TEETH. 3 



grinding-face of the tooth always keeps at the same level, the 

 gradual wear of the superficial parts being compensated by the 

 pushing outwards of the whole tooth in its socket until, as may 

 be seen in the older specimens, nothing but the root is left. 



The permanent teeth are preceded by a temporary or deciduous 

 set of " milk-teeth " {d.i., d.m., &c.) ; in which there are as many 

 incisors as in the permanent set, although there are only three 

 cheek-teeth on each side above and below ; these milk-molars 

 being replaced by the last three permanent premolars. The 

 eruption, or cutting through the gums, of the deciduous teeth 

 commences at about the time of birth, and is completed before 

 the end of the first year, when the young animal has its full set ; 

 the upper teeth, as a rule, appearing somewhat earlier than those 

 of the lower jaw. The first teeth to appear are the central incisors 

 and the molars ; between the first and second months the second 

 incisor appears, and finally (at about nine months) the third 

 (corner) incisor, which completes the milk-dentition. Of the 

 permanent teeth, the first molar appears about the end of the first 

 year, followed by the second molar before the end of the second 

 year ; these teeth being thus in place before any of the milk-teeth 

 have been shed. At about two and a half years the second and 

 third premolars replace their predecessors ; and between two-and- 

 a-half and three years the first permanent incisor appears. At 

 three-and-a-half to four years the fourth premolar, the third true 

 molar, and the second incisor have appeared ; while at four-and-a- 

 half to five years the third (corner) incisor and the canine have 

 cut the gum, thus completing the permanent dentition. Up to 

 this period the age of the horse is clearly shown by the condition 

 of its teeth, and for some years longer indications can be obtained 

 from the wear of the incisors, though this depends to a considerable 

 extent upon the hardness of the food and other accidental 

 circumstances. 



In the specimens exhibited the side view of the teeth of the 

 right side, and the grinding-surface of the teeth of the left side 

 are shown. 



The series of skulls exhibited comprises specimens ranging in 

 age from the unborn colt to a horse of 36 years. 



b2 



