Page 251 - My FlipBook

SYSTEMIC CONDITIONS. 127
shows the Haversian canals cut lengthwise for the most part. In
studying these, it will be seen that nearly the entire substance
of the bone as first formed has been cut away and is replaced in
the form of Haversian systems, and in many of the bones we find
no traces of subperiosteal bone left, except, possibly, on the
outer surface. In this cross section, however, we find many
patches of subperiosteal bone scattered through it, though most
of it is occupied by the Haversian system bone. Each Haversian
canal has its blood vessels. In many bones we find the Haversian
systems have been cut out again and again and new Haversian
systems built in their places. This is not done by removing the
old Haversian systems individually, but by absorptions that
seem to run through the bone at random, often cutting out parts
of these systems and leaving parts by which such additional
cutting and rebuilding is readily recognized.
This is nature's manner, or the physiological plan of making
nutritional changes in the bones; a plan perfectly well known
to histologists and physiologists. There is no such plan for
nutritional changes in the human teeth. Normally, there is no
absorption of the roots of the permanent teeth, nor any absorp-
tional changes going on. Normally, as a physiological process,
however, the roots of the deciduous teeth are removed by absorp-
tion in the shedding process. Figure 153 is a photomicrograph
from a line of absorption at a in a cross section of a deciduous
tooth, showing the peculiar notching known as the lacuna? of
Howshijo, where the dentin and cementum were being removed
by the process of absorption. In the bones the process of absorp-
tion is practically the same in kind and quality, and though we
name the cells which absorb bone, osteoclasts, and those which
absorb the roots of teeth, odontoclasts, there is really no differ-
ence in the two processes physically or physiologically. Figure
154 is a photomicrograph from an example of this in bone, which
may be compared with that in dentin and in cementum. An
absorption of bone is always repaired with bone. It may, if it
is on the surface, be repaired by subperiosteal bone. If within
the bone, it is repaired by Haversian system bone. An absorp-
tion of any portion of a tooth, dentin or cementum, if repaired
at all, is repaired with cementum ; no matter how deeply it may
have cut into the dentin, it is never repaired by dentin. Many
of these repairs are found in the study of the histology of the
teeth, where, for some cause, an absorption has occurred, cutting
deeply perhaps into the root of the tooth. Several pictures show-
ing this are published in my book on "Periosteum and Peri-