OcToBER 26, 1899] 
NATURE 
629 
series of regular contractions which are easy to register. But 
when we quicken up the succession of the stimuli, there comes 
atime when the responsive contractions lose their regularity : a 
normal contraction is followed by a small one, a large one by a 
small one, and soon. Thus we can determine at what rate of 
intermission of the successive stimulations their responses lose 
their regularity: we find that when the intervals between the 
induction shocks are less than the tenth of a second, at the 
normal temperature of the body (39° C. for the dog) the con- 
tractions are no longer regular. Matters now go on just as if, 
after the large normal contraction, there were a refractory 
period, during which the excitability of the nervous system is 
lowered. 
Marey, in his beautiful researches on the heart, had previously 
showed that after a contraction of the heart there is a short 
refractory period during which it is not excitable. So, after the 
stimulation of the brain, a period not exceeding 1/10” inter- 
venes during which it is not excitable, a refractory period. 
Whatever be the temperature of the animal under experiment, 
we always find this refractory period, which, however, becomes 
easier to measure when the temperature falls, for then it 
lengthens out enormously. It is 0-1” at 39°C. ; e718" at 35° C.; 
and if we chill the dog greatly, to 30° C., it rises to 0°6”. Hence 
it is advantageous to chill warm-blooded animals for the purpose 
of these observations. 
It is noteworthy that this refractory period can be demon- 
strated otherwise than by electrical stimuli; mechanical shocks 
will also serve the purpose. If we poison a dog with chloralose, 
it becomes extremely sensitive to every mechanical disturbance. 
The least jolt of the table on which it lies makes it start, and 
though insensible, and not susceptible to pain, it responds to 
every jolt by a start. We can register these starts; and if, 
working with a dog cooled to 30°, we repeat the jars at intervals 
of less than half a second, the starts lose their regularity. 
Under these conditions a big start is followed by a small one, 
and wice versd, though the jolts of the table are quite equal. In 
successful experiments we may even find the second shock 
absent ; so that if the times of the successive jolts be noted as 
a a, a’, a’, at, &c., we only get responsive shocks at a, a2, 
a‘, &c. 
The physicists have given the mathematical and mechanical 
explanation of this phenomenon, which they call the ‘* syz- 
chronisation of the oscillators”; it has recently formed the subject 
of an important memoir by Cornu, which, however, I cannot de- 
scribe even in abstract here. Suffice it to say that these refractory 
intervals presuppose the existence of a refractory period, of a 
negative phase in the nerve-wave. 
The synchronisation of the nervous oscillation with that of the 
stimulus can only be explained by the assumption of the vibra- 
tion of an apparatus (the nervous apparatus) possessing a proper 
period of its own, and with which we regulate and adjust 
the proper period of a second apparatus (the stimulating 
apparatus). 
Thus, by this method we have succeeded in determining the 
duration of the nerve-wave ; and we may state that this is 
1/10’, an exceedingly slow rate as compared with electric 
or luminous vibrations, whose period is measured in 1I-one 
thousand millionths or billionths of 1”. 
We can also determine the form of the wave, and we find it 
approximate to our type 8. If we consider the period of 
o'1” which elapses between the stimulus and the completion of 
the nerve-wave, we find that it may be divided into two periods : 
(A) in the first part a second simulus will augment the effect ; it 
is the “* phase of summation” or positive phase of the wave. (B) 
in the second period the stimulus produces a decreased effect ; 
this is the ** phase of subtraction” or negative phase. Now the 
phase of summation is very small, scarcely more than 0°01’, 
while the phase of subtraction is very long, nearly 0°09” ; but I 
must not go into more detailon this point, lest I should enter on 
matters too strictly technical, which I prefer to avoid. 
VIL. 
In cold-blooded animals the phenomena are quite different ; 
and recent experiments have shown us how imprudent it would 
have been to generalise too hastily. If, indeed, we repeat the 
experiment on a tortoise, we find results apparently contra- 
dictory of those I have just related to you. A stimulus following 
another always appears to produce a stronger response than its 
predecessor. There is no refractory period, there ts a summation 
phase all the time. Of course I mean that the stimuli must not 
NO. 1565, VOL. 60] 
Jlickering ; that is, the images are becoming confused. 
be too far apart; if the interval exceeds 2”, two successive 
stimulations of the brain call forth equal contractions. But 
with intervals of less than 2” summation phenomena are always 
observed, the more marked as the interval between successive 
stimuli is decreased. Finally, as I say, there is no refractory 
period. 
