168 THE STRUCTURE OF THE HORSE. 



The third segment of the hind limb, the foot or 

 pes, has undergone precisely similar changes from 

 the generalized or typical form to those already 

 described in the fore limb. In fact, below the car- 

 pal and tarsal bones (the "knee" and "hock" of 

 the horse respectively) the fore and hind limbs are 

 almost exact repetitions of one another. The great 

 development of the third metatarsal bone, the rudi- 

 mentary; condition of the second and fourth, the 

 complete absence of the first and fifth; the presence 

 of only one digit, consisting of three phalanges, hav- 

 ing almost precisely the same form (except that 

 they are rather narrower in the hind than the fore 

 foot), are common' to both extremities. In this 

 structure of the foot, especially in the possession of 

 but a single toe on each limb, the horse is absolutely 

 unique among mammals. A very small Australian 

 marsupial (Clioeropiis castanotis) has but one func- 

 tional toe (in this case the fourth), on the tip of 

 which it walks, on the hind foot, but three other 

 toes are present, and complete in all their parts, 

 though very minute ;_' and in the fore foot two 

 nearly equally developed toes reach the ground. 



As the first segment" of. the horse's hind limb is 

 so much '.shorter: proportionately thaii that of man, 

 the last is as much : longer, and being habitually car- 

 ried in a totally different position has a very differ- 



