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THE PERICEMESTUM OR ALVEOLO-DENTAL MEMBRANE. 91

pighii of the epithelium are seen to be in columns, and from these
new cells are formed, which flatten and lose their vitality as they
near the outer surface, wdiere they are given off as lifeless scales.
The Pericementum or Alveolo-dental Membrane.
This is a formation of fibrous connective tissue, having its origin
from the outer layer of the sacculus (Fig. 75). It differs from the gum
tissue in that it is not so dense. Tomes speak of it as having a rich
supply of nerve fibers.
Fig. 75.

Alveolar dental mcmbrune (section from ja,\ d kiitfiu : 1, alveolar dental membrane ; 2, bone
of alveolus; 3, dentin.

The pericementum passes into the gum at the tooth neck, w'here it is
thicker than at any other part. It is seen to be everywhere connected
with the periosteal membrane of the alveolar process. The general
direction of its fibers is across, slightly wavy, downward from the
alveolus to the tooth root. In the young tooth there are no breaks in
the continuity. There is no appearance of two separate membranes,
one for the root and the other for the alveolus; but simply a mem-
brane common to both surfaces.