THE STRUCTURE OF THE TEETH 69 



with the general aspect of the horse — the wearing of the 

 centre incisors, the growth and shape of the tushes — will 

 prevent a late four-year-old from being substituted for 

 a five. Though the incisors may be got up a few months 

 before their time, and the tushes a few weeks, the grinder 

 is with difficulty displaced. The three last grinders, as 

 well as the tushes, are never shed, but come at once per- 

 manently. At "five years old ofi"," the corner teeth are 

 still but slightly worn, and the margins inside are rounding. 

 Much of the original stony crust (crusta petrosa) is remain- 

 ing, and the enamel, where bare, is transparent and pearly. 

 The corner nippers are so clean in their coat and mark as 

 to procure them the name of " shell-teeth." 



Six Years Old. — At six years the corner teeth seem set 

 more firmly, the enamel mark is irregular, and the edge 

 uneven. The teeth, too, are getting more square on the 

 external surface. All the edges meet with accuracy. The 

 semi-circle which the teeth formed in the fifth year is 

 widened out. The mark in the centre incisor is now 

 growing indistinct. There will still be a diff'erence of 

 colour in the centre of the tooth. The cement filHng the 

 hole made by the dipping in of the enamel will present a 

 browner hue than the other part of the tooth ; it will be 

 evidently surrounded by an edge of enamel, and there 

 will even remain a little depression in the centre, and also 

 a depression round this case of enamel ; but the deep hole 

 in the centre of the tooth, with the blackened surface 

 which it presents, and the elevated edge of enamel, will 

 have disappeared. Persons not much accustomed to horses 

 have been sadly puzzled here. They expected to find a 

 plain surface of a uniform colour, and knew not what 

 conclusion to draw when there were both discolouration 

 and irregularity. 



In the next incisors the mark is shorter, broader, and 

 fainter ; and in the corner teeth the edges of the enamel are 

 more regular, and the surface is evidently worn. The tush 

 has attained his full growth, being nearly or quite an inch 

 in length, convex outwards, concave within, tending to 

 a point, and the extremity somewhat curved. Th^ third 

 grinder is fairly up, and all the grinders are level. Mr. 

 Youatt says : " Now, or perhaps at a period of six months 

 before, the horse may be said to have a perfect mouth. All 

 the teeth are produced, fully grown, and have hitherto 



