RECORD OF MEDICAL SCIENCE.
ANATOMY AND PHYSIOLOGY
Sensibility of the teeth explained on Hydrostatic principles- By J.
Neill, M. D., Demonstrator of Anatomy in the University of Penn.—
Dr. Neill called the attention of the College to the fact that no satis-
factory explanation had yet been offered of the manner in which ex-
ternal impressions made upon the teeth are conveyed to their nervous
pulp within. There is no difficulty in understanding the manner in
which the nerves of the eye, of the olfactory organs, of the tongue,
the skin, &c., are reached by the impressions for the reception and
transmission of which they are respectively designed. But in the teeth
the nerves are enclosed in a bony case ; and yet experience teaches us
that impressions made upon the teeth do reach the nerve within, and
are conveyed through it to the nervous centre. How then is it that heat,
cold, acids, or pressure when applied to the external surface of the teeth
affect the nervous pulp within them ? Some may suppose that the
impression is transmitted to the nerve through the gums. It is doubtful
whether the gums in their ordinary healthy condition possess any very
great amount of nervous sensibility, at any rate this explanation is un-
tenable. The old doctrine of increased sensibility of the teeth from in-
flammation has passed away ; the teeth have neither blood-vessels nor
nerves, and consequently cannot become the seat of inflammation. Dr.
Neill believed that the transmission of impressions through the teeth
may be explained by a reference to the anatomical structure. The
parietes of the tooth are not perfectly solid ; even the enamel is com-
posed of a congeries of hexagonal prisms, which are not in perfect jux-
taposition. This may be shown by the microscope. The bony por-
tion beneath the enamel is even less solid, it is somewhat allied to bone,
and consists of a number of minute hollow tubules, which pass from
the circumference to the centre of the teeth, with the internal cavity
of which they communicate; these tubules are filled with a fluid secreted
by the pulp. Now if the tubules, thus filled, pass from the circumfer-
ence to the interior of the teeth, may not pressure applied without, by
compressing the enamel and the fluid of the tubes, affect the nervous
pulp within, by subjecting the latter to a species of hydrostatic
pressure, the amount of which can be measured ? Whatever reduces
the thickness of the enamel, or uncovers any portion of the dentine
increases the painful impressions caused by external pressure. Thus
acid, by removing the mucus by which the teeth are ordinarily covered
and perhaps a portion of the enamel, increases the sensibility to exter-
nal impressions. When the teeth have been affected by acids, no dis-
agreeable sensation is experienced until pressure is made upon them.
Mr. Nasmyth, it is true, denies the tubular structure of the dentine, but
it is admitted by all other writers on the anatomy of the teeth. Dr.
Neill presented a specimen which would, he remarked, under the mi-
croscope, show very clearly the tubular structure of the dentine. Of
the conveyance of- impressions to nervous expansions by hydrostatic
pressure, we have an example in the ear. Dr. Neill supposed that all
those substances which are known to relieve tooth ache, when applied
to the affected tooth, acted by coagulating the fluid of the tubuli, and
thus preventing hydrostatic pressure.—Transactions oc the College of
Physicians.