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

NEURO-PHYSIOLOGY. 47
tion between motor and sensory nerves.
Excitants affect
either. Their electrical condition is alike. Toxic agents
only seem to influence them differently because they act
variously upon the parts, muscular or ganglionic, which lie
at their extremities. Their degenerations and regenerations
present no distinct difference, and both are capable
of carrying messages in either direction.*
l^ot less interesting and demonstrative are the cases in
which M. Bert has grafted the tip of the rat's tail into the
dorsal region, and after a time cut the tail oft' at the
normal point of attachment. A year subsequent to this
operation irritations of the end of the tail were felt as pain,
so that its sensitive nerves, now in union with dorsal nerves,
must have carried impressions in a reverse direction.
Where the central end of the lingual and the outer end
of the hypoglossal nerves are united, it is found that irritation
of the lingual causes pain and tongue movements
at one and the same time. The excitation is, in this case,
propagated in both directions, so that it would seem as if
in all cases of nerve irritation or excitement the impression
affects the nerve from end to end, precisely as happens
when a stretched cord is made to vibrate throughout
its length by a force applied to any portion of it.f
This remarkable fact which has now been set in the
clearest light by the experiments just related, as well as
by the previous results obtained by Dr. Du Bois-Reymond,
is full of interest to the pathologist as well as to the pliys-
* Vulpian, op cit., p. 286.
f We may also illusti'ate the case by a tube open at one end, and having
a whistle at the other. Blow into the centre of the tube and the air
moves in both directions, but as it
emerges it makes a sound only at the
end possessing a whistle, so that a careless observer would say that the
air moved only in one direction. We pinch the trunk of a motor nerve
and the muscle moves, but if at the other end of the nerve there were
also a muscle, we should in like manner be able to see it brought into
action.