OcToBER 5, 1899 | 
WM TORE 
559 
this, as in some other respects, the arrangement of the involun- 
tary nervous system is more complex than that of the voluntary 
nervous system ; in the latter the motor nerve-fibres run direct 
to the tissue and have no nerve-cells on their course. The 
nerve-cells which form the intermediate stations for the involun- 
tary nerve-fibres are grouped together into ganglia; and so we 
may call the nerve-fibres which run from the brain or spinal 
cord to the nerve-cells pre-ganglionic fibres, and the nerve- 
fibres which run from the ganglia to the peripheral tissues post- 
ganglionic nerve-fibres. \ 
The involuntary nervous system is divided into at least two 
subdivisions. The most extensive of these is what is called the 
sympathetic nervous System. The pre-ganglionic fibres of the 
sympathetic arise from a limited portion of the spinal cord. 
They arise from that part of the spinal cord which is in the 
region of the chest and the small of the back—z.e. roughly from 
the part which lies between the origin of the voluntary nerves 
for the arms and the origin of the voluntary nerves for the legs. 
The fibres given off by the ganglia of this system—z.e. the post- 
ganglionic fibres—run to the involuntary tissue in all parts of 
the body. 
The Cranialand Sacral Systems.—The second division of the 
involuntary nervous system consists of two parts: one part— 
the cranial—arises from the brain—z.e. above ‘the origin of the 
sympathetic ; the other—the sacral—arises from the end of the 
spinal cord—z.e. below the origin of the sympathetic. 
Each supplies a limited and different part af the involuntary 
tissue of the body, but both together supply a portion only of it. 
Taking the distribution broadly, they supply the muscular coats 
of the alimentary canal and certain structures connected develop- 
mentally with the anterior and posterior portions of it. They 
are especially connected with these terminal portions ; they send 
numerous nerve-fibres to them ; whereas they send but few to 
the intervening portion, and none at all to 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 considered 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 toa 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 
spinal nerves in the sympathetic region and from three spinal 
nerves 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 mammals 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 possession 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 fiow 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 
NO. 1562, VOL. 60] 
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 inhib- 
ition 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 between 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 inhib- 
ition 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 cessation 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 
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 further 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. i 
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 
becomes 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, if the 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 sub-maxillary 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 con- 
traction 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 dilation—z.e. inhibition of the blood-vessels. The 
