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126 PATHOLOGY OF THE HARD TISSUES OF THE TEETH.
of interglobular spaces that dip down on the axial surfaces and
finally reaches the pulp of the tooth. As the atrophy occurs
in all four of the first molars, it is rather rare that some one
or more of them is not destroyed. These are the principal faults
in the structure of the teeth that seem to influence caries. It
will be noted that all of these faults are such as are discoverable
by macroscopic or microscopic examination. No faults in the
chemical structure of the teeth have been found which seem to
influence caries in any marked degree. Even some of those
rare cases in which the cementing substance between the enamel
rods has failed, leaving the enamel rough and chalky, have been
found almost immune to dental caries. It would seem that such
teeth would be especially liable to decay early and quickly, and
they certainly would do so if they were in the mouths of suscepti-
ble persons.

Physiological and Pathological Differences Between Bone
and Dentin.
illustrations: figures 151155.
Some persons have seemed to suppose that the teeth and the
bones, being calcified tissues, should have similar physiological
processes of nutritional change and of repair, and that similar
changes might be expected as results of pathological conditions.
It is well known that, in certain diseases, as in rickets, the bones
become soft and may become hard again; and that nutritional
changes are going on continually in the bones up to an advanced
age, if not during the whole life, and that the bones are very apt
to become hard and brittle as persons grow older.
In the study of the bones, we find that, continuously, or at
least frequently, portions of the bone are being removed by
absorption and replaced by Haversian systems, so that the shaft
of a bone that has been formed largely as subperiosteal bone, is
finally converted almost or quite into bone composed of Haver-
sian systems. This is shown in Figure 151, which is a photo-
micrograph from a cross section from the femur of a young per-
son. In this figure the line drawn from the letter a passes over
laminae of subperiosteal bone, which have not yet been cut away.
The lines drawn from b point to Haversian systems, where the
subperiosteal bone has been cut away and new bone supplied
in the form of circular whorls, with a canal in the center of each,
which is called a Haversian system. In Figure 152, a photo-
micrograph from a section cut lengthwise of the same bone,