MAMMALIA. 519 



second and fourth digits also go, though their metacarpals 

 remain as the two splint-bones down the hinder borders of 

 the large and elongated third metacarpal or "cannon-bone," 

 which bears the third digit and the single hoof. Just as in 

 origin the cannon-bone of the ox is formed of two meta- 

 carpals and that of the horse is one, so they can be imme- 

 diately distinguished by the double hinge-joint at the distal 

 extremity of the former and the single hinge-joint on that 

 of the latter. 



In the hind-limb the femur is recognised by the presence 

 in the horse of a third trochanter on its outer border, and 

 the tibiofibula or tibia, carrying the fused remnant of the 

 fibula, will be seen in the ox to have three articular facets 

 at its distal extremity. The two larger articulate with the 

 astragalus, as in the horse, but the small outer one articu- 

 lates with a small condyle on the calcaneum. The astragalus 

 in the horse has a flat facet for the navicular below it, but 

 that of the ox has a hinge-joint with the naviculo-cuboid 

 bone below it, which gives it a double appearance, a hinge- 

 condyle at each end. In other w^ords, the horse has only 

 a crurotarsal joint, as in most mammals, but the ox has a 

 certain amount of intertarsal movement as well as the 

 crurotarsal. Of the distal tarsals the navicular and cuboid 

 fuse across in the ox to form a fiaviculo- cuboid^ whereas in 

 the horse the navicular commonly fuses with the ecto- 

 cuneiform below it, or remains distinct, but never fuses with 

 the cuboid. There is usually a small middle cuneiform in 

 the horse over the inner splint bone (digit tw^o). The 

 digits of the hind-foot are modified in a closely similar way 

 to those of the fore-foot. 



The metacarpal "cannon-bone" of the ox is distinguished from 

 the metatarsal by the much shallower median groove in the former, 

 and the metacarpal "cannon-bone" of the horse is flattened from the 

 front behind, whereas the metatarsal is round in cross section. 



If it be remembered that the horse's limb is formed from 

 hypertrophy of one digit and the bones in the main axis 

 above it, whereas that of the ox is really bilateral or formed 

 from two digits and the bones above them, which are only in 

 later geological times fusing together to form one, it is easy 

 to account satisfactorily for the persistent calcaneo-fibular 

 joint, for the fusion across the middle line of navicular and 



