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146 PATHOLOGY OF THE HARD TISSUES OF THE TEETH.

cases in which they occur. It is difficult to understand the wide
variations of sensitiveness that occur in different cases, some-
times between teeth in the same mouth, but more commonly
between the teeth of different persons. Occasionally, we may
find that much of this apparent difference between persons is a
difference in the disposition, or ability to endure pain. Apart
from that, however, there is a real and tangible difference among
persons in the sensitiveness of dentin. The teeth of some per-
sons are extremely sensitive, no matter when or how they are
cut, while the teeth of others are not.
The rule is that sensitiveness of the dentin to cutting instru-
ments indicates no especial danger to the pulp of the tooth, no
matter how extreme it may seem at the moment. Generally, it
is all over the moment the cutting is done. More rarely distinct
pain is felt for some time after the cutting is finished. Danger
to the pulp is indicated, however, when there is much mutilation
of the tooth, as by cutting away the enamel with a considerable
amount of dentin over wide areas, as is often done in preparing
teeth for abutments of bridges. Distinct hypersensitiveness of
the pulp tissue itself is developed and also, in many cases, ther-
mal sensitiveness and hyperemia, which may seriously endanger
the life of the pulp.
Sensation in dentin is derived from the dentinal fibrils.
The dentin has no nerves. Sensation is conveyed by the fibrils.
Whenever those of any part are cut off, the dentin, to which the
severed fibrils are distributed, loses sensation at once. These
fibrils are not simply glue-giving fibers, such as the fibers of
ordinary connective tissue, but are prolongations from the odon-
toblasts and are probably formed of the same material as the
nuclei of these cells. I have removed the fibrils from the den-
tinal tubules for a considerable length and stained them together
with the odontoblasts to which they belonged, and found them to
have the same reaction to staining agents as the nuclei of
these cells. Eose has examined this point with great care and
expresses the opinion that the fibrils are not glue-giving mate-
rial, as are ordinary fibers, but, chemically, are closely allied to
nuclein. We conclude, therefore, that the dentinal fibril is a
most highly endowed, living, functionating tissue, really a part
of the odontoblast — a prolongation of its nuclein and a func-
tioning part of it. Therefore, when we have touched a dentinal
fibril, we have touched the prolongation of an odontoblast, a
living part of the cell itself. The odontoblasts are in physiologi-
cal relation to nerve endings, as has been demonstrated by many