— 
PHYSIOLOGY OF THE NERVOUS SYSTEM. 
and of longer duration than the direct current : 
hence the tetanic contractions which remain 
after the interruption of the current. 
It is sufficiently obvious why a contraction 
should occur at the moment of completing 
the circuit in a nerve. But why the same 
phenomenon should occur on breaking the cir- 
cuit is not easily explained. Marianini sup- 
posed that during the passage of a direct cur- 
rent through a nerve a part of the electricity 
accumulated in it, and on qhe interception of 
the current discharged itself, traversing the 
nerve in an opposite direction, and thus giving 
rise to contractions. It is not, however, likely 
that such an accumulation would take place, 
when the conducting power of muscle is so 
much better than that of nerve. And further, 
it is evident that this will not explain the ab- 
sence of contractions in the direct limb after a 
time on breaking the circuit. 
The truth is, that when a continuous current 
has been passed through the limb of a frog 
for some time a different state of excitability 
is established in the nerve of each limb ac- 
cording to the direction which the current 
had taken. That in which the direct current 
passes becomes exhausted in its powers, while 
that in which the inverse current passes has its 
excitability augmented. In the quiescent state 
a nerve maintains a certain state of tension: the 
application of a stimulus modifies this tension 
and causes the nerve to assume a new polar 
state, which displays itself in the contraction of 
muscles or the excitation of a sensation or of 
pain. The electric current is a powerful sti- 
mulus of the nervous force, and the greatest 
disturbance of the quiescent state of tension 
is produced by making the direct current. 
Upon this current beginning to pass, a new 
State of tension is established, which is disturbed 
by breaking the circuit: but if the current have 
continued to pass too long, the maintenance of 
the state of unnatural tension exhausts the ner- 
vous power, and the nerve ceases to respond 
to any stimulus. Whilst, however, the nerve 
of the direct limb has assumed one condition, 
that of the inverse limb has taken on a different 
one, in which the molecules of the nerve may 
be conceived to have a disposition the opposite 
to that which the direct current would produce. 
Hence only two electric stimuli would restore 
the particles of the inverse nerve, and so disturb 
the state of tension into which it had been 
thrown, namely, making a direct current through 
the nerve, or simply breaking the inverse. 
The tetanoid state which results from the 
continued passage of the inverse current through 
@ nerve is a phenomenon resulting from the ex- 
treme augmentation of its polarity. This state 
‘4s never produced by the direct current; and 
the instantaneousness with which it is removed 
by resuming the current, thereby restoring the 
State of tension which had been disturbed by 
breaking the circuit, is highly favourable to this 
Supposition. Anything which weakens the 
force of the current, or diverts a portion of it 
from the nerve, as the contact of muscles with 
the nerve, or of much moisture, or the occa- 
Sional reversal of the current making it direct 
720N 
where it had been inverse, will materially re- 
tard and diminish, or altogether prevent the 
developement of this phenomenon. 
The rapidity with which the changes in the 
nerves, however they may have been excited, 
are propagated, and the precision with which 
they are perceived by the mind in the case of 
sentient nerves, or produced by it in the case of 
motor nerves, are well calculated to excite our 
admiration. If the communication between 
the nerve and the centre be cut off, the will can 
exert no influence upon the muscles supplied 
by the nerve below the section; nor will the 
mind perceive any stimulus applied to parts 
which derive their nerves from below the sepa- 
ration. And this for an obvious reason; be- 
cause the solution of continuity of the nerve 
interrupts the propagation of the change which 
the mental or physical stimulus excites in it. 
In the case of the voluntary nerves, the effects of 
the mental stimulus are propagated no further 
peripherad than the point of section; and in 
that of the sensitive nerve, the change travels no 
further centrad than the same point. That this 
interruption is caused solely by the solution of 
continuity, and not by any alteration in the pro- 
perties of the nerve, is proved by the fact that 
the lower segment of the motor nerve will still 
continue to respond to a physical stimulus. 
Mechanical or chemical irritation, or the pas- 
sage of an electric current along it, will cause 
its muscles to contract. Such a degree of in- 
jury to a nerve as will break the continuity of 
the nervous matter within the tubular fibres is 
likewise sufficient to destroy its power as a 
propagator of nervous change. This effect may 
be produced by tying a ligature very tightly 
round a nerve, or by pressing it with great force 
between the blades of a forceps. The paralysis, 
which results from the compression of a nerve 
by a tumour or in any other way, is, no doubt, 
due to a similar solution of continuity in the 
nervous matter. 
These facts strongly denote the important 
principle in nervous physiology, that, in pro- 
pagating the influence of a stimulus, either 
from periphery to centre, or vice versa, the 
whole extent of the nerve-tibre between the point 
stimulated and its peripheral or central con- 
nection is the seat of change; and that the power 
of developing the nervous force is inherent in 
the nerve-fibre itself is shown by the fact that 
the stimulation of a muscular nerve, which has 
been separated from the centre, below the point 
of section is capable of exciting muscular action. 
The conducting power of a neive, then, results 
from its proneuess to undergo certain changes, 
physical or chemical, under the influence of 
stimuli. 
We may perceive, then, how important it 
must be to the healthy action of nerves to pre- 
serve them in a sound physical condition. A 
morbid fluid impregnating a nerve at any point 
may irritate it, or may suspend or destroy its 
inherent property by modifying its nutrition or 
impairing its physical condition. Thus we 
may paralyse nerves by soaking them in a so- 
lution of opium, or of belladonna, aconite, or to- 
bacco, in sulphuric ether, or other sedative or 
