Injuries of nerves and their consequences - Reflex Sympathetic ...

NEURO-PHYSIOLOGY. 39
nerve branches whicli supply tlie upper portions of a
limb.
Nerves of j^ciin.
— There is a good deal of doubt as to
whether or not the various forms of sensory impressions,
such as heat, cold, tickling, etc., afi'ect the sensorium
through distinct sets of fibres, or whether they travel on a
common track with impressions of touch and pain. These
latter are, certainly, in a manner distinct; but, although
it has been made clear that one or more of the varied
forms of sensory expressions may disappear
without any
other being lost, this is no absolute proof. that separate
fibres are needed for their conveyance to the centres.
Brown-Sequard is disposed to admit the existence of a
number of such sets of nerve tubes ;
but while there is
a good deal in general nervous pathology to make it
probable that the distinction of sensations depends upon
their central discrimination alone, the phenomena of
nerve injuries tend to discredit by negative proof the
theory of the existence of numerous sets of sensory nerve
fibres, as I shall have future occasion to point out.
Of the exact nature of the aflfection of nerve matter
which produces pain we know but little. In general, for
the causing of pain we must have mechanical impressions
far more severe than those which excite the nerves of tact
and temperature ;
while it would also seem that neural
excitation attaining the grade of pain is possible, as in
many neuralgias, without the least evidence of visible
organic alteration of tissue.
In the normal condition, the sense of pain is very unequall}'
distributed as regards the skin surfaces, and bears
no definite proportion to the acuteness of the tact sense.
Indeed, there are certain portions of the tissues in which it
is extremely deficient, as the back and a limited space
over the tibia. As a rule, the thigh
is less sensitive than
the arm, the leg than the forearm; but as no complete