ZOOLOGICAL POSITION AND STRUCTURE 1 3 
are the only ones. In no existing artiodactyle (and, 
for that matter, in no living perissodactyle) is there a 
toe corresponding to the human thumb or to the 
human great toe. 
In the Perissodactyla (Greek perissos, unequal, and 
dactulos), on the other hand, the toe corresponding in 
the fore-limb to the human third or middle finger, 
and in the hind-limb to the human third toe, retains 
the character of being symmetrical to a line drawn 
through its own axis, and of being longer and larger 
than the toes on either side. In the horse and its 
immediate relatives, which constitute the supreme, or 
most specialised, representatives of the odd - toed 
group, the third toe, which is greatly enlarged, is the 
only one remaining. 
It is necessary to add, without entering further into 
the details of the ungulate skeleton, that in the more 
advanced, or specialised, members of the even-toed 
group, the two enlarged and mainly functional toes 
in each limb are supported by an elongated bone — 
the cannon-bone — formed by the fusion of the two 
bones which respectively support the third and fourth 
fingers of the human hand and the corresponding seg- 
ments in the human foot. In the horse, on the 
contrary, the single toe in each limb is supported by 
a cannon-bone of simple, instead of compound, 
structure. 
Both in the ox and in the horse the cannon-bone is 
greatly elongated in comparison with the bones or 
bone by which it is represented in the pig among the 
artiodactyles and the rhinoceros among the perisso- 
dactyles. This elongation of the cannon - bone, 
in both these animals, causes the wrist and ankle 
joints — the former being the so-called " knee " and 
