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PERIDENTAL MEMBRANE. 155

BLOODVESSELS AND NERVES OF THE MEMBRANE.
Bloodvessels.—The blood-supply of the peridental membrane is
very abundant. Several vessels enter the membrane from the bone in
the apical region. These arteries branch and divide, forming a rich
network, from Avhich the capillary vessels are given off. The arterial
network is constantly receiving vessels which enter the membrane
through Haversian canals opening on the wall of the alveolus, and in
this wav the size of the vessels passing occlusally is maintained. Ar-
terial vessels also enter the membrane over the border of the process.
This double or triple supply of the membrane is important, as it main-
tains the health of the membrane when the supply entering through
the apical region is entirely cut ofi' by alveolar abscess. While the
arterial supply of the membrane is very rich, the capillaries in the
membrane are comparatively few. This is, however, a characteristic
of connective-tissue membranes.
The nerves of the peridental membrane have not been sufficiently
studied to be described in detail. Six to eight medullated nerve trunks
enter the apical region in company with the bloodvessels, and they re-
ceive other trunks through the wall of the alveolus and over the border
of the process, but the manner of their distribution and the nature of
their endings are not known.

THE CHANGES WHICH OCCUR IN THE MEMBRANE WITH AGE.
When a tooth is erupted the roof of the bony crypt in which it was
inclosed in the body of the bone is removed by absorption and the
crown advances through the opening. The diameter of the alveolus at
that time is, therefore, greater than the greatest diameter of the crown,
and the peridental membrane which fills the space is very thick. By
the formation of bone on the wall of the alveolus and the formation
of cementum on the surface of the root the thickness of the membrane
is reduced. In the young membrane most of the large bloodvessels are
found in its outer half, forming a rather defined vascular layer near its
centre. In the old membrane most of the bloodvessels are found very
close to the surface of the bone, often lying in grooves in its surface.
Both young and old membranes are illustrated in Figs. 138 and 139,
which are taken from the temporary teeth of a sheep, one just after
eruption and the other shortly before the time of shedding.