THE LIMBS 



157 



called c third trochanter,' as mentioned in the first 

 chapter, is characteristic of all known Perissodactyles, 

 and is also found in some rodents, but not in man or 

 in mammals generally. 



The second segment of the skeleton of the hind 

 limb is represented in the horse almost entirely by the 

 tibia. The fibula, indeed, is present, and a distinct bone, 

 but only appears as a slender styliform rudiment of the 

 upper portion attached to the outer side of the tibia. 



The third segment of the hind limb, the foot or pes, 

 has undergone precisely similar changes from the gene- 

 ralised or typical form to those already described in the 

 fore limb. In fact, below the carpal 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 

 rudimentary condition of the second and fourth, the 

 complete absence of the first and fifth ; the presence of 

 only one digit, consisting of three phalanges, having 

 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 (Chce- 

 ropus castanotis) has but one functional toe (in this case 

 the fourth), on the tip of which it walks, on the hind 



