March 8, 1883] 
NATURE 
437 
in talking of animals, we say that the stimulus to the 
sensory nerves of the hand or cheek, by contact with the 
wet soap or with the curtain, caused in us a reflex spasm, 
which was inhibited by the stimulus applied to our optic 
nerves. A similar occurrence is observed in frogs, and 
the reflex actions produced by stimuli applied to the feet 
are much stronger when the inhibitory effect of the optic 
nerves upon them is removed by covering up or destroy- 
ing the eyes, or by removal of the optic lobes.! 
Regarding the optic lobes, we will have a good deal 
more to say presently, for they have been considered to 
be special inhibitory centres, and are often known by the 
name of Setschenow’s centres. 
If we try to explain all those instances of inhibition by 
the assumption of special inhibitory centres for each 
action, we must suppose, in connexion with every sen- 
sory nerve, that centres exist which lessen or abolish the 
ordinary reflexes produced by stronger or weaker stimula- 
tion applied to the nerve. Besides this, we must suppose 
other centres which inhibit motor actions in other parts 
of the body: as for example, when irritation of the 
extensor lessens reflex excited by irritation of the flexor 
surfaces, or vice versd, or when the irritation of one 
sciatic stops reflex action from mechanical irritation of 
the other foot. A special inhibitory centre must be 
placed also in the optic lobes in connection with the optic 
nerves. This complication reminds us of the multitude 
of inhibitory centres which one must imagine in glass, in 
order to explain the occurrence of Newton’s rings by 
them, but it seems to me that all these cases are readily 
explained on the hypothesis that the motor and sensory 
cells concerned in them are so placed with relation to 
each other that the stimuli passing from them produce 
interference under normal or nearly normal conditions of 
the organism. 
A spot of light may be caused to disappear by throwing 
another ray upon it, so as to interfere with it, but it may 
be also made to disappear from the place where it was, 
by simply reflecting it somewhere else. 
A similar occurrence to this takes place in the body, 
and although two stimuli may interfere with and destroy 
each other, we not unfrequently find that the apparent 
abolition of the effect of a stimulus is simply due to its 
diversion into some other than the usual channel. In 
very many cases, where we have inhibition we have also 
diversion ; and it is not at all improbable that when the 
stimulus is very strong complete inhibition may be impos- 
sible by interference alone, and can only be effected by 
diversion of part of the stimulus. We have already said 
that two waves of sound will neutralise each other and 
produce silence, but this only occurs when the waves are 
not too powerful. When they reach a certain intensity 
they produce secondary waves which give resultant tones, 
and several facts seem to point to an analegous condition 
in animal organisms. 
We have hitherto considered cases in which the inhibi- 
tion was probably brought about by interference of two 
stimuli, so that the one counteracts the other in much the 
same way as tworays of light interfered with one another 
in Newton’s rings. In one case which we have men- 
tioned, the movement of the hand when it is tickled is 
entirely arrested by a strong effort of the will, and the 
hand is allowed to remain perfectly passive and limp. 
Here we suppose the impulse sent down from the motor 
centres in the brain to interfere with that which has 
originated in the cord by irritation of the sensory nerves, 
and to counteract it so that no muscle whatever is put in 
action. But very frequently we find that a result ap- 
parently similar is produced by a different mechanism, 
viz. by diversion of the stimulus into other channels. In 
the former case the arm is felt to be quite limp, but in the 
latter though it is quite quiet, it is perfectly rigid—all the 
* Langendorff, Arch. f. Anat. u. Physiol. 1877; Von Boetticher, Ueber 
Reflexhemmung, Inaug. Diss., Jena, 1878, p. 12. 
muscles being intensely on the stretch. Here the stimulus 
which would usually have excited convulsive movements 
of the arm, and probably of the body, resulting in a con- 
vulsive start, have been diverted from the body into other 
muscles of the same limb. 
A similar power of diverting a stimulus is seen in the 
instinctive muscular efforts which any one makes when in 
pain. One of the most common of these is clenching 
the teeth, and it used to be a common practice in the 
army and navy for men to put a bullet between the teeth 
when they were being flogged, and at the end of the 
punishment this was usually completely flattened. <A 
| patient seated in a dentist’s chair usually grasps convul- 
sively the arms of the chair, or anything which may be 
put into his hand ; and there can be little doubt that pain 
is better borne, and appears to be less felt, when the 
sensory stimulus occasioning it can thus be diverted 
into motor channels. In children the motor channels 
into which diversion usually takes place are those con- 
nected with the respiratory system, and the sensory 
stimulus works itself off in loud yells. At a later 
age the stimulus is often diverted into those motor chan- 
nels through which reaction occurs between the indi- 
vidual and his surroundings. Thus most people probably 
remember how a kick in the shins at football often 
served simply to accelerate their speed; and during the 
heat of battle the pain of a wound is often but little 
felt, the stimulus having been diverted into motor 
channels. 
Many more instances might be given of the effects of 
diversion of stimuli, but having discussed this subject at 
length in a former paper,! I shall not pursue it further 
here. 
Sensory stimuli are also capable of inhibition by inter- 
ference. Hippocrates* noticed, and it is a matter of 
general observation, that pain in one part of the body 
may be lessened or removed by the occurrence of pain in 
another. In many instances, the removal of the pain to 
one part may be indirect, through the action exerted on 
the vessels by the pain in the other part. But in some 
instances it may be, and probably is, due to the direct in- 
terference of sensory impressions. 
This question of the removal of pain by the interfer- 
ence of waves in the sensory nerves or nerve-centres has 
been very fully and clearly discussed by Dr. Mortimer 
Granville.* Starting from the hypothesis of interference, 
he has also devised a plan of treatment which appears to 
give satisfactory results. By means of a small hammer 
moved by clockwork or electricity, he percusses over the 
painful nerve in order to induce in it vibrations of a dif- 
ferent rhythm to those which are already present and 
which give rise to the pain. Thus he percusses rapidly 
over a nerve when the pain is dull or grinding, and per- 
cusses slowly when the pain is acute, in order to produce 
interference if possible. In many instances the treat- 
ment is successful, and its success affords additional 
support to the hypothesis on which it is based. 
We have hitherto spoken of reflex inhibition in the 
cerebro-spinal axis alone, but we find also reflex inhibition 
of motor actions produced by irritation of sympathetic 
nerves ; and, wéce versd, we find inhibition of the move- 
ments of internal viscera produced by irritation of cerebro- 
spinal nerves. Thus strong irritation of the sensory nerves 
of the liver, intestine, uterus, kidney, or bladder, occa- 
sionally abolishes the power of walking or standing. 
Irritation of a sensory nerve will frequently arrest the 
movements of the heart. 
The phenomena which occur in swallowing afford an 
excellent example, not only of inhibition occurring in 
parts innervated by the sympathetic system, but also of 
* Brunton, ‘‘Inhibition, Peripheral and Central,’’ West Riding Asylum 
Reports, 1874. 
2 Hippocrates, Aphorisms, sec. i. 46; Sydenham Soc. Ed. vol. ii. p. 713. 
3 Mortimer Granville, Nerve Vibration and Excitation. (London: 
Churchill, 1883.) 