Hence we may conclude that in cold-blooded animals (at least 
in the tortoise) the nerve-wave has a different form from that of 
the dog ; after the displacement from the primitive position of 
equilibrium there is only a slow and gradual return, without any 
such backward oscillation as explains the negative phase in 
the dog. This form of wave we have described under the third 
type of damping (type y) (Fig. 3). 
This type of wave is exceedingly slow; if the tortoise be 
chilled by the use of suitable stimuli, we can estimate its 
duration at 2”. But with normal animals at 15° C. the period 
may perhaps be taken as 1’’. 
This difference of tenfold is not surprising ; there was no ante- 
cedent improbability in conceiving that the nervous phenomena 
of a tortoise are ten times as slow as those of a dog. 
VIII. 
The fact that the nerve-wave lasts one-tenth of a second 
in the dog, as it probably does approximately in man, opens up 
a field of interesting considerations which confirm the results of 
direct experimental physiological observation. 
If the nerve-wave lasts 1/10’, it follows that two nerve-waves 
cannot remain completely dissociated when they follow at shorter 
intervals than this. Suppose that a stimulus of light calls forth 
a nervous reaction, a sensation ; this reaction, this sensation, 
will last at least one-tenth of a second ; and consequently when 
a fresh stimulus follows on the first, its sensory response will 
not be clearly distinct unlessjthis interval at least separates the 
two. If they follow more closely, they will blend together. 
Well, a classical and well-known experiment tells us that we 
cannot receive more than ten or eleven distinct retinal sens- 
ations in a second. At eleven per second, we already experience 
This, 
the persistence of retinal images, is the familiar principle of the 
cinematoscope, which has latterly received such elegant popular 
applications on a large scale. 
No such exact studies have been made on the confusion of 
acoustic or tactile stimuli. But the very remarkable and con- 
cordant results of retinal sensation are enough to prove that the 
cerebral vibration consequent on a stimulation of the retina has 
a period of 1/10”. 
If we turn to the case of a voluntary movement, determined 
also by a cerebral nerve-wave, we find the same figure. Schafer 
in 1886 determined that distinct successive muscular con- 
tractions, voluntary or called forth (as reflexes) by electrical 
stimuli, very rarely exceeded 11-12 per second. Herringham 
found a frequency of 9-12 in pathological tremors. In the case 
of shivering from cold, I have determined frequencies of 10, II, 
12, 13 per second. Griffiths determined a frequency of 10 for 
the muscles of the thumb, and 14 for those of the arm. The 
Swedish physiologist, Loven, found that the electric oscillations 
of the cord determined by very frequent stimuli were only 8-10 
per second. 
Yet we know that if muscles be stimulated directly by rapidly 
alternating currents, they will contract with much greater fre- 
quency. The numerous physiologists who have studied the 
subject are agreed that we may thus determine as many as thirty 
or forty muscular contractions per second. If then we can only 
produce some ten voluntary contractions in the time, the cause 
lies, not in the muscles, but in the cerebral apparatus, which 
cannot vibrate more rapidly. Its period is o'r"; it can only 
vibrate ten times in a second—can only order ten distinct 
voluntary movements of the same muscle in a second. It is not 
that the muscle cannot obey, but that the central nervous 
system cannot give its orders at a greater speed. 
Now I will give you an experiment that you can all try for 
yourselves, which proves most clearly that the vibration of the 
nerve-centres determining a psychological phenomenon lasts 
about one-tenth of a second. When I thought over the various 
modes of obtaining a very rapid muscular motion, it occurred to 
me that perhaps the best was the articulation of some sentence 
pronounced with the greatest possible rapidity. We may admit 
that every syllable articulated represents a distinct muscular 
contraction, and consequently a distinct effort of the will. On 
trying what was the greatest speed of articulation, I found it 
