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VITAL, PHENOMENA IN CARIES. 147

authorities, hence the sensation arising from an injury to a
dentinal fibril is transmitted to the brain and registered as pain.
This is illustrated in the diagram, Figure 159. An injury to den-
tinal fibrils near the enamel, e, is transmitted through the thick-
ness of the dentin, d, to the odontoblasts, o, without nerves,
because the cell is continuous through the dentin. The impression
is then transmitted to the proper conveyors of sensation— the
nerves in the pulp, p, and by them to the brain.
Sensation is a function of living cells especially. Nerves,
while they may have sensation of themselves, as other living
tissue, are, functionally, the conveyors of sensation from, or
impulses to, the cellular elements of the body. An injury to a
sensitive nerve trunk is usually represented in the sensorium as
coming from the tissue to which its fibers are distributed. Most
persons have felt the shock of an injury to the ulnar nerve in
the little finger and the half of the third finger next to it, when
they have struck the so-called "crazy bone" near the elbow.
This is because the nerve injured is distributed to these parts.
Impulses are conveyed outward by motor nerves, the same as
the impressions of pain or touch are conveyed inward by sensory
nerves. A muscular fiber is often very long, yet there is gen-
erally but a single nerve plate, or nerve ending, on a muscular
fiber. An impulse through that nerve plate may, however, set
the whole length of the muscular fiber into contraction. This is
the analogue of the transmission of the sense of pain through
the thickness of the dentin without nerves.
Besides the sensory function, the dentinal fibrils have also
the function of the maintenance of the integrity and qualities
of the dentin. It goes no farther than this. The dentin has no
reparative functions. The fibrils, though highly endowed with
sensation, do not exhibit any function of repair. But that there
is a power of maintenance of the qualities of the dentin, seems
clear from clinical evidence. There is a slow but distinct dete-
rioration in the qualities of the dentin in pulpless teeth, which
is too apparent to escape the clinical observation of a close
observer of long experience. This becomes especially apparent
in careful laboratory study. The enamel is much easier to split
off from the dentin in teeth that have long been pulpless. The
dentin very slowly loses strength as compared with neighboring
living teeth from the same or other mouths. It loses much of
its translucency, loses elasticity and becomes more brittle. The
younger the person at the time of the loss of the pulp, the more
rapidly these changes occur. This occurs, however, much less