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Formation of the Teeth. 27
formed or being formed in the jaws, namely, twenty tempo-
rary teeth and twenty-eight permanent ones.

OP THE MANNER OP THE FORMATION OP THE TEETH.
The teeth are formed in a manner peculiar to themselves,
differing from the mode observed in the formation of all other
bones. The pulp of the crown of a forming tooth is surround-
ed by two membranes—an internal and an external. The
internal membrane secretes the bony structure of the tooth in
concentric layers or strata, one within another, first forming
a shell of bone, of the shape of the crown. As the ossification
goes on* the internal membrane contracts till the whole
tooth is formed
; it then composes the nerve of the tooth, as it
is commonly called; or in other words, it lines the cavity of
the tooth, and forms a medium for the ramification of the dental
nerves and vessels, which enter the cavity, through the fangs.
As soon as a shell of bone has been formed on the pulp, the
external membrane takes on a new action and secretes the
enamel upon the bone. The deposition of enamel goes on
till the tooth has made such progress as to cause the absorption
of the membrane. The enamel, when it is first, formed is
quite soft ; it soon however grows hard and seems to undergo
a process similar to crystallization, for it takes a regular and
peculiar form. The enamel is composed of a great number of
crystalline fibres, which are so arranged as to pass in a direc-
tion from the external surface, to the centre of the tooth ; by
this disposition of its fibres, the enamel acquires a greater de-
gree of strength than could be obtained by any other arrange-
ment. When perfect, the enamel of a tooth is so hard, that in
cutting it a file is soon worn smooth, and sparks may be elicit-
ed from it with a bit of steel. The enamel is very thick on
the grinding surfaces of the teeth and gradually grows thinner
towards the gum, where it almost insensibly terminates. It is
much thicker on the teeth of some individuals than upon those
of others, also more compact. Sometimes it is defective in
formation, being of a dirty yellow color and crumbling, or