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THE SIXTEENTH CEXTURf 1S3

To remove even the slightest doubt and to put an end to any controsersx'
on such a point, only one fact is sufficient, which is revealed to us h\
anatomical dissection, and that is, that the teeth which appear about the
seventh \ear are not onl\- not united to those which tall at the same
period, but cannot e\en be m contact w itii them, owing to the presence of
a thin osseous partition.
In the following chapter' Kustachius speaks of the central cavitv of
the teeth and of the substance contained in it. In young teeth, he savs,
the dental cavity is very large, in proportion to the size of the tooth.
According to some anatomists, the central cavit\' of a tooth is coated b\'
a ver\' soft and thin membrane, formed b\- a tissue of ver\' small vessels
and nerves; and besides, this cavitv is filled with marrow, like hollow
bones. The observations of the author, however, do not agree with these
statements. The dental cavitv does not contain any fatty substance
analogous to the marrow of bones. As to the above-mentioned mem-
brane, Eustachius doubts its existence. The large hollow existing in
children's teeth contains, he sa\s, a mucous substance, somewhat hard,
and ver}" smooth at its surface—almost like a cuticle—but which has rather
the appearance of a concretion than of a membranous tissue. At an\
rate, adds Eustachius, if the substance alluded to is made to dry up in the
shade, it acquires an appearance not unlike that of a membrane. It
is certain, however, that at an early age the substance contained in the
dental cavity does not adhere to the walls of the latter after the manner
of a periosteum, but is found in simple contact with the same, and can,
therefore, be separated from them with the greatest ease.
As years pass by, the dental cavity becomes narrower and narrower,
because the substance contained inside the tooth gradually becomes
ossified at the surface, adhering to the dental scale previously formed, in
the very same manner as the internal or woody part of a tree adheres
to the bark. Of the two hard substances which make up a tooth, the
outer one is white, tense, and dense, like marble, the underlying one,
instead, is somewhat dark, rough, and less compact. To observe accu-
rately the above-mentioned facts, the author advises searching for them,
first, in the molar teeth of the ox or the ram, and then in human teeth,
and likewise, first in children or in recently born animals, and then in
adults.
Chapters XIX and XX are, comparativel\ speaking, of lirrle impor-
tance. In the former the author undertakes especially to examine the
opinions of Galen on dental bloodvessels and nerves, and discusses whether
it were known to him that these vessels and nerves penetrate into the
internal part of the teeth. In the latter, Eustachius speaks ot the great

' Chap, xviii,
p. 54.