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606 DENTAL EMBRYOLOGY AXD HISTOLOGY.
I have no doubt that if the enamel had been examined it Mould
cells.
have shown a similar arrangement.
The amount of pigment found in enamel varies very considerably in
different animals, and generally bears a close relation to the density of
the enamel ; but whether it has any influence upon the hardness or not
I am unable to say. Those teeth Mhich have the greatest amount of
pigment have the most compact formation, and rice versa. Pigmenta-
tion after eruption comes from without and may be due to various
causes : the use of tobacco is the most probable source of external pig-
mentation. The interprismatic cement-substance becomes dissolved by
the acids of the mouth to a greater or less depth, and the pigment
lodges in the spaces so formed, also coating the surface.
This pigmentation must not be confounded with the change in the
color of the enamel which results from death of the pulp. In a vast
majority of cases the enamel in human teeth, by reason of its translucency,
transmits the varying changes in the color of the dentine. That there
is no pigmentation of enamel itself in these cases may be proved by cut-
ting out the underlying dentine and filling with chloride of zinc or some
other white filling. Indeed, this mode of " mechanical bleaching " has
come into almost general practice.
This naturally brings us to the consideration of the dense polished
outer coat of enamel.
This layer shows a distinct characteristic variation from the prismatic
layer underneath. It is the outer capping of the prisms, and furnishes
a polished surface for the coat of mail which is best adapted for its
office. Smooth, polished surfaces are known to resist the eroding action
of acids longer than rough surfaces. The character and shape of the
ameloblasts change before this layer is formed. From a membrane
comj)osed of prismatic cells, they now assume a horizontal direction.
I have studied these changes in the form and direction of the axis of
the ameloblasts in sections of incisor teeth of rodents, where it was
very plainly shown. Enamel is here being continuously formed on the
labial side, at the base of the tooth, to supply loss by attrition. All
the stages of calcification, as well as the changes which occur in the
ameloblasts, are plainly demonstrated in one tooth. Below a certain
point the enamel-prisms are yet unfinished, and here the ameloblasts
are seen to be prismatic in form ; toward the surface they gradually
shorten and widen until near the margin of the gum, when thev change
from a position at right angles with the axis of the tooth to a longitu-
dinal direction, Mrs, Emily Whitman also noted these changes in
studying the development of the teeth in the ray and rabbit. Mrs.
""
Whitman says
" The cuticula of the mammalian tooth has several times been found
to have the same structure, and it has been possible, in transverse and
longitudinal sections, to trace the gradual transition of the enamel-cells
into a perfectly homogeneous membrane (Fig. 338), the cylindrical cells
growing shcM'ter as they approach the croAvn of the tooth, until, instead
of being columnar, they are almost square, and finally flattened, and
at last the outlines of the cells quite disappear, and there is left a
perfectly homogeneous membrane.
606 DENTAL EMBRYOLOGY AXD HISTOLOGY.
I have no doubt that if the enamel had been examined it Mould
cells.
have shown a similar arrangement.
The amount of pigment found in enamel varies very considerably in
different animals, and generally bears a close relation to the density of
the enamel ; but whether it has any influence upon the hardness or not
I am unable to say. Those teeth Mhich have the greatest amount of
pigment have the most compact formation, and rice versa. Pigmenta-
tion after eruption comes from without and may be due to various
causes : the use of tobacco is the most probable source of external pig-
mentation. The interprismatic cement-substance becomes dissolved by
the acids of the mouth to a greater or less depth, and the pigment
lodges in the spaces so formed, also coating the surface.
This pigmentation must not be confounded with the change in the
color of the enamel which results from death of the pulp. In a vast
majority of cases the enamel in human teeth, by reason of its translucency,
transmits the varying changes in the color of the dentine. That there
is no pigmentation of enamel itself in these cases may be proved by cut-
ting out the underlying dentine and filling with chloride of zinc or some
other white filling. Indeed, this mode of " mechanical bleaching " has
come into almost general practice.
This naturally brings us to the consideration of the dense polished
outer coat of enamel.
This layer shows a distinct characteristic variation from the prismatic
layer underneath. It is the outer capping of the prisms, and furnishes
a polished surface for the coat of mail which is best adapted for its
office. Smooth, polished surfaces are known to resist the eroding action
of acids longer than rough surfaces. The character and shape of the
ameloblasts change before this layer is formed. From a membrane
comj)osed of prismatic cells, they now assume a horizontal direction.
I have studied these changes in the form and direction of the axis of
the ameloblasts in sections of incisor teeth of rodents, where it was
very plainly shown. Enamel is here being continuously formed on the
labial side, at the base of the tooth, to supply loss by attrition. All
the stages of calcification, as well as the changes which occur in the
ameloblasts, are plainly demonstrated in one tooth. Below a certain
point the enamel-prisms are yet unfinished, and here the ameloblasts
are seen to be prismatic in form ; toward the surface they gradually
shorten and widen until near the margin of the gum, when thev change
from a position at right angles with the axis of the tooth to a longitu-
dinal direction, Mrs, Emily Whitman also noted these changes in
studying the development of the teeth in the ray and rabbit. Mrs.
""
Whitman says
" The cuticula of the mammalian tooth has several times been found
to have the same structure, and it has been possible, in transverse and
longitudinal sections, to trace the gradual transition of the enamel-cells
into a perfectly homogeneous membrane (Fig. 338), the cylindrical cells
growing shcM'ter as they approach the croAvn of the tooth, until, instead
of being columnar, they are almost square, and finally flattened, and
at last the outlines of the cells quite disappear, and there is left a
perfectly homogeneous membrane.
