May 5, 1870] 
NATURE 3 
furthest point of the two inches of nerve. The motor 
impulse has then to travel along the two inches of nerve 
before it reaches the point at which, in the former ex- 
periment, it was first set up. 
On examination, it is found that the interval of time 
elapsing between the setting up of the motor impulse 
and the commencement of the muscular contraction is 
greater in this case than in the preceding. Suppose it is 
4 of a second—we infer from this that it took the motor 
impulse ;}, of a second to travel along the two inches of 
nerve: that is to say, the rate at which it travelled was 
one inch in 5}, of a second. 
By observations of this kind it has been firmly estab- 
lished that motor impulses travel along the nerves of 
a frog at the rate of 28 metres a second, and by a 
very ingenious application of the same method to the 
arm of a living man, Helmholtz and Baxt have ascertained 
that the velocity of our own motor impulses is about 
33 metres a second.* Speaking roughly this may be 
put down as about 100 feet in a second, a speed which 
is surpassed by many birds on the wing, which is nearly 
reached by the running of fleet quadrupeds, and even by 
man in the movements of his arm, and which is infinitely 
slower than the passage of a galvanic current. This is 
what we might expect from what we know of the complex 
nature of nervous action. When a nervous impulse, set 
up by the act of volition, or by any other means, travels 
along a nerve, at each step there are many molecular 
changes, not only electrical, but chemical, and the analogy 
of the transit is not somuch with that of a simple galvanic 
current, as with that of a telegraphic message carried along 
a line almost made up of repeating stations. It has been 
found, moreover, that the velocity of the impulse depends, 
to some extent, on its intensity. Weak impulses, set up 
by slight causes of excitement, travel more slowly than 
strong ones. 
The contraction of a muscle offers us an excellent ob- 
jective sign of the motor impulse having arrived at its 
destination ; and, all muscles behaving pretty much the 
same towards their exciting motor impulses, the results 
obtained by different observers show a remarkable agree- 
ment. With regard to the velocity of sensations or 
sensory impulses, the case is very different ; here we have 
no objective sign of the sensation having reached the 
brain, and are consequently driven to roundabout methods 
of research. We may attack the problem in this way. 
Suppose that, say by a galvanic shock, an impression is 
made on the skin of the brow, and the person feeling it 
at once makes a signal by making or breaking a galvanic 
current. It is very easy to bring both currents into con- 
nection with a revolving cylinder and levers, so that we 
can estimate by means of a tuning-fork, as before, the 
time which elapses between the shock being given to the 
brow and the making of the signal. We shall then get 
the whole “ physiological time,” as it is called (a very bad 
name), taken up by the passage of the sensation from the 
brow to the brain, by the resulting cerebral action, in- 
cluding the starting of a volitional impulse, and by the 
passage of the impulse along the nerve of the arm and 
* Quite recently M. Place has determined the rate to be 53 metres per 
second, This discordance is too great to be allowed to remain long unex- 
plained, and we are very glad to hear that Helmholtz has repeated his ex- 
riments, employing a new method of experiment, the results of which we 
ope will soon be published, 
hand, together with the muscular contractions which make 
the signal. We may then repeat exactly as before, with 
the exception that the shock is applied to the foot, for 
instance, instead of the brow. When this is done, it is 
found that the whole physiological time is greater in the 
second case than in the first ; but the chief difference to 
account for the longer time is, that in the first case the 
sensation of the shock travels along a short tract of nerve 
(from the brow to the brain), and in the second case 
through a longer tract (from the foot to the brain). We 
may conclude, then, that the excess of time is taken 
up by the transit of the sensation through the distance 
by which the sensory nerves of the foot exceed in length 
those of the brow. And from this we can calculate the 
rate at which the sensation moves. 
Unfortunately, however, the results obtained by this 
method are by no means accordant ; they vary as much 
as from 26 to 94 metres per second. Upon reflection, 
this is not to be wondered at. The skin is not equally 
sentient in all places, and the same shock might produce 
a weak shock (travelling more slowly) in one place, and a 
stronger one (travelling more quickly) in another. 
Then, again, the mental actions involved in the making 
the signal may take place more readily in connection with 
sensations from certain parts of the body than from 
others. In fact, there are so many variables in the data 
for calculation that though the observations hitherto made 
seem to show that sensory impressions travel more rapidly 
than motor impulses (44 metres per second), we shall not 
greatly err if we consider the matter as yet undecided. 
By a similar method of observation certain other con- 
clusions have been arrived at, though the analysis of the 
particulars is not yet within our reach. Thus nearly all 
observers are agreed about the comparative amount of 
physiological time required for the sensations of sight, 
hearing, and touch. If, for instance, the impression to be 
signalled be an object seen, a sound heard, or a galvanic 
shock felt on the brow, while the same signal is made in 
all three cases, it is found that the physiological time is 
longest in the case of sight, shorter in the case of hearing, 
shortest of all in the case of touch. Between the appear- 
ance of the object seen (for instance, an electric spark) 
and the making of the signal, about 1; between the 
sound and the signal, ; between the touch and signal, 
+ of a second, is found to intervene. 
This general fact seems quite clear and settled ; but if 
we ask ourselves the question, why is it so? where, in the 
case of light, for instance, does the delay take place? we 
meet at once with difficulties. The differences certainly 
cannot be accounted for by differences in length between 
the optic, auditory, and brow nerves. The retardation in 
the case of sight as compared with touch may take place 
in the retina during the conversion of the waves of light 
into visual impressions, or may be due to a specifically 
lower rate of conduction in the optic nerve, or may arise 
in the nervous centre itself through the sensations of light 
being imperfectly connected with the volitional mecha- 
nism in the brain put to work in the making of the signal. 
One observer (Wittich) has attempted to settle the first 
of these questions by stimulating the optic nerve, not by 
light, but directly by a galvanic current, and has found 
that the physiological time was thereby decidedly less- 
ened ; while conversely, by substituting a prick or pressure 
