THE HORSE TRIBE 



1075 



structure may be imitated by taking the finger of a glove and pushing in the top, 

 and afterward filling the whole of the inside with wax. 



The skull of the horse differs from that of all other living Odd-Toed 

 Other Charac- _ 



teristics Ungulates in having the socket of the eye completely surrounded by 



bone. In all existing horses the number of toes on each foot is re- 

 duced to one, which is inclosed in a large solid hoof. This toe, which corresponds 

 in the fore-limb to the human middle finger, is supported by a single long canon 

 bone. On the sides of this canon bone there are, however, small splints represent- 

 ing the remnants of the second and fourth toes; and in certain extinct forms (as 

 shown in the figures on p. 743) these lateral toes were complete and furnished with 

 hoofs, although they were much inferior in size to the middle toe, and could have 

 been of little, if any, functional importance. In defining the Horse family, it must 

 accordingly be stated that although the toes may vary from one to three in number, 

 it is only the middle one that is functionally important. Another distinctive feature 

 of the family is that in the fore-limb the ulna is represented only by its upper ex- 



UPPER MOI.AR TEETH OF THE ANCHITHERE (A), THE HORSE (B), AND THE HIPPARION (C). 



(A is from the left, and B and C from the right side of the jaw.) 



t 



tremity, which becomes united with the radius; while in the hind-limb the remnant 

 of the fibula becomes similarly fused with the tibia. 



So far as their extremely specialized organization is concerned, 

 Specialization thg horses hold a pos i t i on amO ng the Odd-Toed Ungulates precisely 



analogous to that occupied by the true Ruminants, or Pecora, in the Even-Toed 

 division of the order, and it is curious to observe how the two groups have 

 undergone an almost exactly parallel development, although differing so essentially 

 from one another in the structure of their limbs and teeth. In both, for instance, 

 the cheek-teeth have acquired tall crowns, with complicated foldings of the enamel, 

 and the front teeth are separated by a long gap from those of the cheek series. 

 Then again, both have the feet supported by a long canon bone, although in the 

 Ruminants this is formed by the fusion of two distinct elements, and in the horses 

 of but one original constituent. Moreover, both groups have the two bones of the 

 lower segment in the fore and hind-limbs fused together, and in both the process 

 by which the second vertebra of the neck articulates with the first has assumed a 

 spout-like form. 



