438 
NATURE 
[March 8, 1883 
partial diversion of stimuli. Kronecker has found that 
when we swallow, the food or water is sent down at once 
into the stomach by the contraction of the muscles of the 
pharynx, and that afterwards a peristaltic contraction of 
the cesophagus occurs. When several attempts to swallow 
are made one after the other, however, the cesophagus 
remains quiet until they are ended, and then it occurs at 
the same interval of time after the last, that it would have 
done after a single act of swallowing. 
B 
R “ 
eae 
A 
Fic. 2.—Diagram to illustrate Sir J. Herschel’s cbservaticns cn interference. 
Adapted from his article on ** Absorption of Light.” Phil. Mag. 1883, 
p- 405. 
If we now refer again to our diagram (Fig. 2, which 
for convenience we repeat here) we will see that it 
answers just as well for the contractions of the cesophagus 
as for the tides at Batsha by simply giving a different 
meaning to the letters. Let R now instead of represent- 
ting a reservoir or the open sea represent the ganglia of 
the pharynx, A and B the nerve fibres which conduct 
nervous impulses from these ganglia to P, and let P be the 
ganglia of the cesophagus which stimulate its muscular 
fibres to peristaltic action. A single wave passing from 
R causes two waves at P, one succeeding the other, but a 
number of waves from R under the conditions supposed 
also cause only two waves: one at the beginning and one 
at the end, for during all the intermediate period they 
neutralise each other. 
It might perhaps seem that the two stimuli should cause 
two contractions of the muscular fibres of the cesophagus, 
But it frequently happens that a single stimulus is unable 
to produce muscular contraction. It only increases the 
excitability of the contractile tissue to a second stimulus, 
and when this is applied contraction ensues. The effect 
of the first wave then would be to increase excitability, 
that of the second wave to cause contraction. This is well 
shown in the accompanying tracing from the contrac- 
éak current 
Fic. 6.—Showing the increasing contracticns of the tissue of medusa when 
stimulated by repeated weak induction shocks of the same intensity. 
tile tissue of medusze, which I owe to the kindness of 
my friend, Mr. Romanes. He has found that when very 
slight stimuli, such as from weak Faradaic shocks, are 
applied, the first has no apparent action, but the effect of 
each successive stimulus is added to that of the preceding 
ones, until contraction is produced. Two shocks were 
applied before the first small contraction shown in the 
tracing occurred, and the shocks are all of the same 
strength, although the last ones produce the maximal 
contraction of which the tissue is capable, and the first 
had apparently no effect at all. This relation of the con- 
tractile tissue to stimuli is usually expressed by saying 
that the tissue has the power of summation. 
At the same time that a stimulus is sent down from the 
pharynx to the cesophageal ganglia, which has an inhib‘tory 
action, there appears to be another sent to the medulla 
oblongata, which acts on the roots of the vagus nerve. 
This latter stimulus has a very curious effect, viz. inhi- 
biticn of inhibition. The vagus usually exercises an 
inhibitory action on the heart, rendering its beats less 
rapid than they would otherwise be, but during swallowing 
this inhibitory action is removed and the heart pulsates 
at nearly double its normal rate.! Here we seem to have 
a stimulus one part of which passes along one path, while 
another part is diverted and passes along another. Each 
part interferes with the nervous actions which would 
occur in its absence, but one part interferes so as to 
prevent, and the other so as to increase muscular activity 
in the cesophagus and heart respectively. 
The same diversion of a stimulus which we find in the 
case of the cesophagus seems to occur frequently through- 
out the body. Thus we find it almost invariably in rela- 
tion to the vascular changes which occur on stimulation 
of a sensory nerve. When a sensory nerve going to any 
part of the body is irritated, the vessels of the district 
which it supplies usually dilate, while those of the other 
parts of the body contract.2. The stimulus in this case 
passes to the vasomotor centre, and thence is reflected 
as an inhibitory stimulus in one direction and as a motor 
stimulus in another. 
Some results of the greatest interest have recently been 
obtained by Dastre and Morat, in some experiments 
which they have made on the subject of vascular dilata- 
tion or inhibition. 
In many cases the stimulation and inhibition of vascu- 
lar nerves take place in the medulla oblongata, or in the 
spinal cord, and the inhibitory and motor centres are 
close to each other; but in other cases, such as those ex- 
perimented on by Dastre and Morat,’ we find the inhibt- 
tory and motor centres separated from one another, some 
of the motor centres being in the cord and some of the 
inhibitory in a ganglion situated nearer the periphery. 
It was previously known that in some cases, as in the 
dilatation of the vessels of the submaxillary gland on 
irritation of the chorda tympani, small ganglionic struc- 
tures were situated at the terminal branches of the nerve, 
and it was supposed that these ganglia, by their interposi- 
tion between the nerve and the structure on which it was 
to act, converted its motor power into an inhibitory one. 
The experiments of Dastre and Morat are much more 
definite on this point. Excitation of the cervical sympa- 
thetic nerve has the effect of causing the vessels of the 
ear to contract very greatly in the rabbit, but irritation of 
the same nerve causes in the dog enormous dilatation of 
the vessels of the mouth. Moreover, in the rabbit this 
constricting action on the vessels of the ear is exerted 
only when the nerve is irritated between the first cervical 
ganglion and the ear. When the nerve is irritated be- 
yond the cervical ganglion, instead of causing constric- 
tion, it produces dilatation. 
In order to explain this action, the authors suppose that 
the fibres of the sympathetic, in passing through the 
ganglion, end in the ganglionic cells, and thus suspend 
the tonic action which they exert on the constricting 
fibres which issue from the ganglion and pass to the ear. 
It seems to me, however, that a more satisfactory expla- 
nation of this fact also is afforded by the hypothesis of 
interference. 
In the cerebro-spinal system, cells being ranged above, 
below, and around one another with free communication 
between them, we have ample provision for the passage 
of two stimuli along paths of such different length, as to 
enable them to interfere with and inhibit each other. 
1 In my own case the proportion is 120 to 76. — 
2 Ludwig and Loven, Ludwig’s Arbeiten, 1866, p. 17- 
3 Archives de Physiologie, 1882, tom. x. p. 326. 
