OcrToBER 26, 1899] 
NATURE 
627 
Well,.as I say, just two years after this unfortunate prophecy 
of Johannes Miiller, Helmholtz ascertained that, if you deter- 
mine the time of response by stimulating a nerve at a given 
point, you can determine the rate of transmission by stimulating 
the same nerve at a measured distance, say a decimetre, above 
that point; for, as in this case, the response will be delayed, 
the period of delay measures the rate the nerve impulse has 
taken to travel over ten centimetres. Since then countless de- 
terminations have been made of the speed of the nerve-current. 
It has been found to vary with the temperature and with the 
character of the nerve stimulated ; it is less rapid in the nerve- 
centres than in the peripheral nerves, less in cold-blooded than 
in homeceothermic (or so-called warm-blooded) animals. But it 
never differs much from thirty metres per second. 
Moreover, this nerve current has been found to be always 
transmitted in both directions from the point of stimulation. I 
will not dwell on the exceedingly technical proof of this law, 
but merely recall the fact that whether the nerve stimulated be 
motor or sensory, the nerve current travels both ways along it, 
both towards the periphery (skin, muscle, &c., as the case may 
be) and towards the central nervous system. 
A most important fact is that an electrical disturbance ac- 
companies every stimulation of a nerve. If in the undisturbed 
condition we place the poles of a circuit with an interposed 
galvanometer at two points of a nerve (one on its surface, the 
other on a cut end), to ascertain its electric condition, we find 
that there is an electric tension between them, that there exists 
in the nerve a certain current. If we then stimulate the nerve, 
the current is seen to be reversed, or, as we say, undergoes a 
“‘negative variation,” and the rate at which this change is 
transmitted is sensibly the same as that of the nerve-wave. 
Matteucci, Du Bois Reymond, Bernstein, Waller have studied 
all the complex details of this process; so that it now ranks 
among the best known phenomena in physiology. 
We ask :—Are there, concurrent with this electric variation, 
modifications in the chemical and thermic condition of the 
nerve or nerve-centre? Yes, in all probability ; but the answer 
is not certain. Schiff thought that by stimulating the retina of 
the pigeon he induced a change in the temperature of the brain. 
Mosso also thought he could find localised areas of higher 
temperature in the brain after stimulating certain points; but 
the elevation of temperature is, to say the least, of low in- 
tensity and difficult to determine. 
In this rapid sketch, the last law I have to formulate is the 
law of the integrity of the organ. The physical and mechanical 
union may be maintained; but if its organic continuity be 
severed as by a cut, even when the two ends are joined up, the 
nerve-current is no longer transmitted. 
IV. 
Several hypotheses may be put forth as to the nature of this 
phenomenon. 
Formerly, when words were accepted in place of facts, it was 
said that there was a transference of ‘‘ animal spirits” (a con- 
ception due to Descartes); this was the current expression in 
the sixteenth, seventeenth and eighteenth centuries. A curious 
apparent confirmation was found in Richard Lower’s experiment: 
he tied a nerve, and saw that it swelled above the seat of lig- 
ature ; this, said he, was the accumulation of the animal spirit, 
arrested by the tightened thread. The experiment was a per- 
fectly valid one; and you see that from it it was possible to 
deduce conclusions that were perfectly false. The swelling was 
due to the increase of blood pressure and to inflammation. 
We may drop this old hypothesis of ‘‘ animal spirits,” and 
pass to four theories put forward to explain the nature of the 
nerve-current. 
(1) Mechanical Hypothests.—lIf, as is probable, the semi-fluid 
protoplasm of the nerve-cell and its prolongations form one con- 
tinuous whole, it follows that a mechanical disturbance of this 
liquid mass will be propagated to a distance along the whole 
length. Suppose a capillary tube filled with mercury; a dis- 
turbance of the mercury will be propagated the length of the 
tube, so that at the far end we perceive a vibration started from 
the opposite end. In this case the nerve-wave would be the 
molecular disturbance of a liquid enclosed in a capillary tube. 
