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610 DENTAL EMBRYOLOGY AND HISTOLOGY.
When it has become checked by constant breathing through the mouth,
it always remains chocked. Then, again, I have no doubt that teeth
the enamel of which has softened—during pregnancy, for instance
—may appear to harden ; but in such cases the softening has been
superficial, and this thin layer, by the processes of attrition, becomes
worn away, and the underlying layer of enamel, which has not been
alfected, becomes polished and cannot be told from the outer layer by
ord inary observat ion
Finally, then, enamel is the product of an organ which in the erup-
tion of the tooth ceases to exist, and which, if it does remain, is lifted
by the eruption beyond the source of nutrient supply and cannot there-
after exercise any influence over the physical condition of the enamel.
The many varieties of calcified tissues are due to the variations in the
form and nature of the matrices and the conditions and positions in
which the lime salts are laid down. They are not dependent upon any
variation or special vital function exerted upon the crystallizing prod-
ucts, but are due to the form of crystallization and the special salts of
calcium which enter into their formation. I do not think that the
theory ad\'anced that secondary changes do occur in enamel by recrys-
tallizatiou can be demonstrated.
Development of the Teeth.
It is with the feeling that no easy task lies before me that I enter
upon this section. Frey has well said that tooth-development is the
most difficult subject that embryologists are called upon to demonstrate.
The study of developing hair and glands is comparatively simple, and
cah'ified products ahjue do not seem intricate ; but when w'e approach
the consideration of both these conditions in one structure, we seem to
stand before an unfathomable mystery. It is only as we approach the
problem from the standpoint of general histology that we get anything
like a full and true interj^retation of the phenomena of tooth-devel-
opment. A microscopic examination of the intimate tissue concerned
in tooth-devel()j)ment led to the discovery that the teeth are developed
in the mucous membrane ; that, instead of standing in close relation to
the bony skeleton of the body, they are a part of its outer, or dermal,
system ; that they are developed in a similar manner to hair, nails, glands,
etc., by a jirocess of involution. Besides conducting us to new truths
concerning the natnre and constitution of the teeth, the microscope has
shown us how to apply this knowledge in a better system of hygienic
rules which aim at rendering their decay less rapid, their life more
vigorous, and their loss less frequent.
All who wish successfully to prosecute pathological in(piiries regard-
ing decay of the teeth will do well to acquaint themselves not only with
the histological character of the teeth of the human subject, but with
that of some of the lower animals.
I have comjmred tooth-development in the human with that found
in embryo pigs, calves, and lambs, and find that the porcine embryo
varies very little from the human, l^ig embryos are so easily obtained,
and in any size desired, that I am confident that they will come to be
610 DENTAL EMBRYOLOGY AND HISTOLOGY.
When it has become checked by constant breathing through the mouth,
it always remains chocked. Then, again, I have no doubt that teeth
the enamel of which has softened—during pregnancy, for instance
—may appear to harden ; but in such cases the softening has been
superficial, and this thin layer, by the processes of attrition, becomes
worn away, and the underlying layer of enamel, which has not been
alfected, becomes polished and cannot be told from the outer layer by
ord inary observat ion
Finally, then, enamel is the product of an organ which in the erup-
tion of the tooth ceases to exist, and which, if it does remain, is lifted
by the eruption beyond the source of nutrient supply and cannot there-
after exercise any influence over the physical condition of the enamel.
The many varieties of calcified tissues are due to the variations in the
form and nature of the matrices and the conditions and positions in
which the lime salts are laid down. They are not dependent upon any
variation or special vital function exerted upon the crystallizing prod-
ucts, but are due to the form of crystallization and the special salts of
calcium which enter into their formation. I do not think that the
theory ad\'anced that secondary changes do occur in enamel by recrys-
tallizatiou can be demonstrated.
Development of the Teeth.
It is with the feeling that no easy task lies before me that I enter
upon this section. Frey has well said that tooth-development is the
most difficult subject that embryologists are called upon to demonstrate.
The study of developing hair and glands is comparatively simple, and
cah'ified products ahjue do not seem intricate ; but when w'e approach
the consideration of both these conditions in one structure, we seem to
stand before an unfathomable mystery. It is only as we approach the
problem from the standpoint of general histology that we get anything
like a full and true interj^retation of the phenomena of tooth-devel-
opment. A microscopic examination of the intimate tissue concerned
in tooth-devel()j)ment led to the discovery that the teeth are developed
in the mucous membrane ; that, instead of standing in close relation to
the bony skeleton of the body, they are a part of its outer, or dermal,
system ; that they are developed in a similar manner to hair, nails, glands,
etc., by a jirocess of involution. Besides conducting us to new truths
concerning the natnre and constitution of the teeth, the microscope has
shown us how to apply this knowledge in a better system of hygienic
rules which aim at rendering their decay less rapid, their life more
vigorous, and their loss less frequent.
All who wish successfully to prosecute pathological in(piiries regard-
ing decay of the teeth will do well to acquaint themselves not only with
the histological character of the teeth of the human subject, but with
that of some of the lower animals.
I have comjmred tooth-development in the human with that found
in embryo pigs, calves, and lambs, and find that the porcine embryo
varies very little from the human, l^ig embryos are so easily obtained,
and in any size desired, that I am confident that they will come to be
