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HISTOLOGY AND CHEMISTRY OF THE TEETH. 147

quantity of organic sul)stance near the enamel and cement may be
increased by numerous small interglobular spaces (stratum granu-
losum) filled with protoplasm. Larger interglobular spaces are
also often found in dentine, especially in teeth of poor structure.
The sheaths of the tubules are remarkable for their great
power of resistance to acids and to putrefying agents. Accord-
ing to Hoppe-Seyler/^ the tubules oC many fossil teeth may be
sufficiently isolated as to permit of examination under the micro-
scope. The isolation is performed by extracting the lime-salts,
washing carefully with water to remove the salt and acids, then
boiling for a certain length of time in water.
The enamel covers the crown of the tooth somewhat like a
thimble. At the margin, that is, at the neck of the tooth, it is
very thin, thicker toward the grinding-surface, and thickest on
the cusps of the molars, where it sometimes has a thickness of
2.5 mm.
It is the hardest tissue of the human and animal body. Ac-
cording to Hoppe-Seyler, only the siliceous urinary calculi which
occur in ruminants, and, perhaps, the siliceous sheaths of bacil-
laria, exceed it in hardness.
In the fissures of molars and bicuspids, as well as in the fora-
mina coeca of molai's and superior lateral incisors, the formation
of the enamel-cap is very imperfect. These localities conse-
quently form loci minoris resistentiae which but feebly resist
the attack of caries. Indeed, they rather induce it by the reten-
tion of food-particles. The fissures, cracks, etc., of old enamel
act in the same way, although in a minor degree.
Morphologically, enamel consists of four- to six-sided straight
or undulating, usually parallel prisms, which are separated by
an extremely thin layer of intervening substance (binding sub-
stance). The spaces between the prisms are, however, under
normal conditions, by far too narrow to permit the entrance of
micro-organisms.
The enamel-cuticle (iSTasmyth's membrane) forms a thin, trans-
parent, glueless layer on the crown of every tooth, in so far as
the latter is not worn down by mastication. It resembles the
sheaths of the dentinal tubules in the high power of resistance
which it shows to the action of acids and putrefactive agents.