30 THE HORSE AND ITS RELATIVES 



will have completely disappeared ; and the section 

 will represent ( i ) a central core of soft wax, corre- 

 sponding to the comparatively soft dentine, (2) a 

 ring of kid, equivalent to the hard enamel, and (3) 

 an irregular coat of sealing-wax, corresponding to 

 the external layer of cement. A horse, like most 

 mammals, grows two sets of incisor teeth in both 

 jaws ; firstly, a baby, or milk set, and secondly, a 

 permanent set. The three teeth of the first set are, 

 however, not all shed at once, but one by one, when 

 they are as gradually replaced by their permanent 

 successors, which grow up beneath. And it is by 

 knowing how this replacement occurs, and noting 

 the extent to which the central mark is worn away, 

 that the age of a horse can be approximately ascer- 

 tained up to six or seven years old. The " mark," 

 it should be added, is common to both the temporary 

 and the permanent set of incisors ; but is deeper in 

 those of the upper than in those of the lower jaw. 



The jaws of a quite young colt show only the 

 first and second pairs of milk-incisors, both above and 

 below ; but after a time the third pair appears on 

 their outer sides. In a horse of about three years old 

 the first pair of permanent incisors (recognisable 

 by their larger size and unworn crowns) will have 

 pushed out and replaced the corresponding baby- 

 teeth. At an age of between three and a half and 

 four years the second or middle pair of milk-incisors 

 will have been similarly replaced by the permanent 



