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TRANSACTIONS OF SECTION I, 
its blood-vessels. Thus parts of the involuntary tissue of the body receive a 
double supply of nerve-fibres, whilst parts receive a single supply only. Amongst 
the latter are all the involuntary tissues of the skin, the blood-vessels of the limbs 
and trunk, and of most of the viscera. 
The cranial and sacral divisions of the involuntary nervous system are con- 
sidered by some observers to be simply portions of the sympathetic system separated 
from it by the development of the nerve-centres for the arms and for the legs. I 
may give one reason why I do not take this view. The middle portion of the 
spinal cord, which is the region that sends fibres to the sympathetic, always sends 
fibres to a given spot by more than one nerve, and usually by four or five. The 
fibres passing by the several spinal nerves never differ in the kind of effect they 
produce, but only in the degree of effect ; the difference is in quantity and never 
in quality. If, then, regions above and below were mere separated parts of this 
sympathetic region, we should expect that when one of these regions and the 
sympathetic region sent nerves to the same spot, the effect produced by both sets 
of nerves would be the same in kind, though it might differ in extent. But this 
is often not the case. Thus certain blood-vessels may receive nerve-fibres from 
four spival nerves in the sympathetic region and from three spinal rerves in the 
sacral region ; all the former cause contraction of the blood-vessels, all the latter 
cause dilation. And thus it seems to me probable that in the evolution of mam- 
mals the sympathetic nerves have developed at one time, and the cranial and 
sacral involuntary nerves have developed at another time. 
Inhibition. A. striking feature of the involuntary nervous system is its posses- 
sion of nerve-fibres, which, when excited, stop some action at the time going on. 
The most striking example is perhaps the cessation of the heart-beats brought 
about by excitation of the vagus nerve. Such nerve-fibres are called inhibitory 
nerve-fibres, and the stopping of the action is called inhibition. 
So far as has been definitely proved inhibitory nerve-fibres only run to 
involuntary muscle and to nerve-cells, and to these, so far as has been certainly 
shown, only in particular cases. It is true that when fear or other emotion causes 
the tongue to cleave to the roof of the mouth, there is a cessation of the customary 
flow from certain glands, but this flow is itself the result of nervous impulses 
passing in ever rising and falling intensity from the central nerve-cells, and its 
cessation is due to inhibition of nerve-cells and not to inhibition of glandular cells. 
The inhibition of nerve-cells has only been proved to take place in the central 
nervous system. When a group of nerve-cells of the central nervous system is 
engaged in sending out nervous impulses, other nervous impulses reaching them 
by way of other nerve-cells may diminish or stop their activity. The theory which 
is commonly advocated now to explain this inhibition makes the activity of the 
nerve-cells depend upon their receiving stimuli from the minute endings of other 
nerve-cells, and the cessation of the activity to depend upon these minute endings, 
either withdrawing themselves out of range, or having something interposed be- 
tween them and the nerve-cells, so that the impulses can no longer pass. This 
theory I do not wish to discuss to-day ; it is sufficient to say that if it is true, the 
inhibition of nerve-cells is an entirely different process from that of the inhibition 
of involuntary muscle. 
Turning to the inhibition of involuntary muscle, there is a source of confusion 
which we must first guard against. Nearly all the unstriated muscle in the body 
is kept in a state of greater or less tone, or contraction, by the central nervous 
system. A diminution or cessation of this contraction may then be caused by a 
diminution or cessation of the activity of the central nervous system. This cessa- 
tion of contraction is, of course, not what we mean by an inhibition of the 
unstriated muscle. It is usually spoken of as an inhibition of the nervous centre. 
The inhibition we mean is that which is caused by stimulating the peripheral end 
of a nerve outside the spinal cord. 
I have said that this inhibition can only be obtained in certain cases, and it is 
not easy to find anything in common with regard to these cases. But on the 
whole it appears that the more a tissue is able to work by itself, the more likely it 
is to be under the control of inhibitory fibres, The heart, stomach, and the 
