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DEVELOPMENT OF CORD FOR PERMANENT TEETH. 645
upon a certain condition of the calcific material. They do not occur
persistently, as do the fibrillse of the odontoblasts. I have, under favor-
able circumstances, succeeded in demonstrating them in sections of pigs'
teeth, where they showed very plainly indeed, being nearly or quite as
lono; as the ameloblasts themselves and several times lono;er than the
enamel was thick. As a rule, however, the ameloblasts separate from
the forming enamel so as to leave a comparatively smooth line or plate
—that is, provided the sections have been sufficiently thin, so as not to
show a ragged edge from the overlapping of the cells themselves. I
have never been able to demonsti'ate processes that would lead me to
infer the least analogy between them and the fibrillte of the odonto-
blasts. That the enamel organ exists in the commencement of the
development of the teeth is now generally admitted. There are certain
classes of teetli, however, that do not possess enamel, and in which,
although there is an enamel organ developed, the stellate reticulum
fails to appear. In all cases where there is to be a deposit of enamel
we find a stellate reticulum fully developed, and my observation leads
me to believe that the calcification of the enamel-matrix is due to the cal-
cific material stored in the meshes of the stellate cells of the enamel organ.
The subject of calcification has already been considered, and will be
referred to here only in a general manner. After the temporary teeth
are developed and have served their purpose, they are then removed by
resorption of their roots and their places taken by the permanent set.
The process of resorption is physiological, and is accomplished through
the agency of giant-cells. This part of the subject has been considered
quite fully under the head of Physiological Action of Cells, in the ojjen-
ing chapter. The comparative stages of decalcification of the temporary
and calcification of the permanent teeth have been so well delineated by
Prof. Pierce in his chart in the Dental Cosmos (August, 1884) that I
cannot do better than reproduce it here (see p. 647), together with the
explanatory text accompanying the same
" In the microscopical examination of dense animal tissues, or such
tissues as are impregnated with the salts of lime, it becomes evident
that they, like vegetable structures, have periods of growth and of rest,
Avhich are illustrated by concentric layers or zonal shades, and that,
while these conditions are normal, they are both modified and intensified
by the genius presiding over the function of nutrition. Unfortunately,
however, in dating the progressive solidification of tissues, we can with
a degree of certainty mark the beginning and the end only, the inter-
mediate lines merely approximating the conditions Mdiich we attempt to
illustrate ; yet they are near enough to exactness to give a comprehen-
sive idea of the condition of the average tooth at a certain age, and in so
doing they serve as an important guide in the performance of many
necessary dental operations (Fig. 1).
" From the tabular statement or chart to which we have alluded
above we see that by the seventh week of intrauterine life, and when
the embryo is less than one and a quarter inches in length, preparation
is made for the development of the enamel-germ or matrix, followed in
the ninth week by the dentine-germ, these germs continuing in or
througli their progressive stages until the seventeenth week, when we
DEVELOPMENT OF CORD FOR PERMANENT TEETH. 645
upon a certain condition of the calcific material. They do not occur
persistently, as do the fibrillse of the odontoblasts. I have, under favor-
able circumstances, succeeded in demonstrating them in sections of pigs'
teeth, where they showed very plainly indeed, being nearly or quite as
lono; as the ameloblasts themselves and several times lono;er than the
enamel was thick. As a rule, however, the ameloblasts separate from
the forming enamel so as to leave a comparatively smooth line or plate
—that is, provided the sections have been sufficiently thin, so as not to
show a ragged edge from the overlapping of the cells themselves. I
have never been able to demonsti'ate processes that would lead me to
infer the least analogy between them and the fibrillte of the odonto-
blasts. That the enamel organ exists in the commencement of the
development of the teeth is now generally admitted. There are certain
classes of teetli, however, that do not possess enamel, and in which,
although there is an enamel organ developed, the stellate reticulum
fails to appear. In all cases where there is to be a deposit of enamel
we find a stellate reticulum fully developed, and my observation leads
me to believe that the calcification of the enamel-matrix is due to the cal-
cific material stored in the meshes of the stellate cells of the enamel organ.
The subject of calcification has already been considered, and will be
referred to here only in a general manner. After the temporary teeth
are developed and have served their purpose, they are then removed by
resorption of their roots and their places taken by the permanent set.
The process of resorption is physiological, and is accomplished through
the agency of giant-cells. This part of the subject has been considered
quite fully under the head of Physiological Action of Cells, in the ojjen-
ing chapter. The comparative stages of decalcification of the temporary
and calcification of the permanent teeth have been so well delineated by
Prof. Pierce in his chart in the Dental Cosmos (August, 1884) that I
cannot do better than reproduce it here (see p. 647), together with the
explanatory text accompanying the same
" In the microscopical examination of dense animal tissues, or such
tissues as are impregnated with the salts of lime, it becomes evident
that they, like vegetable structures, have periods of growth and of rest,
Avhich are illustrated by concentric layers or zonal shades, and that,
while these conditions are normal, they are both modified and intensified
by the genius presiding over the function of nutrition. Unfortunately,
however, in dating the progressive solidification of tissues, we can with
a degree of certainty mark the beginning and the end only, the inter-
mediate lines merely approximating the conditions Mdiich we attempt to
illustrate ; yet they are near enough to exactness to give a comprehen-
sive idea of the condition of the average tooth at a certain age, and in so
doing they serve as an important guide in the performance of many
necessary dental operations (Fig. 1).
" From the tabular statement or chart to which we have alluded
above we see that by the seventh week of intrauterine life, and when
the embryo is less than one and a quarter inches in length, preparation
is made for the development of the enamel-germ or matrix, followed in
the ninth week by the dentine-germ, these germs continuing in or
througli their progressive stages until the seventeenth week, when we
