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128 PATHOLOGY OF THE HABD TISSUES OF THE TEETH. ;
dental Membrane." The photomicrograph, Figure 155, shows
the repair of an absorption which had occurred in the root of a
tooth in which the full contour is rebuilt with cementum.
These illustrations are brought prominently forward in this
place as the histo-physiological expression of the fact that den-
tin possesses no means of physiological repair. Additions may
be made to it by the action of other tissues, but dentin never
repairs itself. It is never repaired by dentin under any circum-
stances, excepting such as may be done by calcifications occur-
ring in the pulp chamber. These may sometimes effect a repair
of an exposure of the pulp. This latter is a physiological process,
however, in which the pulps of teeth are inclosed by further
deposit of calcified matter, by cells whose physiological purpose
has been the building of the dentin originally. Their sphere of
action is always within the pulp chamber, never elsewhere. The
suggestion has been made that some additions of calcium salts
may be made on the walls of the dentinal tubules, narrowing their
caliber. This is plausible, but, as yet, no sufficient series of
measurements have been made to determine the facts. An
injury of any character occurring to the dentin during its devel-
opment remains an injury for life. In the study of atrophy, we
find sheets of interglobular spaces passing throughout that por-
tion of the dentin being formed at a time of malnutrition. These
are never repaired. They form an injury that remains perma-
nently. Dentin, or enamel, once formed, is formed for all time
it never can be re-formed, changed, or improved in its character
or qualities. It is fixed material; nature has furnished it with
no physiological means of repair or betterment.
Studies by Dk. J. Leon Williams.
ILLUSTRATIONS: FIGURES 156-158.
Dr. J. Leon Williams, of London, published a series of
studies (Dental Cosmos, 1897) of faults in the teeth of animals
as compared with faults in the teeth of man, and of the beginning
of caries under plaques formed on the surfaces of teeth, which
he found to be composed mostly of microorganisms agglutinated
together. He found the structure of the human teeth much
more perfect than that of the teeth of the animals. The faults
in structure were less frequent in man and generally of less con-
sequence, notwithstanding the fact that animals do not suffer
from caries of the teeth, except in a few rare instances of captive
animals kept in cages, some domesticated house-dogs, etc.