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868 PATHOLOGY OF THE DENTAL PULP.

narro^A'ing of the canals, but I have seen very few. In those cases in
which the root-canal is obstructed I have generally found it to be with
pulp-nodules, cylindrical calcifications, or general calcification of the
tissues of the pulp. In Fig. 463, at c, c, there is a blocking of the

Reduction of the Size of the Pulp-chambev by deposit of secondary dentine excited by abrasion.
A, Diagram of a Lower Molar badly worn, showing narrowing of the pulp-chamber.
B, Illustration of the Tissue of the Secondary Deposit: a, «, ber, from which the secondary growth has begun ; in the rootwise portion theie iippears a second
line of beginning; b, gloliuhir dentine, in which a few dentinal tubes may be seen traversing the
the globules c, irregular crystalline deposits.
;
entrance to the root-canals by calcific deposits in the form of irregular
crystalline masses. In this figure, at A, is represented a lower molar
with the crown very much abraded by mastication, the pulp-cavity of
which is very much reduced by the growth of secondary dentine. The
lines «, rt, o, a point out the original outlines of the pulp-chamber, and
the new tissue formed is seen to be quite regular in its structure. The
rootwise portion of the new formation shows two lines of the beginning
of new growth, showing that there had been a cessation and rebeginning
of the process. In passing I will call attention to the peculiar structure
at 6, in which a series of globules are seen to have a few dentinal tubes
passing through them. Here the secondary growth has become mark-
edly abnormal ; and tliis abnormality is expressed in some form in
almost every case in which the pulp has died from exhaustion follow-
ing large deposits of secondary dentine.
It is found that these growths of secondary dentine caused by abra-
sion are not confined to the particular teeth worn, but if there is con-
siderable wear of the teeth generally, those that may have escaped
abrasion will have the growth of secondary dentine in very nearly the
same degree as those that have actually suffered from the wear.
Grovths of Secondary Dentine excited by Carief^ present some features
that differ markedly from those excited by abrasion. As has been seen,
deposits of secondary dentine excited by abrasions are very generally
equally distributed on the inner walls of the pulp-chamber. In their
structure and in the direction of their tul)ules they resemble very closely
the normal dentine. In the study of secondary dentine the growth of
Avhich lias been excited by the irritation of caries this is quite different
in a large proportion of cases. The irritation is confined to a.smaller