Periscope of Medical and General Science in their Relations to Dentistry. [Volume: 2, Issue: 12, July, 1861, pp. 681-694]

PERISCOPE OF MEDICAL AND GENERAL SCIENCE.

683

and the peripheral ramifications which connect them with the surface of
the body are no longer considered as successive portions of the same
great apparatus, but as organs totally distinct from each other; and, notwithstanding the juxtaposition of these different parts, their fibres are no
longer held to be continuous.
" These premises being laid down, let us proceed to examine the properties of the spinal cord with reference to the faculty of perceiving impressions from without. That physiologists should differ in their views
on this subject, will certainly not astonish you. Have we not seen how
different the results of the same experiment may prove to be, when the
conditions in which it is performed by various observers do not happen to
be identical? The controversy which formerly arose with respect to recurrent sensibility affords a notable instance of this.
"An experiment which, considering the novelty of its results, may
fairly be viewed as one of fundamental importance, is due to M. Van Dell.
The spinal cord being laid bare in a considerable portion of its extent, is
found to enjoy a high degree of sensibility in its various parts, more especially so in its posterior tracts; provided always that proper precautions
have been taken not to exhaust the animal's strength. Let the posterior
roots now be divided within certain limits: the corresponding region of
the spinal cord will be found to be totally insensible, while the power of
sensation persists in its remaining parts, which have not been deprived of
their sensitive nerves. The conclusion which evidently results from this
decisive trial is, that the sensibility of the spinal cord is exclusively derived from the minute ramifications which it receives from the posterior
roots.
"The ingenious author of this experiment believes the peripheral
nerves, which arise from the cells contained within the spinal cord, to be
alone endowed with the property of receiving impressions from external stimuli, while the central axis exclusively perceives the phenomena of
nervous action; in other words, although cold, heat, electricity, and other
external agents produce marked effects upon the peripheral nerves, the
spinal marrow remains indifferent to their action, but is highly excitable,
when its own proper stimulus is applied. The impressions which it perceives are in every case the result of nervous vibrations, (if we may be
allowed to use the expression.) In this manner alone does it perceive
outward impressions, and transmit them to the encephalic mass; and it may
in this respect not unaptly be compared to nerves endowed with a special
sense. Allow us, for instance, to remind you of Magendie's experiments
on the retina: he destroyed it in animals, without provoking the slightest
manifestation of pain; he pricked it twice in the human subject in the
course of an operation for cataract, without the patient's perceiving the
experiment of which he unconsciously became the subject. The sensation
produced by the division of the optic nerve, in cases of total extirpation
of the eyeball, is that of intense light, but the patient feels no pain. The
retina, the optic nerve, are instances of highly sensitive organs which
remain indifferent to the action of ordinary stimuli; light alone exerts a
powerful action over them. Entirely similar to these are the special
properties of the spinal cord. External agents, when they come in contact with it, do not arouse its dormant powers; it is only capable of receiving second-hand impressions, so to speak.
" We shall therefore start in future from this principle, —that the nerves
are organs which extend from the surface of the body to the central parts