BOOTS OP THE SPINAL NERVES.
518
latter roots have been previously divided, no mechanical irritation of
the spinal cord itself, or of the posterior roots connected with it,
excites the slightest muscular contractions.
The experiments with the galvanic stimulus of a single pair of
zinc and copper plates are equally conclusive.
The application of galvanism to the anteriqr roots of the spinal
nerves, after their connection with the cord is divided, excites
violent muscular twitchings; the same stimulus applied to the
posterior roots is attended ivith no such effect. This result is very
remarkable, and is what I did not at all expect: for I imagined that,
although the posterior roots are endowed with sensation merely, they
might still conduct the galvanic fluid to the muscles; and when a
powerful galvanic pile is employed, this is inevitably the case (as in
Magendie’s experiments), the strong galvanic current being con¬
ducted by the posterior root of the nerve as by any animal substance.
The stimulus of a single pair of plates, however, while it causes the
anterior roots of the nerves to give rise to muscular contractions, has
no such influence when applied to the posterior roots. In this ex¬
periment it is necessary to be very cautious that the plates are
brought into contact with no other parts than the nerves.
The experiments may be performed on frogs in the manner adopted
by Sir C. Bell and M. Magendie, and the results will be as decided
as in those just detailed. If in the same frog the three posterior roots
of the nerves going to the hinder extremity, be divided on the left
side, and the three anterior roots on the right side, the left extremity
will be deprived of sensation, the right of motion. If the foot of the
right leg, which is still endowed with sensation but not with the
power of motion, be cut off, the frog will give evidence of feeling
pain by movements of all parts of the body except the right leg itself,
in which he feels the pain. If, on the contrary, the foot of the left leg,
which has the power of motion, but is deprived of sensation, is cut off,
the frog does not feel it. This experiment is the most striking of all,
and the result is decisive; because, on account of the small number
and large size of the roots of the nerves going to the posterior ex¬
tremity in the frog, we can be certain that all are divided.
The foregoing experiments leave no doubt as to the correctness of
Sir C. Bellas theory.
I may further remark that the section of the posterior roots is fre¬
quently attended with very distinct manifestations of the sensation of
pain in the anterior part of the body.
The difference with regard to motor and sensitive properties,
which has been established so clearly in the case of the anterior and
posterior roots of the nerves, has not been by any means demon¬
strated to exist between the anterior and posterior columns of the
spinal cord.