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CHEMICAL ABRASION. 37 ;

CHEMICAL ABRASION.

This consists in a gradual destruction of the entire
substance of the crown of the tooth—the enamel and
the dentine. It is an affection of comparatively rare
occurrence. It assails the superior more often than

the inferior teeth, though both are subject to it. It
begins upon the points of the central incisors, wasting
them away most rapidly at the median line, from

which it progresses each way, involving the lateral
incisors, cuspids, and sometimes the bicuspids, so that
a curved line is presented by the edges of the teeth,
of greater or less inclination, according to the rapidity
of the process. When the superior teeth only are
affected, the opening between the ends of the upper

and of the lower front teeth, when closed, is a semi-
ellipsis. If the inferior teeth are affected, as is some-
times the case, then the opening will be an ellipsis.
In the case of Mr. Gr., the affection had been in
process about two years and a half; the wasting away
extended to the first bicuspids both above and below
and when the jaws were closed, the ends of the upper
and of the lower central teeth were about one third
of an inch asunder, and the opening was of the

elliptical form. It was a mystery to him. Two years
and a half before, his anterior teeth shut close together