OSSEOUS SYSTEM OF MAMMALIA. 



309 



the second, ii, and fifth, v, digits are appended to the coalesced 

 metatarsals of the functional pair of toes, in and iv. 



In the miocene fossil Horse (^Hipparion,^g. 194) a similar pair 

 of ^ spurious ' hoofs, ii, iv, dangled behind the main toe, in, com- 

 pleting the digits, ii and iv, indicated by the ' splint-bones ' or 

 proximal parts of the metatarsals in the modern Horse, fig. 193, 

 II, IV, but stunted in growth. In the eocene Palceotherium 

 these digits were nearly equal in size to the middle one. The 

 Rhinoceros at the present day preserves these proportions of the 

 toes, II, III, IV, but with shorter and more massive proportions of 

 the whole foot. Accordingly, in fig. 193, it will be seen that the 

 * mesocuneiforme,' cm, and ' cuboides,' b, have a larger propor- 

 tional size than in the Horse ; but the structure of the tarsus is 

 essentially the same : the cuboid, b, articulates directly with the 

 calcaneum, cl\ the naviculare, &, intervenes between 

 the two cuneiform bones and the astragalus, a. The 

 aflinity of these ' perissodactyles ' is obvious, and the 

 closer links of affiliation are supplied by the extinct 

 forms above cited. In like manner we find the affinity 

 of the Ox and Hippopotamus illustrated in the struc- 

 ture of the hind-foot, the Hog holding a similar inter- 

 mediate step in the developement of the toes, iv and 

 V. In the tarsus the cuboid, b, and naviculare, s, show 

 the same near equality of size, but they are distinct 

 bones in the Hippopotamus as in all Artiodactyles 

 except the restricted or horned Ruminants : a mesocu- 

 neiforme, cm, now supports the metatarsal of the toe, 

 II, that of the fifth, v, articulates with the cuboid. 

 In the Elephant the innermost digit, i, is present — the 

 last to appear in the ungulate as in the unguiculate 

 series, and the tarsal group shows the completeness 

 which it manifests in Man. The human anatomist will 

 recognise the astragalus, a, calcaneum, cl, naviculare, s, extended 

 transversely and presenting articular facets to the three ' cunei- 

 form ' bones, * internal,' * middle,' and ' external,' which for con- 

 venience of definition I have called ' entocuneiform,' ci, * meso- 

 cuneiform,' cm, ' ectocuneiform,' ce ; the ' cuboides,' b, supports as 

 usual the metatarsals of the fourth and fifth toes. The toe, i, has 

 a short metatarsal and some bony representative of a phalanx 

 imbedded in the innermost part of the hoof: the other toes have 

 the normal complement of phalanges, which, in Mammalia, do 

 not exceed (save in Cetacea) three in number, nor two in the 

 innermost digit, I, in both pectoral and pelvic limbs. 



Foot of extinct 



Horse 



iHippurian). 



