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TEETH OF THE VERTEBRATA. 359

extended over the entire crown of the tooth, and becomes continuous
with its outermost layer in the vicinity of the neck. It is one of those
excessively thin membranes (not over -g- ^"^^^ "^ thickness, accord-
oto'o
ing to Kolliker) which are peculiarly indestructible and resist the ac-
tion of the strongest acids and alkalies. When stained with the nitrate
of silver, it shows a peculiarly reticulated structure resembling epithe-
lium, which is believed by Tomes to be due to the pitted surface on its
interior, by which it is applied to the enamelrprisms. Encapsuled
lacunae are likewise found in its substance, which would be difficult to
explain if it were not a part of the cementum layer. Tomes has like-
wise traced its connection with the outer layer of the cementum on sev-
eral occasions, and is therefore firmly of the opinion that it is a continua-
tion of this tissue.
Enamel.—The excessively hard, shiny substance investing the crown
of the tooth is the enamel. It is by far the hardest tissue to be met
with in the animal body, being at the same time the poorest in organic
constituents. Where it exists at all, it generally forms a cap of varying
thickness over the exposed part of the tooth, except in those instances
where there is an excessive development of cementum in this situation,
which causes it to occupy a position between the cementum and dentine,
as seen in the most exclusively herbivorous feeders, of which the horse,
cow, and elephant are good examples. Even here palseontological evi-
dence is quite conclusive in support of the proposition that their earlier
representatives possessed teeth with naked enamel-covered crowns. This
condition of nudity of the enamel is coincident with shorter cusps and
less elevated ridges of the crown, and, as we have good reasons to infer
from analogy, with more omnivorous habits of feeding. It can thus be
shown that this anomalous arrangement of the tissues is one acquired
comparatively late in the development of these forms for the exclusive
purpose of giving greater strength to the lengthened cusps, thereby
affording immunity from fracture during the act of mastication.
Von Bibra gives the following chemical analysis of the enamel of an
adult human tooth :

Calcium phosphate and fluoride 89.82
Calcium carbonate - . • 4.37
Magnesium phosphate ' ' 1-34
Other salts 88
Cartilage 3.39
Fat . 20
The proportion of the organic to the inorganic material is therefore 3.59
to 96.41, while in dentine it is 28.01 to 71.99. Its structure consists
of minute hexagonal prisms, known as enamel-fibres or enamel-prisms,
whose long axes, broadly speaking, have a direction at right angles to the
surface of the tooth. It is a comparatively rare occurrence to find the
fibres pursuing a perfectly straight course from the dentine to the sur-
face, but such Is found to be the case in the enamel of the manatee or
sea-cow and several other forms. Usually, they are tortuous, and fre-
quentlv decussate, as in the human subject, which renders it difficult to
trace the course of an individual fibre. A variety of patterns is pre-