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216 THE MICRO-ORGANISMS OF THE HUMAN MOUTH.

^ PREDISPOSING CAUSES OF DENTAL CARIES.
Ill contradistinction to the exciting causes of caries, we char-
acterize as predisposing such conditions of individual or of all
teeth which divest them of their normal power of resisting ex-
citing causes, or by which they otter them especial points of
attack.
Predisposing conditions are only found in the teeth them-
selves, in their development, position, etc., while, on the other
hand, all external agencies are to be considered as exciting
causes. It is therefore not logical to regard gout, e.g.., as a pre-
disposing cause, because it is accompanied by an acid reaction
of the saliva. The action of an acid on the teeth will always be
the same, whether it is secreted by the mucus or salivary glands,
or formed in the mouth by fermentation, or introduced from
without; it is invariably an exciting, not a predisposing cause.
1. The structure of the teeth plays the most important part as
a predisposing cause of dental caries. Poorly developed, soft,
porous teeth, with many large interglobular spaces, are highly
predisposed to caries. As a lump of table-salt dissolves more
rapidly in water, on account of its porosity, than an equally
large piece of rock-salt, porous dentine is more rapidly decalci-
fied than well-developed, firm dentine, because the acid may the
more readily penetrate the tissue, and because less acid is
required to decalcify a porous than a hard tooth. It may easily
be proved by experiment that poorly developed dentine is much
more rapidly attacked by acids than sound dentine. (See page
196.) Xot only does the decalcification, but also the destruction
of the cartilage, advance more rapidly in the former case, be-
cause the micro-organisms, in many cases at least, enter the
interglobular spaces and more readily pervade and destroy the
entire softened tissue.
2. As a second predisposing factor I designate abnormally
deep fissures or blind holes (foramina cceca) in molars and super-
ior lateral incisors, especially in cases where the enamel also is
poorly developed. By their continual retention of food-particles,
such points directly induce caries, and offer but little resistance
to it in consequence of the absence of an intact protecting cover
of enamel.