308 



ANATOMY OF VERTEBKATES. 



The successive gradational steps by Avliich the pentadactyle 

 condition of the limb or appendage is attained are selected from 

 the aeries of hoofed Mammals in fig. 193. 



The pelvic limb, fig. 195, shows the same monodactyle sim- 

 plicity as the pectoral one, in the Horse. The ossicles developed 

 in the connective substance between the second and third prin- 

 cipal segments of the long-jointed ray, are the ' astragalus,' a, 

 ' calcaneum,' cl, ' naviculare,' 5, ' mesocuneiforme,' cm, ' ectocunei- 

 forme,' ce, ' cuboides,' h. The metacarpal supporting the three 

 joints or 'phalanges' of the digit articulates chiefly with the ecto- 

 cuneiform, e, which accords in size. The largely developed digit, 

 or continuation of the main limb-ray, fig. 193, answers to the third, 

 III, of the pentadactyle foot. At its base are rudiments of the 

 metatarsals of the second, ii, and fourth, iv, digits. In the Ox 

 the naviculare, ib. s, is connate with the ' cuboides,' h : and, as this 

 supports one-half of the single metatarsal, such half is held to be 

 the developed homologue of the rudimental fourth metatarsal 

 in the Horse : whilst the half supported by the ' ectocuneiforme,' 

 ce^ in the Ox, is held to answer to the metatarsal of the developed 

 digit. III, in the Horse. 



Elephant. Hippopotamus. Rhinoceros. 



Embryology here lends partial proof to this view : the so-called 

 ' cannon-bone ' being developed from a single centre and epi- 

 physes in the Horse, and from a pair of shafts or centres and 

 epiphyses in the Ox : it accordingly supports a pair of toes, 

 which answer to the third, iii, and fourth, iv, in the penta- 

 dactyle foot. The Camel and Giraffe have not rudiments of 

 any other toes : in the Ox such rudiments of the distal parts of 



