886 . kRePor'tT—1899. 
intestines work when no longer connected with the central nervous system, and 
these are especially liable to inhibition. 
There has been a marked tendency amongst physiologists, in considering the 
question of inhibitory nerve-fibres, to take what may be called the view of the 
equal endowment of the tissues. Because some arteries have inhibitory nerve- 
fibres, therefore it is to be held as in the highest degree probable that all have. 
And many would go farther and say that it is therefore in the highest degree 
probable that all unstriated muscle, and glands, and even the voluntary muscles 
have such fibres. This view seems to me a mistaken one. There is hardly room 
for doubt that the motor fibres are supplied in most unequal measure to the 
unstriated muscle and glands of the body. ‘There are veins in the body 
containing unstriated muscle, which show no visible contraction from any nerve 
stimulation, And there are a number of glands which no nerve—so far as we 
know—excites to secretion. Since, in the course of the evolution of the organism, 
a universal development of motor fibres has not occurred, it is, I think, to be 
expected that the development of inhibitory fibres should be still less universal. 
For up to a certain point the results of inhibition can be obtained in most cases 
without inhibitory nerve-fibres, by a simple diminution in the impulses travelling 
down the motor fibres. The only, and the final, test is of course experiment. 
But not all experiments are decisive, and theory inevitably colours interpretation. 
This theory of the equal endowment of the tissues has, it seems to me, caused a 
number of quite inconclusive experiments to be accepted as offering satisfactory 
evidence for the existence of inhibitory nerve-fibres. 
Passing from this question, we may consider briefly how far we can get on the 
way to understand what occurs during inhibition. The external characteristic 
feature of inhibition is that a certain state of activity ceases ; a muscle contracting 
at short intervals ceases to contract, or a muscle in a steady state of contraction ~ 
loses this state. The tissue in either case hecomes flabby. 
The activity of a tissue may obviously be due to its receiving some stimuli from 
the nervous system or to its own inherent qualities. In the former case, ifthe tissue 
were only active when receiving nervous impulses, we should naturally look to some 
interference with these impulses as being the cause of inhibition. The blood- 
vessels of the submaxillary gland appear to me to offer sufficiently clear evidence 
with regard to the inhibition of blood-vessels. The superior cervical ganglion is 
the local centre from which the nerve-fibres bringing about contraction run to the 
blood-vessels of the gland. When this ganglion has been removed and the nerve- 
fibres from it have degenerated, the vessels receive no nervous impulses causing 
them to contract. But stimulation of the inhibitory nerve will still cause dila- 
tion—i.e. inhibition ef the blood-vessels. The inhibition must then be due toa 
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 changeinit. In this chemical 
change some energy is set free as work—shown by the contraction of the muscle— 
and some as heat. It is conceivable that tlhe nervous stimulus which cuases inhi- 
bition should cause all the energy set free by the chemical change to take the form of 
heat. In that case the inhibitory nerve would bea calorific nerve. The amount of 
chemical change is indicated by the amount of carbonic acid given off to the blood. 
No experiments 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 prevent a 
particular chemical change is to induce a different one. We have seen that the 
