as indicative of the Aye of the Animal. 291 
tooth. This is the enamel. If the tooth has been recently put 
up, this substance forms also its cuttinj^ edge. It is thicker on 
the front surface of the crown than on the back, an arrangement 
which tends to keep up a sharp edge to the tooth. It extends 
also downwards to the neck, where it suddenly ceases. It is 
harder and more compact than the dentine, and has as much, 
according to the statements of our most celebrated chemists, as 96 
or 98 parts of earthy matter in every 100. The enamel not only 
covers the exposed surface of a tootli, but in some teetli it enters 
deeply into their interior. It matters not what may be the shape 
or size of a tooth, or how numerous its projections, the whole of 
these are originally covered with a 
layer of enamel. In many teeth these Fig. 3* 
projections, technically called cus])S, 
are very numerous. They are seen 
most to advantage in teeth which 
have been recently cut. Fig. 3 gives 
a view of a cap of enamel as removed 
from the sixth molar of a pig on the 
eve of its appearing through the gum. 
Its irregular suiface admirably adapts 
it lor the comminution of tlie food. 
When viewed microscopically with a magnifying power of 
400 to 500 linear, enamel is found to consist of an assemblage 
of rods or bars lying side by side, and piled also in layers the 
one above the other proportionate to the thickness of the S})ecimen, 
These rods, which have been named the enamel prisms or 
fibres, are evidently the structures upon which the hardness of the 
substance depends. In different parts of a tooth the fibres are 
somewhat differently arranged, but they are always placed end- 
ways upon the surface of the dentine on which they rest. On 
the apex of the crown the prisms proceed directly upwards, but 
on its sides they incline a little, becoming more and more 
oblique as they approach the neck of the tooth.j Between each 
bar, and running parallel with its course, a minute canal can be 
detected in very recently-formed enamel, seemingly produced by 
the memi)ranous Avails of elongated cells which have coalesced 
* Fig. 3. Enamel cap of the sixth molar of a pig removed from the jaw 
just prior to the tooth being cut. Natural size. 
t It should be added that their course outwards is not perfectly straight, but 
slightly -waving, as shown in fig. 4 a. I am inclined to think that the separate 
layers of enamel at times decussate, and that this explains the well-known cir- 
cumstance, that brown spots of irregular outline are observed in this structure 
when examined with a low magnifying power. The hexagonal appearance 
(fig. 4, b), which is only seen here and there in thin sections of enamel, I have 
also thought might depend on an oblique cutting of a superimposed layer of the 
prisms, or perhaps upon the decussation of these layers. 
