of the fore 

 limb, the pes is formed upon one or other of two distinct 

 types, each characteristic of one of the sub-orders. 



In the Perissodactyla, the third digit is the largest, in the 

 centre of the foot, and symmetrical in itself; the second 

 and fourth are smaller, and nearly equal in length, but some- 

 times quite rudimentary. A line drawn through the centre 

 of the foot passes through the axis of the third digit, and 

 the middle of the external cuneiform, navicular, and as- 

 tragalus. The distal surface of the astragalus has a large 

 articular surface for the navicular, and a very small one for 

 the cuboid; which bone is of comparatively less importance 

 than in the Artiodactyla, The calcaneum does not articu- 

 late with the low r er end of the fibula. 



The Rhinoceros (Fig. 118) and Tapir have all the usual 

 bones of the tarsus well developed. The internal cuneiform 

 has a curved process projecting backwards. The middle 

 cuneiform (c 2 ) is very small. The whole foot is compara- 

 tively short and broad. The second and fourth toes are 

 well developed, being nearly as long as the middle toe. The 

 phalanges resemble those of the fore limb. In the Tapir 

 the pes differs from the manus in wanting the fifth digit. 



In the Horse (Fig. 119), the middle toe is greatly 

 enlarged, and the second and fourth reduced to slender 

 styliform metatarsals, about three-fourths the length of the 

 second, but supporting no phalanges. The navicular (n) 

 and the external cuneiform (&) are very broad and flat. 

 The cuboid (cb) is small, and the internal and middle 

 cuneiform bones are small and united together. 



Various gradational stages between the complete tridac- 



