CHAPTER XI

THE NERVOUS SYSTEM

Twenty-five years ago a new process was introduced for colouring the

elements which by their combination make up the nervous system. With

its aid anatomists discovered the inadequacy of their conceptions

of nerve-cells. It was already known that a nerve-fibre—that is to

say, its essential part, its core—is a part of a cell, the body and

other parts of which are situate within the brain or spinal cord, or

in one of their dependents, a ganglion. But the new method showed the

nerve-cell as more elaborate in form than anything which had been

imagined hitherto; and since the word “cell” was often loosely used

when the cell-body alone was referred to, it seemed worth while to give

the unit of structure a new name. The term “neurone” was introduced

to emphasize its functional individuality. The nervous system is an

association of neurones.

By the extremely simple expedient of placing a small block of

nerve-tissue in bichromate of potassium, and then transferring it to

nitrate of silver, jet-black pictures of nerve-cells are obtained

showing with amazing completeness all the details of contour of their

bodies and all the intricacies of branching of their limbs. The most

surprising feature of the process is the absence of confusion in its

results. Dyes were in use which stained one kind of cell better than

another, or picked out a particular part—usually the nucleus—of every

cell. If the chrome-silver process had acted in the same way, a dense

black preparation in which no details could be distinguished would have

been the result. But instead of treating all cells alike, the process

blackens one cell here and another there, leaving hundreds or thousands

untouched. It shows no preference for any particular kind of cell. In

one section large cells are picked out, in another small ones; in a

third no nerve-cells are blackened, but connective tissue is brought

into view. When the block of tissue soaked with bichromate of potassium

is immersed in a solution of nitrate of silver, the chromate escapes

from it into the surrounding liquor much more quickly than the nitrate

gets in; and when at last the nitrate of silver enters, it finds that

some of the cells have fixed the chromate in their substance. This

retained chromate combines with silver. The product is rapidly reduced

to a black subchromate. No explanation of the fixing of the chromate

by individual cells has yet been offered. It is a remarkable fact that

another process which similarly makes choice amongst the elements

has since been introduced, giving even more valuable results. Pieces

of fresh tissue are placed in a very dilute solution of methylene

blue. When staining is satisfactory, nerve-cells alone take up the

dye. The selection of individual nerve-cells is not carried so far as

it is by the chrome-silver method, but it is exhibited to a certain

extent. It is probable that nerve-cells live (in a physiological

sense) longer than other tissue-elements. Methylene-blue contains some

easily removable oxygen of which the oxygen-starved nerve-cells take

advantage. The reduced methylene-blue remains in their substance, so

that when the preparation is reoxidized by exposure to air the pattern

of the nerve-cells is rendered conspicuous. When a few cells are

selected, it is, presumably, because they were the only ones alive at

the time when the dye entered the tissue. Preparations made from the

wall of the alimentary canal seem to justify this simple explanation.

They show patches in which muscle-fibres are stained, patches in which

there is no staining, and intermediate zones in which nerve-cells are

coloured and muscle-fibres are not. But the hypothesis is inadequate

to meet all cases. When first employed, the blue was injected into

the animal in successive doses until it killed it. The staining was

believed to occur _intra vitam_. Subsequently it was found that its

application to fresh tissue, or, for certain results, to tissue which

has been kept for some hours, is equally effective.

Without an understanding of the nature of the two new processes, and of

the character of the results which they yield, it would be impossible

for the reader to realize the extraordinary advance in our knowledge of

the finer structure of the nervous system which has marked the period

during which they have been employed.

The chrome-silver process is the more useful for the central nervous

system. Methylene-blue gives better results with tissues containing

minute nerve-cells and the branches of nerves. The latter method has

revealed such a profusion of nerve-twigs as would never have been

suspected but for its use. Consider, for example, the lining epithelium

of the lungs (p. 168). Every one of its flattened cells has its own

nerve twig or twigs. They lie between the cells. They give branchlets

which enter them. A similar statement might be made regarding the

richness of the nerve-supply of the muscle-fibres of the alimentary

canal, or of the cells of glands, and possibly of other tissues.

Each fresh success achieved in the application of the method makes a

further revelation of the abundance in which nerves are distributed,

increasing our sense of the dependence of all vital processes upon

nervous control, and our appreciation of the unifying and integrating

importance of the nervous system.

The term “neurone” is used by certain writers with a view to

emphasizing their belief, not in the functional individuality alone

of the unit of structure, but also in its anatomical isolation. The

peculiarity of the methods of coloration which we have described lies,

as already pointed out, in their selecting the cells which happen to

be in a particular nutritive condition, and ignoring their neighbours.

