370 
ON THE TEETH. 
of the herbivora are composed of alternate concentric layers of bone 
and enamel, which, wearing away irregularly, give that mill-stone 
like surface which so peculiarly adapts them for their office ; but 
this very fitness required that they should also be so constructed 
that there should be a process of growth sufficient to supply the 
waste by wear. The molar tooth, when fully completed, consists, 
as I have before observed, of concentric layers of bone and enamel. 
The enamel is produced during the development of the tooth, and 
extends downwards so far as the body of the tooth is solid, but does 
not enter into the structure of the fang, which is a prolongation of 
the bony layers only. The enamel thus formed is not afterwards 
added to, and the length of the enamel is about equal to the wear 
of the tooth during the ordinary life of the animal ; for in very old 
horses the molar teeth are found to be composed of hard bone 
only, the layers of enamel having been worn out. This applies 
both to the molars and incisors ; but from the incisors being less 
used, they grow longer, and do not shew so perfectly or frequently 
the same fact ; but I have seen this condition of the teeth in old 
horses too often to doubt of its being the natural result of age. 
These changes are, of course, very gradual, and it is the knowledge 
of the progressive changes which enable us to tell the age of such 
animal after the period of removal of the temporary teeth has passed 
away. The teeth are thus, as I think I have clearly shewn, always 
growing, but the growth during their development is, of course, 
much more rapid, and the degree of solidity of the tooth is different 
in different individuals, as some are very much more dense, which 
causes them to carry a youthful appearance of mouth, while all 
other indications shew a much more advanced age. On the other 
hand, some are so soft as to shew the reverse ; but, practically, these 
variations are not sufficiently numerous to greatly embarrass a 
careful practitioner, for the shape and general character of the tooth 
is a much better criterion of age at all periods of life than mere 
marks. It is not my intention in these remarks to point out the 
proper indications of particular ages as shewn by the teeth, but to 
state generally the result of my observations on the physiology of 
the reproduction and general growth of the teeth. The ovoid in- 
cisor, with its longest axis more or less across the median line of 
the body, becomes in advanced age of an elongated triangular form, 
with its longest axis more or less parallel with the median line. 
I shall simply state the times at which the changes take place of 
the temporary for the permanent teeth. At two and a half years 
the two upper centre incisors are shed, then the two lower centre ; 
at three years the temporary molars are thrown off; at three years 
and a half the second pair of incisors, first above, then below ; at 
four years the tushes appear, and the last or sixth molar is com- 
