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88 Diseases of the Teeth,
where the enamel is thinest, and tartar and other matter most
liable to collect.
There is no difficulty in distinguishing external caries : on
the sides of the teeth, a depression or hole more or less exten-
sive will be seen, of a greenish or brown color: when the
disease has attacked the molares or bicuspides, on their conti-
guous sides, it will be indicated by a dark or bluish color, ap-
pearing through the enamel, between them. In the incisores,
it is at once seen, and cannot be mistaken.
The predisposing causes of external caries, are the same as
those of the first species, or of internal decay. To these may
be added, defective enamel, presenting what is called the
honey-comb appearance : such teeth are sometimes cf a yel-
lowish color, and of a brittle texture, and are strongly pre-
disposed to decay.
The exciting causes, are chemical and mehanical agents.
TARTAR OF THE TEETH OR SALIVARY CALCULI.
This substance, which is secreted with the saliva, often col-
lects ahout the teeth in great quantities ; particularly, about
those near the entrance of the salivary ducts. When it is first
secreted, it is quite soft like slime, and may be easily removed
with a tooth-brush and dentrifice ; but if neglected, it soon be-
comes concrete and hard. Tartar of the teeth i« a common
cause of external caries, by acting chemically on the enamel,
and exposing the bone of the teeth, to the action of foreign
matter, and to inflammation. It discolors, erodes, and de-
composes the enamel, as is always seen with teeth, that have
been covered with it for a length of time. In this case, the
teeth next the gums are found rough, and of a yellow color,
and frequently, the enamel is completely destroyed across
their necks. .
PUTREFACTIVE AGENTS.
By putrefactive agents are meant, the particles of food lodg-
ed in the interstices of the teeth, which if not removed, undergo