720M 
and no contractions are produced. They may, 
however, be reproduced by inverting the 
direction of the current by transposing the 
conducting wires of the battery. The cur- 
rent will now be inverse in B, and direct in 
A, fig. 398a. Or the fact may be illustra- 
ted by another disposition of the legs of the 
frog. Let both feet be immersed in one vessel 
and the pelvis in the other. The direct current 
may now be passed along the nerves in both 
limbs at the same time, until the phenomena 
of contraction on making or breaking cease. 
Inverse the current, and the contraction will 
again become manifest. This fact was first 
discovered by Volta, and this mode of exhibiting 
it has been described under the title Alternatives 
Voltianes. If the inverted current continue 
some time, exhaustion will be produced ; but on 
inverting it again or restoring it to its former 
course, the actions will recommence. 
4. These effects cannot be produced unless 
the nerves be in a state of integrity. If a liga- 
ture be tightly applied to the nerve of either 
limb close to the muscles, the contractions in 
that limb will no longer take place. Or to give 
a more striking illustration of this important 
fact, if adrop or two of pure sulphuric ether be 
applied to a point of either nerve, the contrac- 
tions in the limb of that side will be suspended 
until the effects of the ether pass off. These ex- 
periments unequivocally shew that the nerves 
are not merely conductors of the electrical cur- 
rent, but that the passage of the current through 
them developes in them a change which influ- 
ences the contractile force of the muscles. 
5. The influence of the galvanic current af- 
fords the most striking results when motor 
nerves are made the subject of the experiments, 
but Matteucci has shown that sensitive nerves 
are affected in an analogous way by the inverse 
and direct current. Ina living rabbit the sciatic 
nerves were exposed, and one nerve was devoted 
to the direct current, the other to the inverse. 
Opening and closing both currents were accom- 
nied with marked signs of pain, which, 
owever, were greatest at the closure of the 
inverse current. After a short time, the signs of 
pain are manifested only on opening the direct 
current and closing the inverse. 
The reader will scarcely fail to observe that 
both as regards the sensitive and motor nerves, 
the effect of the electric current, whether in 
causing pain or in producing contractions, is 
greatest when the current passes through the 
nerve in the course in which the nervous force 
would naturally proceed in the ordinary nervous 
actions. It is further worthy of notice that the 
continuance of the direct current exhausts the 
power of the nerve, while the reversal of the 
direction of the current, if not too long delayed, 
restores it. The continuous passage of the 
current, however, is not marked either by con- 
tractions or by pain. The interruption of 
the current by any means at once developes 
these phenomena; or even the diversion of a 
portion of it produces the same effect, as Mari- 
anini showed long ago. If, for instance, the two 
vessels in which the frog’s paws are immersed 
be connected by a conductor, as an arc of copper 
PHYSIOLOGY OF THE NERVOUS SYSTEM. 
or silver wire, contractions will take place on 
making or breaking the connection ; or if the 
wires of the battery be connected by a third wire 
of the same material before they dip into the _ 
cups, the same effects will be produced. 
Me continued transmission of an inverse 
current through a nerve increases to a remark- 
able extent its excitability. This is shewn by 
the following experiments: let the limbs of a 
frog be placed in two vessels of water and the 
current be passed through them in the manner 
above described, and let this be continued for 
a few minutes. After the lapse of this period, 
if the circuit be broken by taking one of the 
wires out of the water, the limb in which the 
current was inverse will be thrown into a state 
of tonic or tetanoid spasm for a few seconds, 
the tetanus ceasing with a clonic convulsion on — 
the renewed completion of the circuit:* 
That these phenomena are due to a change 
developed in the nerve (not to any affection of 
the muscles) by the passage of the galvanic — 
current, is clearly demonstrated by applying the | 
galvanic current to a muscle directly, having 
first removed as much nerve out of it as pos-— 
sible. The -muscle will contract equally on 
making and breaking the circuit, whatever 
be the direction of the current; nor is it 
sible to produce tetanic spasm, however long: 
the current may have been continued through 
it. The following experiment, suggested by 
Matteucci, also strongly confirms this view. 
Let the current be passed through the limbs 
of a frog in the ordinary way. After the curren 
has passed for 25 or 30 minutes, cut the nervé 
traversed by the inverse current, at the poin 
where it plunges into the thigh, and there wil 
instantly ensue a violent contraction of thal 
limb, which ceases very quickly. If, howeve 
instead of this the nerve be cut where it issue 
from the spinal cord, so as to leave a certai 
length of A nerve attached to the thigh, the 
will be a violent contraction of the mu 
which will be followed by others, and the lim 
will remain in a tetanic state for 10 or 1 
seconds or longer.t i 
The tetanoid contractions of the muselk 
may be produced by a rapid series of curren 
passed through the nerve alternately in the it 
verse and direct course, as by the electro-ma 
netic or the magneto-electric instrument. Th 
are always greatest and last longest if a port 
of the nervous centre remain connected 
the limbs. E. H. Weber has lately made 
very interesting series of researches by mes 
of the magneto-electric rotation instrumgé 
developing the peculiar mode of action of p 
ticular muscles.{ 7 
We cannot explain these remarkable phe 
mena on any other principle than on that 
supposes the developement of the nervous fo 
to be associated with the assumption of a pt 
condition by the molecules of the nerves un 
the influence of certain stimuli. The in 
current excites a polar state of greater int 
we 
DOS=— 
* Mattencci, Phil. Trans. 1846. 
+ Comptes Rendus, March 15, 1847. 
¢ Wagner, Worterbuch. Art. Maskell 
