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132 HUNTER ON THE TEETH.

either in a child, or an adult, begin by four or five ossifications,
one on each point, the external always the first. Whore the

ends with the tissue of tlie bulb, by their peripherical ends with the
small quantity of bulb matter which extends beyond them. These den-
tinal cells are transformed into ivory : the change extends from the
periphery of the cell to the central end. " When the alteration into the
solid condition is once effected, the cell becomes for ever unrecognisable,
the body having gradually changed from the condition of a soft sub-
stance to that of ivory, and the nucleus becomes atrophied during the
general invasion." Robin and Magitot therefore are of opinion that the
matrix of the dentine is formed by the calcification of the dentinal cells. (1)
With regard to the formation of the cement, according to Kolliker,
it proceeds from that part of the tooth sac which is situated between the
pulp and the enamel organ. It commences as soon as the fang begins
to be formed. " At this period the tooth sac becomes elongated at its
lower part, is closely applied to the developing fang, and, from its rich
network of vessels, furnishes—as the periosteum does during the growth
in thickness of the bones—a soft blastema, in which nucleated cells
become developed, and which then immediately ossifies. Accordingly
the cement is not formed by the ossification of the tooth sac itself."
Upon the dentine the membrana praeforrnativa becomes covered by the
deposition of the cement, and is then no longer to be recognised. On
the enamel, on the contrary, it remains as Nasmyth's membrane or the
cuticle of the enamel. (2)
Professor Hannover, of Copenhagen, entertains some original views on
the subject of the formation of the dentine and cement. According to
this observer the dental follicle, in addition to the germs for the produc-
tion of the different dental tissues, consists of what he terms the mem-
brana intermedia, a membrane which separates the cement from both
the enamel and the dentine. On the crown of the tooth, the enamel
cells are attached to the inner surface of this membrane, and in those
animals in which cement exists on the crown, it there separates the
enamel from the cement. At the root of the tooth, where there is no
enamel, the membrane lies between the dental germ and the cement
germ. Dr. Hannover thinks that in the fully-formed tooth this mem-
brana intermedia is metamorphosed into a stratum intermedium.— the
" granular layer" of other writers. On the crown, Dr. Hannover believes
that the so-called " cuticle of the enamel" is subsequently formed by
this membrana intermedia. According to this observer, the dentinal
tubules are formed by the dentinal cells, which arrange themselves in
(1) Robin and Magitot, op. cil.
(2) Kolliker, op. cit., p. 307.