Hence pictures of separate and discrete units are obtained. This proves

the nutritive autonomy of the cells, but it does not necessarily follow

that A is not structurally connected with B, and B with C. Impulses are

passed along the chain from A to C. Functionally, therefore, they are

linked together; but until the question as to the way in which contact

is established is settled, it is as well to think of the neurones as

anatomically discrete.

It would be impossible in this book to describe all the varieties of

neurone, for nothing is so characteristic of these elements as their

enormous range both in size and form. It may be truly described as

having no limits. Each of the two electric organs of Malapterurus is

governed by a single neurone. Its cell-body is a fifth of a millimetre

or more in diameter—large enough to be seen with the naked eye—and

traversed by capillary bloodvessels. The axon of this nerve-cell—its

single nerve-fibre—ramifies to supply a separate branch to each of the

2,000,000 chambers of the electric organ, and each branch breaks up

into a bunch of twigs within the chamber. Contrast with such a giant

cell as this one of the granules of the retina or cerebellum, the

smallest cells to be found in the body, yet each a perfect neurone,

exquisitely elaborate in form.

[Illustration: FIG. 19.—A NERVE-FIBRE CONSISTING OF A, THE UNDIVIDED,

FIBRILLATED AXON OF A NERVE-CELL, WITH ITS VARIOUS WRAPPINGS.

In segment 1 the wrappings comprise B, a tube of

phosphatic fat (myelin), interrupted at H, a node

of Ranvier; C, a delicate membrane (sarcolemma);

D, connective tissue; E, the rind of the axon;

F, a tubular space containing lymph, between the

axon and its sheath of myelin; G, nucleus of

an enwrapping cell. At I the myelin is seen to

be divided into overlapping conical rings. 2,

The medullated nerve-fibre, running an isolated

course, is merely enclosed in a tube of connective

tissue containing lymph. 3, As a “grey” or

“non-medullated” fibre, the axon has lost its

myelin sheath.]

As types for description we may take one of the motor cells of the

spinal cord and a granule of the cerebellum. Every nerve-fibre which

supplies a group of voluntary muscle-fibres is a thread drawn out from

a large cell-body which lies in the grey matter of the spinal cord or

of the axis of the brain. The fibres pass out in the anterior root of a

spinal nerve or in a cranial nerve. The cell-body may have a diameter

of as much as 100 µ (1 µ = 0·001 millimetre). In shape it is like a

very irregular starfish, owing to its being continued into several,

usually four or five, thick tapering branching limbs or processes,

known as dendrites, in addition to its slender thread-like axon. From

its origin in a cell-body to its destination in a muscle—it may be a

few inches, or it may be a yard away—the axon is an unbroken thread.

A short distance from the cell-body it enters a tubular sheath, which

protects and insulates it, recalling the covering of gutta-percha

in which the wires of a telegraph cable are enclosed. The sheath

is of a phosphatic fat, invested and held in place by a delicate

transparent membrane, neurilemma. Beneath this membrane nuclei occur

at regular intervals, and midway between each two nuclei the sheath

is cut across by a septum. Such interruptions or nodes show that the

sheath is not a part of the nerve, if the term is used in the most

restricted sense. Each internode is a cell which has been wrapped round

the nerve for its protection. The axon with its sheath is spoken of

as a nerve-fibre. A large number of nerve-fibres bound together by

connective tissue constitute a nerve. In some cases the axon before it

leaves the spinal cord, but after it has entered its myelin sheath,

gives off one or two lateral branches (“collaterals”), which return to

arborize in the grey matter of the cord. It does not appear that they

are always present in the case of the motor neurones of the spinal or

cranial nerves—probably they are usually omitted—but collaterals

are important features of the large neurones of the cortex of the

cerebrum and cerebellum (Figs. 23, 24). Usually two, three, or four

such branches start off at right angles from the axon, and after a time

turn back towards the surface, dividing into a few extremely slender

branches. Their purpose is an enigma. Possibly they bind a group of

cells together in functional unison. Such an explanation would seem

reasonable in the case of an arrangement of collaterals on the plan we

have just described; but in various situations in the brain cells are

seen of which the axons, instead of becoming nerve-fibres, break up

completely into collaterals, which branch repeatedly.

[Illustration: FIG. 20.—A GANGLION OF A LEECH.

Pear-shaped cells are set round a felt-work of

nerve-fibrils (neuropil). A neuro-sensory cell

is shown with one fibre directed peripherally,

branching on the surface; and one directed

centrally, ramifying in the neuropil. Several very

slender fibrils from the neuropil pass up the stalk

of each ganglion-cell. They join a network near its

surface. This net is connected by radiating fibrils

with a coarser net which surrounds the nucleus.

