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DENTIN. 121

Dentin belongs to the connective-tissue group, and is made up of
a solid organic matrix impregnated with about 72 per cent, of inorganic
salts ^ and pierced by minute canals or tubules, which radiate from a
central cavity which contains the remains of the formative organ, or
pulp. The minute cauals, or dentinal tubules, are occupied in life
by protoplasmic processes from the odontoblastic cells which form the
outer layer of the pulp. Dentin contains two kinds of organic matter,
the contents of the tubules and the organic basis of the matrix. The
dentin matrix, after the removal of the calcium salts by acids, yields
gelatin on boiling and resembles the matrix of bone, reacting in a similar,
though not identical, way with staining agents. The portion of the
matrix immediately surrounding the tubules shows different chemical

Fk;. KCx

Dentin showing tubules in cross-section: D/, denlinal tubules; i), dentin matrix; S, shadow of
sheaths of Neumann. (About 1150 X-)

characteristics from the rest of the matrix, resembling elastin, and re-
sisting the action of strong acids and alkalies after the rest of the
tissue has been destroyed. This portion of the matrix surrounding the
tubules and lying next to the fibrils is known as the sJicafhti of Neumann.
The dentinal tubules are from 1.1 to 2.5 microns in diameter, and
are separated from each other by a thickness of about 10 microns of
' Von Bibra gives the following analysis of dentin
:
(Organic matter 27.61
Fat 40
Calcium phosphate and Huorid 66.72
Calcium carbonate 3.36
Magnesium phosphate 1.08
Other salts 83