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40 HUNTER ON THE TEETH.
When it is broken it appears fibrous or striated ; and all the
fibres or striae are directed from the circumference to the center
of the Tooth.
This, in some measure, both prevents it from breaking in
mastication, as the fibres are disposed in arches, and keeps the
Tooth from wearing down, as the ends of the fibres are always
acting on the food, (n)
The Enamel is thickest on the grinding surface, and on the
cutting edges, or points, of the Teeth ; and becomes gradually

struck with steel. (1) It is also more brittle than the cement and
dentine. We have already referred to the worn surface of the Elephant's
molar, as well exemplifying the relative density of the dental tissues.
It presents a series of transverse cylindrical ridges of enamel, each
enclosing a depressed surface of dentine, whilst the plates so formed are
separated from each other by still more deeply depressed valleys of
cement.]
(«.) [The external surface of the enamel appears smooth, but almost
always presents, on careful examination, delicate, closely-approximated
transverse ridges, and in some instances .umular elevations. We have
already adverted to the fact that it is covered by a delicate calcified
pellicle (Nasmyth's membrane, cuticle of the enamel). According to
Kolliker, " Nasmyth's membrane " is a calcified structureless membrane,
0-0004"' to 0-0008'" in thickness, which on its internal surface is fur-
nished with depressions for the reception of the extremities of the
enamel fibres. Its connection with the enamel is so intimate, that it
can only be demonstrated by the employment of hydrochloric acid.
Professor Huxley believes that the membrane in question is the altered
membrana preformativa of the dental pulp. To this subject we shall
have, however, to recur, when stating the various opinions current on
the subject of the development of the dental tissues.
In very thin sections, enamel is translucent and of a bluish colour.
Its substance is generally allowed to be composed of the so-called
enamel fibres (enamel-prisms, enamel-needles)—hard, dense, microscopic
structures, consisting almost entirely of earthy matter. These fibres
were described by Retzius as being solid hexagonal prisms. They are
not, however, regular in outline. Mr Tomes observes, that in a trans-
verse section he has as frequently found them approach a square or an
irregular circle as any other form. They are long, extending for the
most part throughout the thickness of the enamel, the inner end resting
on the dentine, the outer being in contact with Nasmyth's membrane.
The position of the enamel fibres being generally vertical to the surface

(1) Nasmyth's Researches, p. 81.