as indicatine of the Age of the Animal. 
309 
wear immediately upon such a tooth being cut. Such, however, is 
not the case. In fig. 17, which gives a representation of the third 
temporary molar of a calf, 
taken from out of its 
socket a few days after it 
was cut, it will he seen 
that the cusps a, a, a, are 
very pointed, and that no 
other structure, except 
that which covers the 
body of the tooth and 
these projections, is as 
yet apparent. The cusps 
of molar teeth, as well as 
the edge of the incisors, 
are cased with enamel, as 
was shown when speak- 
ing of the formation of this 
structure. The hardness of the enamel combined with the irregu- 
larity of the shape of the cusps is sufficient for all the purposes 
needed at the first by the young tooth, as a grinding organ. Con- 
tinuous use, however, soon wears away these projections, and after 
a time the caps of enamel are completely cut tlirough. 
It will also be remembered that the enamel passes from the outer 
surface of the body of a tooth, as a covering to the dentine into deep 
hollows, which are originally produced by a separation of the pa- 
pilla, and that it lines them throughout. The wisdom of nature's 
arrangement in thus blending the several structures together now 
becomes more evident. By the wearing away of the enamel caps 
the outer reflection of this structure is separated from the inner, 
and thus, instead of there being fewer points of resistance to attri- 
tion, there is a positive multiplication of them. Between the 
separated layers of enamel, the dentine is exposed, and as the 
enamel cups are filled with crusta, so we see that all three struc- 
tures are now in wear together. The tooth will therefore present 
on its surface the appearances depicted in fig. 18, which may 
be supposed to represent the preceding tooth (fig. 17) after its 
upper part had been removed by continuous use. The irregularity 
also of the wearing surface by the blending together of the 
structures is very nicely shown in the engraving, where K repre- 
sents the outer layer of enamel, and E*, the inner; c, the crusta; 
and D, the dentine. 
In speaking of the production of the Dentine it was observed 
* Fig. 17. The third temporary molar of the calf sho%\ ing its general form, and 
likewise that its cusps, a, a, a, are covered with enamel v/hen first brought into 
use. 
