AZ0- 
distances they have to travel remain the same, the effect 
of the beams on each other will be altered by any change 
in their rate of travel such as would be effected by 
altering the media through which they pass. 
This is a most important point in regard to the hypo- 
thesis -of the causation of inhibition by interference of 
vibrations in the nervous system. It may therefore be 
useful to illustrate this further, and probably it could not 
be done better than by using, with a little modification, 
the example given by Sir J. Herschel in his article on 
Light in the Encyclopzedia Metropolitana. ‘ Let R bea 
reservoir of water, from which the channels A and B pro- 
B 
vas aK 
Haeal| P 
see 
~ mete oo 
Fic. 2,—Diagram to illustrate Sir J. Herschel’s observations on interference. 
Adapted from his} article {on |‘ Absorption of Light,”’ Phil. Mag. 1883, 
P- 405. 
ceed, to join each other at P; they are supposed to be equal 
in every respect except that Bis longer than A, Ifa wave 
from the reservoir enters the openings of A and B at the 
same time and travels at the same rate along them, the 
wave which passes through A will reach P sooner than 
the one which passes through B, so that the water at that 
point will be agitated by two waves in succession. But 
let the original cause of undulation be continually 
repeated so as to produce an indefinite series of equal 
and similar waves. Then if the difference of lengths of 
the two canals A and B be just equal to half the interval 
between the summits of two consecutive waves, it is evi- 
dent that when the summit of any wave propagated along 
A has reached the point of intersection P, the depression 
between two consecutive summits (viz., that corre- 
sponding to the wave propagated along A and that of the 
wave immediately preceding it) will arrive at the inter- 
section P by the course B. Thus in virtue of the wave 
along A the water will be raised as much above its natural 
level as it will be depressed below it by that along B. Its 
level will therefore be unchanged. Now as the wave 
propagated along A passes the intersection P, it subsides 
from its maximum by precisely the same gradations as 
that along B, passing it with equal velocity, rises from its 
minimum, so that the level will be preserved at the point 
of intersection P undisturbed so long as the original | 
cause of undulation continues to act regularly.) So soon 
as it ceases, however, the last half-wave which runs along 
B will have no corresponding portion of a wave along 4 | 
to interfere with, and will therefore create a single 
fluctuation at the point of concourse P.” 
It is obvious that if everything else remains the same, 
the effect which the waves have upon each other at P will 
be altered if the rate at which they travel is increased or 
diminished. 
The more the speed is increased the less effect com- 
paratively will the greater length of B have in retarding the 
wave which flows along it, so that its crest will no longer 
coincide with the trough or sinus of the waves in A, but 
will, on the contrary, coincide more nearly with the crest 
of one of the waves in A. 
The more the speed is diminished, the more will the | 
wave in B lag behind that in A, so that its crest, instead 
of coinciding with the trough between two crests of the 
waves from A will gradually come to coincide with the | 
as ° . . i 
* This actually happens in the harbour of Batsha, into which the waves ! 
pass from the open sea through two channels of unequal length. 
NATURE 
— [March 1, 1883 
crest succeeding the trough, and thus double its magni- 
tude instead of destroying it. 
We see, then, that under the conditions we have sup- 
posed either increase or diminution in the rapidity of 
their transmission may convert the interference of waves — 
into more or less complete coincidence, and the effect of 
the two waves may thus be doubled instead of neutralised 
by their superposition. 
The alteration which is produced in the mutual effect 
of two waves by increase or diminution of their rate of 
transmission along channels of constant length supplies 
us I think with a test by which we may ascertain the 
truth of the hypothesis that inhibitory phenomena in the 
animal body are due to interference. For if it be true 
we ought to find that a nerve which produces inhibitory _ 
phenomena when excited under normal conditions will 
gradually lose this power when the rate of transmission 
along it is increased or diminished, as, for example, by 
the influence of heat or cold, and will gradually acquire 
an exactly contrary or stimulating action. This, I think, 
is shown to be the case by our experimental data so far 
as they go. 
Several authors have pointed out the analogy between 
inhibitory phenomena in the animal body and the effects 
of interference of waves of light or sound. This has been 
done with special precision by Bernard! and Romanes.? 
The tendency to do away with the idea of distinct inhibi- 
tory centres is gradually spreading, but hitherto no attempt 
has been made to bring all the phenomena of inhibition 
under one general rule or to explain the mode in which 
they are affected by the action of drugs. The object of the 
present paper is to gather together some instances of inhi- 
bition which we find in the body, and to see whether by 
the theory of interference it is not possible to explafn 
both the curiously perplexing exceptions which we meet 
with in physiological experiments, and the still more 
perplexing action of drugs on inhibitory phenomena. 
One of the most striking examples of reflex action and 
of inhibition, is the effect of a slight touch or touches, and 
of firm pressure upon the palms of the hands, the soles of 
the feet, or the axilla, and in some persons also the 
knees. In many persons a very slight touch or succes- 
sion of touches upon these parts is sufficient to throw first 
the respiratory muscles, and then the whole body into 
violent convulsions. Indeed, it is stated that during the 
persecution of the Albigenses by Simon de Montfort, 
several people were tortured to death by tickling the 
soles of their feet with a feather. The stimulus here 
applied, and the consequences it produces, appear to be 
out of all proportion to one another; the stimulus being 
almost infinitesimal, and the consequences enormous. 
In the case of Newton’s rings it might be possible with 
much trouble to throw a different beam into such a con- 
dition that it would interfere with one of the beams in 
the rings. and produce darkness, but in the rings a 
similar, effect is produced in a very much simpler way by 
alteration of part of the same beam. A similar occur- 
rence is to be observed in the inhibition of the reflex 
action on tickling. 
By a very powerful effort of the will we may completely 
arrest the reflex movement which would otherwise occur, 
and allow the limb to remain perfectly passive. But the 
same effect is produced in a much simpler way by apply- 
ing a firm pressure instead of a slight touch. The firm 
pressure neutralises the effect of the touch in regard to 
motion, and not only are no reflex convulsive actions 
produced, but no tendency whatever to them is felt. 
But while the pressure has neutralised the tendency to 
motion, and has altered the character of the sensation, it 
has not neutralised sensation. On the contrary, it has 
rendered it more definite, so that one can distinguish 
with much greater certainty the particular point of the 
1 Bernard, La Chaleur Animale, Paris, 1876, p. 371. 
= Romanes, Phil. Trans. 1877, p- 730. 
