a. 
PHYSIOLOGY OF THE NERVOUS SYSTEM. 
Effects of the galvanic stimulus—The most 
perfect and powerful! physical stimulus of motor 
nerves, and that which most nearly imitates the 
natural mental stimulus, is the galvanic current. 
That the nerve should be duly excited by the 
galvanic current it is necessary that the current 
should pass along its fibres for however short a 
distance. If it pass across the fibre, and at right 
angles to it, it will produce no effect upon the 
muscles; but if it travel along it, even for the 
twentieth or a smaller portion of an inch, it will 
effectually excite the nerve and its muscles, 
just as when the will stimulates it to action. 
The influence of the galvanic current upon 
nerves is so remarkable that it deserves the 
careful study of physiologists and of practition- 
ers in medicine who often have recourse to the 
galvanic stimulus with the hope of rousing the 
dormant energies of nerves. Ht is to the Italian 
school of Physiciens that we owe the highly 
interesting series of facts which have been col- 
lected upon the influence of the galvanic cur- 
rent upon nerves, to Galvani, Valli, Volta, 
Marianini, Nobili, and, although last not 
least, to my distinguished friend, Professor 
Matteucci, of Pisa, by whose well-devised ex- 
riments and researches a flood of light has 
n thrown upon this hitherto obscure and 
difficult subject. 
Ishall content myself here with briefly no- 
ticing the points most deserving of attention as 
bearing upon the laws of action of the nerves. 
1. When agalvanic current is passed for how- 
ever short a distance along a nerve which 
contains motor fibres, muscular contractions 
will be excited at the moment of completing 
as well at that of breaking the circuit, but not 
while the current is passing. These phenomena 
take place whatever be the direction in which 
the current be passed, whether from the nervous 
centre towards the periphery, (when the current 
is distinguished as the direct current, ) or from 
the periphery towards the centre (when the 
current is styled the inverse current ). 
These effects may be produced in warm as 
well as in cold-blooded animals. In the former, 
however, the physical conditions necessary for 
the display of the vital forces continue for so 
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brief a period that cold-blooded animals should 
be selected for the experiments. On this ac- 
count, as well as because of their peculiar sus- 
ceptibility to the galvanic current, frogs are ge- 
nerally employed for this purpose. The most 
striking way of exhibiting the influence of the 
current, direct and inverse, upon the nerves is 
illustrated by the annexed woodcut. It repre- 
sents a frog prepared in the manner adopted by 
Galvani. The integuments have been removed 
from the lower extremities, which have been 
separated from the trunk by the division of the 
lumbar region of the spine. The lumbar nerves 
are carefully raised from the muscles on which 
they lie, but are suffered to retain their con- 
nection with the spinal cord and with the 
thighs. The pelvic bones, however, are re- 
moved so as to admit of the more free separa- 
tion of the extremities, as well as to isolate the 
nerves more completely. Each leg is immersed 
in a glass or cup of water, and the current is 
made to pass through the limbs by immersing 
each wire of the battery in the water of the 
cups. It is obvious that in one limb the current 
is direct, whilst in the other it is inverse. 
The advantage of this arrangement is that it 
affords great facility in making and breaking the 
current without bringing the conducting wire 
of the battery into actual contact with either 
limb. One wire may be left constantly in the 
water, while the other can be alternately intro- 
duced or removed from it as we wish to ob- 
serve the effects of completing or of breaking 
the current. 
2. If the current be allowed to pass for a 
short time through the nerves of a frog, pre- 
pared as before-mentioned, contractions will no 
longer take place in both limbs at the same 
time, but only in one upon completing the 
circuit, in the other on breaking. And we 
shall always find that the contractions occur on 
making in the limb in which the current is direct, 
on breaking in the limb in which the current is 
inverse. I find it useful to adopt the following 
formula to impress this fact upon the memory ; 
MD, BI, making direct, breaking inverse. 
3. If the current continue to pass for some 
time longer, these phenomena cease completely 
Fig. 398a. 
Lower extremities of the prepared Frog. 
P, positive wire of the battery; N, negative ditto. In the limb A the current will be sade in . it will be direct. 
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