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SENSITIVENESS OF THE DENTINE. 1009

Pathologists have generally regarded the nerve-endings as receiving
the initial impression or injury in the production of pain on touch. It
is a subject, however, that has always been veiy obscure—in fact, unde-
nionstrable. Nerve-tissue is certainly itself sensitive, for pain may be
produced by the injury of sensory nerves ; but pain so induced is referred
to the tissue to which the nerve is distributed, not to the nerve itself.
Indeed, so far as consciousness goes, we would never know that we had
nerves. In most positions in the body nerve-endings and the cellular
is impossible to injure one
elements are in such close relation that it

\~d

Group of Odontoblasts with their Processes (dentinal fibrils): a, odontoblasts; b, fibrils; c, nerve-
supply. Irritation at the point d produces pain.
without injuring the other. The experimental isolation of a tissue-
injury from injury to the nerves (these being intact) for the purpose of
determining which receives the initial lesion causing pain does not seem
to have been thought of or to have been possible to other positions than
the one under discussion, and the general fact that without nerves there
is no pain seems to have led physiologists to adopt the supposition that
the nerves or their endings received the initial lesion giving the sensa-
tion of pain.
A close study of the literature of the subject of sensory nerve-endings
and their relation to the various sensations will serve to demonstrate the
obscurity of the subject and show how inaccurate is our knowledge of it.
In practice the cases in which pain is developed—that is, demonstrably
due to the direct wounding of nerves or nerve-endings—are very few
indeed. In this statement I have not reference to pain or perturbations
of functions that may result from disease of the nerve-centres ; in this
case we would deal with the nerve-cells (ganglion-cells) individually,
and their processes. These are protoplasmic bodies in which specialized
function is developed to such a w^onderful degree that their injury pro-
duces exceedingly complex results.
The striped or voluntary muscles furnish an example of the propaga-
tion of impulse along proto])lasmic bodies which is like that I have sug-
gested in case of dentinal fibrils, except that it is an eiferent instead of
an afferent impulse. There is but one motor nerve-ending in conjunc-
tion with a single muscular fibre, no matter what its length (Krause,
Koelliker) ; this is sufficient to communicate the impulse to contraction
to the whole fibre, though it may be much longer than the dentinal
fibril. Here it will be seen that the passage of an impulse along a
Vol. T.—64