560 
NATURE 
[OcToBER 5, 1899 
inhibition must then be due to a direct action on the tissue, and 
not to an interference with other nerve-impulses. The evidence 
with regard to the inhibition of the beat of the heart and of the 
tone or peristalsis of the alimentary canal is more complex, but 
there is good reason to believe that the contraction is in both 
cases due to their inherent qualities. And if this be granted, it 
follows that here also inhibition must be due to a direct action 
upon the tissue. 
The contraction of a muscle is due to a chemical change in it. 
In this chemical change some energy is set free as work—shown 
by the contraction of the muscle—and some as heat. It is con- 
ceivable that the nervous stimulus which causes inhibition should 
cause all the energy set free by the chemical change to take the 
form of heat. 
calorific nerve. 
the amount of carbonic acid given off to the blood. 
ments have been made as to the amount of carbonic acid given 
off to the blood by an inhibited tissue, but it appears very 
unlikely that the amount is increased, and we may take this 
view of the action of an inhibitory nerve as improbable. 
If the nervous impulse does not act in this way it must in 
some way stop the particular chemical change associated with 
contraction from taking place. It does not stop all chemical 
change, for blood passing through an inhibited tissue loses some 
of its oxygen. The simplest way for a nervous impulse to pre- 
The amount of chemical change is indicated by 
vent a particular chemical change is to induce a different one. 
We have seen that the tissues which are inhibited have a great 
tendency to contract of themselves—that is, they forin certain 
very unstable substances. 
not inhibited this tendency exists but little or not at all. 
In closely related tissues which are 
The 
Dar dae 
Fic. 1. 
proximate cause of inhibition might then be that the nervous 
stimulus causes certain molecules of the tissue to form more stable 
combinations. This need not be associated with any general 
assimilation ; it would simply make the muscle adopt for a time 
a mode of life more like that of other closely related muscle. 
Number of Relay Stations.—I have already mentioned that 
the nerve-fibres which pass from the central nervous system to 
the involuntary tissues do not run to it direct, but end in groups 
of nerve-cells or ganglia from which fresh nerve-fibres are given 
off. Now, in most cases, there are anatomically several ganglia 
on a nerve in its course from the spinal cord to the periphery. 
For example, the nerve-fibres which cause the hairs of a cat’s 
tail to stand on end, giving the tail the appearance of a bottle 
brush, leave the spinal cord in the lower part of the back, and 
enter a nerve-strand which is beaded with ganglia. They leave 
this strand near the root of the tail. Between the point where 
the nerve-fibres enter and the point where they leave the strand 
there are seven or eight ganglia. The fact offers us a problem 
of some difficulty. With how many of these ganglia are the 
nerve-fibres connected? Or, in other words, how many relay 
stations are there—eight or one, or some intermediate number ? 
Further, do all kinds of involuntary nerve-fibres in all parts of 
the body have the same number of relay stations, or do some 
have one, some two, some three, and so on? It would take 
too long to discuss this question here. But the experimental 
evidence is, I think, fairly decisive in favour of the simple view 
that the nerve-impulse passes through one relay station only. 
There is, however, evidence that the nerve-fibres which pass 
from the spinal cord branch, so that we may take the element 
by reduplication of which the involuntary nervous system is 
builtiup to be diagrammatically as in Fig. 1. 
NO. 1562, VOL. 60] 
In that case the inhibitory nerve would bea | 
No experi- 
Reflexes.—Another point of view is given by a comparison 
of the groups of nerve-cells of the peripheral ganglia with the 
groups of nerve-cells of the brain and spinal cord. The proper 
working of the body depends upon an agile response by the 
central nervous system to what is going on in the periphery. 
Now the peripheral ganglia are made up of nerve-cells and 
FIG: 2. 
nerve-fibres which differ less in general characters from some 
of the cells of the central nervous system than these differ 
from one another. The nerve-cells of the spinal cord can 
receive impulses from many groups of nerve-cells both near and 
remote ; they do not simply receive impulses from one quarter 
alone—say, the cortex of the cerebral hemispheres—but from 
many quarters, and notably direct from the periphery. Hence 
it has been supposed that the peripheral ganglia have similar 
wide connections, that they receive impulses direct from the 
periphery, that each is connected with other ganglia, and that 
impulses received from the periphery, or elsewhere, bring 
separate ganglia into coordinate action. And this view, which 
has been taken on general grounds, has been supported by 
microscopical observations. 
The evidence against this view is of two kinds. In the first 
place, it can be shown that in a number of individual cases the 
nerve-cells of one ganglion have no connection with the nerve- 
cells of another ganglion, so that anything like a universal scheme 
of connection is out of the question. And, secondly, it can be 
shown that whenever an action occurs, which might be referred 
to such connection, it is an action which is bound to occur in 
consequence of some other known arrangement, and that there- 
fore it is unnecessary to seek for a further cause. 
The evidence of the first kind we need not enter into; the 
evidence of the second kind we may hastily touch on. If we 
accept the conclusion stated above, that the pre-ganglionic nerve- 
fibres branch, and the branches run to different nerve-cells, it 
follows that a stimulus applied to one branch will stimulate a 
Sp.Cc 
number of nerve-cells ; this follows since a nerve-impulse set up 
in any part of a nerve travels over the whole of it. Thus actions, 
resembling reflex actions, will inevitably be obtained whenever 
nerve-fibres are stimulated which send branches to different 
ganglia. The mechanism in this case is confined to motor 
nerve-fibres and nerve-cells. The action, for lack of a con- 
