POSITION AND STRUCTURE OF HORSE 13 



All the members, whether living or extinct, 

 of the family, as thus restricted, come under the 

 denomination of what naturalists term highly special- 

 ised animals ; the specialisation in this instance 

 taking the form of adaptation for the attainment 

 of great speed in running, and likewise for grazing 

 on grass or other herbage. 



Their specialisation is best observed in the 

 skeleton and teeth ; the former departing almost 

 as widely as possible from that of a generalised 

 animal, such, so far as the structure of the limbs 

 is concerned, as a bear. 



In the latter animal each foot terminates 

 in five complete and functional toes, usually 

 armed with claws ; and in walking the entire 

 sole of the foot, inclusive of the heel-bone, or 

 calcaneum, in the hind-pair, is applied to the 

 ground. 



In the horse and its existing relatives, together 

 with certain extinct species, such as the one of 

 which the skeleton is shown in plate xxiii., each 

 foot terminates in a single toe (encased during 

 life in a hoof), upon which alone the animal 

 walks ; this single toe, as already mentioned, 

 and as shown in the figure on page 6, corre- 

 sponding to the middle or third one of the 

 generalised five-toed type. Consequently the heel, 

 or ankle, technically known as the tarsus, in place 

 of resting on the ground, as in the bear, is raised 



