March t, 1883] 
NATURE 
421 
surface which has beentouched. Increased pressure has 
thus inhibited motion but increased sensation. 
In a paper on “Inhibition, peripheral and central,” 
which I wrote in the West Riding Asylum Reports in 
1874, I tried to explain these phenomena in the following 
manner : “It appears to me to be in all probability due 
to there being two sets of ganglia in the cord itself, one 
motor and one inhibitory. The motor is more readily 
excited than the inhibitory, and causes violent move- 
ments, which the inhibitory centres of the brain cannot 
restrain without the greatest difficulty, though they are 
readily controlled by the inhibitory ganglia in the spinal 
cord. A slight titillation excites the motor, but not the 
inhibitory spinal ganglia ; a stronger pressure stimulates 
the inhibitory centres also, and thus arrests the move- 
ments without any action being required on the part of 
the inhibitory centres in the brain. We may try to ex- 
plain this, by supposing that there are two distinct sets 
of nerves proceeding from the skin to the cord, one 
of them having the power to excite inhibitory, and the 
other to excite motor centres. Further, we must suppose 
that these sets of fibres are endued with different degrees 
of excitability, the motorial ones being stimulated by a 
slight touch, but the inhibitory ones only by a stronger 
impression. 
Fic. 3. 
“This is represented in Fig. 3, where s is the skin; a, 
the fibres proceeding from it to the motor ganglion, 7 
and e, those going to the inhibitory ganglion, 1; 7 is the 
fibre by which I arrests the action of 7, and7z that by 
which the brain exerts a similar action. The different 
fibres by which 7 acts on the muscles have not been 
introduced into the diagram. 
Fic. 4. 
“This hypothesis, however, is a very clumsy one, and 
we explain the facts quite as well by supposing that there 
is only one set of afferent nerves (a, Fig. 4) from the 
skin to the cord, which transmit a slight impression only 
to the motor ganglia, 7, but convey a stronger one along 
@ to the inhibitory ganglia 1, also, which then react 
through Z upon the motor ones. This latter supposition 
renders intelligible the fact that it is only when some- 
thing is drawn quickly and lightly across the skin, so as 
to make a slight and transient impression on the ends of 
many sensory nerves, that tickling is felt. If the pressure 
on the skin is heavier, or if the motion over it is slow, the 
effect is quite different, and this is just what we might 
expect if a short and slight impression travels only to the 
motor ganglia, and a stronger or more lasting one goes to 
the inhibitory beyond them.’’ 
These diagrams themselves are suggestive of interfer- 
ence ; but I did not in that paper say anything regarding 
it, contenting myself only with the term inhibition. One 
reason that prevented me from considering inhibition in 
animals as corresponding closely to the interference of 
light, was that the rapidity of transmission of nervous 
impulses was differently given by different observers, and 
indeed, according to Munk, it varies along the course of 
the same nerve.! 
Unless the rate of transmission of impulses is constant, 
one cannot expect interference to produce inhibition. But 
in his observations on Medusz, Mr. Romanes found that 
when the circumference of the bell in a medusa was cut 
into a long spiral strip, leaving only the centre of the bell 
uninjured, stimuli applied to the extreme end of the strip 
passed along it, and were delivered to the centre of the 
bell, just as if they had been applied to the central part 
itself—all passing at the same rate they did not interfere 
with one another. But when the strip was pressed upon 
or stretched, the passage of impulses was interfered with. 
This seems to show that the rate of transmission of a 
stimulus along a conducting structure is a definite one, 
provided the structure remain under the same conditions. 
But still more instructive on this point are the experi- 
ments with the Ton-inductorium, invented by my friend 
Prof. Hugo Kronecker. Other observers have found that 
when a muscle is irritated by an interrupted current 
applied to its nerve, the tetanic contraction into which it 
would be thrown by twenty interruptions per second 
ceased when the interruptions became as frequent as 250 
per second. By using an interrupted current induced by 
the vibrations of a magnetic rod, which gave out a 
definite tone, Kronecker and Stirling were able to throw 
the muscle into tetanus with no less than 22,000 inter- 
ruptions per second. This success is probably to be 
attributed to the regularity and equality of the stimuli 
applied by Kronecker’s method, while the fact that their 
predecessors got no tetanus with more than 250 inter- 
ruptions per second is probably due to interference of the 
stimuli they applied. Kronecker’s observations show, I 
think, how definite must be the rate of transmission of 
stimuli alorg a nerve so long as it remains under the 
same conditions and give us a basis for extending the 
theory of interference from waves of light and sound to 
vibrations in nervous and muscular tissues.3 
We are justified, I think, by these experiments in con- 
sidering that interference may occur in the nervous 
system, and that one part may exercise an interfering or 
inhibitory effect upon the other, which is constant under 
normal conditions, but will be modified when these condi- 
tions are altered. 
Let us now try to apply this hypothesis to the reflex 
action which we have just been discussing. 
Fig 5. 
Let s,S’ and s” be three sensory cells in the spinal 
cord, M, M’ and M’ motor cells, s and s’ sensory nerves, 
and 7 wz’ motor nerves. SB is a sensory and MB a 
® Archiv. f. Anat. u. Physiol. 1860, p. 798 . 
? It must be borne in mind, however, that the overtones of such a vibrat- 
ing rod are in the ratio of , 3%, 5 72, &c., and not in that of #, 2#, 42, like 
those of a vibrating string or pipe. Quincke (Poggendorff’s Annalen, 1866, 
vol, viii. p. 182) Failed to silence the sounds of such a rod by means of an 
interference apparatus. 
3 Vide Hermann’s Handbuch d. Physiol Bd. 1. 
i. Th, i. p. 39. 
Th. i. p. 44, and Bd. 
a) aes 