This hypothesis would afford a fair explanation of the elec- 
trical phenomena involved ; for we know that the friction of a 
fluid in a capillary tube produces electricity. However, this 
mechanical explanation presents certain difficulties, for in a 
capillary tube the narrower its calibre the more rapidly the 
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NO. 1565, VOL. 60] 
vibration is damped ; consequently, it is hard to conceive that a 
vibration could be transmitted so as to be appreciable at the far 
end of a tube one or two metres long. It is true that we can 
form no supposition as to the absolute measurement of such 
perturbation ; and perhaps almost infinitesimally small forces 
are adequate. 
On the other hand, the electric disturbance that accompanies 
the nerve-wave does not lose intensity as it travels : on the con- 
trary, Pfliiger and other physiologists declare that it grows like 
an avalanche. Hence, taking all considerations into account, 
the nerve-wave is a phenomenon other than a mechanical 
vibratory molecular disturbance of the semi-fluid protoplasm. 
(2) Chemical Hypothests,—The transmission of the nerve- 
wave along a nerve has been compared to the explosion of a 
train of powder, or of mixed gases in a tube; and this you 
know is transmitted relatively slowly, nay, very slowly if the 
tube be of capillary dimensions. If, say, an explosive mixture 
of oxygen and hydrogen be contained in a very narrow tube, 
and a flame or spark applied at one end, the combustion will 
not be instantaneous, but will pass as a wave along the tube, 
and that a very slow wave, if the tube be narrow. 
What at first sight would give some plausibility to this hypo- 
thesis is the fact that a very feeble stimulus may call forth a very 
strong response. Take the amount of energy received by a 
surface of r sq. cm. from a candle 300 metres distant; it is 
I/10,000 millions of the total light-giving energy of the candle, 
a quantity whose absolute value is in one sense a negligible 
quantity, but which is adequate to give a sensory stimulus to 
the retina, The retina must be supposed to contain a quantity 
of accumulated energy susceptible of explosive liberation, so 
that the amount freed would be far in excess of the energy of 
the stimulus. 
But there is one very serious objection to this hypothesis ; it 
demands that the explosive tissue should be reconstituted afresh 
immediately after each explosion. It is not easy to see how the 
moment after the explosion, in the hundredth of a second, the 
nervous substance could be reconstituted afresh. Though 
serious, the objection is not irrefutable, for we know too little of 
the speed or slowness of the chemical changes of the organism 
to use this as an argument against any theory whatever. 
(3) Electrolytec Hypothests.—Certain chemical changes are 
characterised by their allowing of an immediate reconstruction 
after their occurrence, such are the phenomena of electrolysis. 
When a current passes through a saline solution, it is believed 
that, as it passes along, the salt is decomposed from place to 
place, and immediately reconstituted as soon as the current has 
passed, The passage of the electrolytic current is sometimes 
exceedingly slow. There is nothing to prevent our accepting 
some such explanation of the nerve-wave ; it has the advantage 
that it can be brought more or less into harmony with the 
chemical and the electrical hypothesis, and can indeed reconcile 
them. 
(4) Electric Hypothesis.—This supposes that an electrical 
current passes along a peculiar form of conductor—the nerve. 
The chief objection that has been urged, in the extreme slow- 
ness of the nerve-wave—30 metres per second—as against 700 
million metres, the alleged rate of electricity. But this omits 
to take account of the fact that electricity travels at this speed 
in good conductors only. Electricity passes along a conducting 
wire, ten thousand, a hundred thousand, times as fastas along a 
badly conducting tube; it is only reasonable to admit that the 
transport of electricity may be enormously retarded in a capillary 
tube filled with a very bad conductor. It has also been urged 
that, since different nerves can transmit very different sensations 
simultaneously to the different parts of the nervous system, 
there should be a blurring and confusion from the imperfect in- 
sulation of the tubes if it were electricity that they conducted. 
“* How, for instance,” we are asked, ‘‘ could nerve-cells of 
the cord and the brain communicate their electrical disturbances 
in narrowly localised groups with that extraordinary precision, 
without the neighbouring cells feeling the effect ?” 
We do not attach much weight to this objection because, in 
the first place, the axis cylinders have an insulating covering of 
myeline, as have also the cells of the brain; and again, in 
electric fishes, electric shocks one hundred thousand-fold as 
strong pass between certain organs without the rest being at all 
affected, so perfect is the insulation. ‘ : 
Thus the hypothesis that the nerve-wave is an electric phe- 
nomenon is fairly satisfactory, especially if we admit that it 
resembles electrolytic action, 