From the central net a relatively stout fibril

passes to muscle-fibres.]

By various methods it may be shown that dendrites, cell-body, and

axon contain fibrils (Fig. 22). These neuro-fibrillæ lie parallel

to one another in the axon. Where it divides they are distributed

amongst its branches. Possibly they also branch. In the neurones of

Malapterurus, already referred to, this would appear to be inevitable.

The discovery of neuro-fibrillæ seemed to carry us a step nearer to

a comprehension of the physics of nervous conduction. They clearly

indicate that particles of the substance of a nerve-fibre are

oriented in the direction in which impulses pass. It is a structural

differentiation similar to the fibrillation of muscle, and probably

of the same order—a response to the same demand. But when we examine

the arrangement of the fibrils in a cell-body and its dendrites, the

appearances which we discover serve to perplex us. They complicate

instead of simplifying our mental picture of the conduction of

nervous impulses. The coarsest and most distinct neuro-fibrillæ are

to be found in annelids, the ganglion-cells of a leech, for example,

affording excellent preparations. These cells are pear-shaped, with

a single stalk. As is usual in invertebrate animals, they do not

exhibit separate dendrites and axon, but dendrites and axon pass out

from the cell in the common stalk. The bodies of the cells are set

round a felted mass of nerve-filaments, into which their stalks break

up. Just beneath the surface of the stalk of one of these cells two

or three very fine neuro-fibrillæ are to be seen. A single, much

coarser fibril occupies its axis. The fine fibrils join a network at

the periphery of the cell-body. The thick fibril is connected with a

coarser network which surrounds the nucleus. Radiating threads unite

the finer with the coarser net. It has been suggested that afferent

impulses ascend the fine fibrils, pass from the finer to the coarser

net, and take their exit by the thick fibril, which can be traced into

a motor nerve. Such a transit could not, so far as one can imagine,

have any effect upon the distribution of the impulses which pass

through the neurone; besides, there are reasons for believing that the

course taken by impulses which are delivered to the ganglion by sensory

nerves is determined by the felt-work in its centre, the neuropil. It

is probable that during their passage through the cell-body impulses

acquire the energy requisite to discharge the muscles to which the

motor-fibre carries them. In vertebrate animals, sensory nerves are

branches of neurones of which the cell-bodies lie in cranial or spinal

ganglia. They resemble the ganglion-cells of the leech in as much as

they are unipolar; both branches, the one which collects impulses from

sense-organs, and the one which distributes them to the spinal cord,

come off from the cell in a common trunk which afterwards divides,

although the unipolar condition of the cell of the spinal ganglion is

not primitive, but acquired. In the earliest stages of its growth the

cell is bipolar. Its two ends subsequently grow together for a certain

distance, the common portion being the vertical limb of the =T= (_cf._

Fig. 21, which shows the growth of a granule of the cerebellum). The

body of the cell contains a network not unlike the network of the

leech. It is probably related to what may be termed the charge of the

neurone, the development of a suitable degree of force in the impulses

which pass through it.

The neuro-fibrillæ of a large nerve-cell, such as a motor cell of

the spinal cord, are exceedingly slender (Fig. 22). They branch and

reunite. A certain number gather towards the axon; but the majority

pass through the cell from one dendrite to another, or from one branch

of a dendrite to another branch. It is very tempting to suppose that

neuro-fibrillæ are connected with conduction. When first discovered

they were regarded as conducting strands; but it is evident that

they are not comparable with telephone wires or other isolated or

separate conductors. There are good reasons for regarding dendrites

as collecting processes, taking up impulses from the end-twigs of

the nerves which branch in the grey matter around them, passing

them through the cell-body into the axon. The continuation of

neuro-fibrillæ from dendrite to dendrite seems to be irreconcilable

with the hypothesis that they are disposed in the lines of conduction.

In common with those of various other types of neurone, the dendrites

of spinal motor cells are beset with “thorns.” These projections are

not rugosities or serrations, but short, delicate threads which stand

out at right angles from the dendrites (_cf._ Fig. 1). About a dozen

years ago, the writer made a careful investigation of these structures;

at a time when most anatomists regarded them as artifacts. He found

that their claim to be regarded as parts of the neurone is as good

as that of its axon or its dendrites; although never seen on certain

types of cell, the thorns, of cells which carry them, are perfectly

definite in arrangement and spacing. In some kinds of cell they are

more numerous, in others less. Neuro-fibrillæ, as we now know them, had

not been discovered at the date when this investigation was undertaken;

but on various grounds the conclusion was arrived at that thorns are

the cell-ends of fibrils which pass from the end-twigs of arborizing

axons into dendrites. Upon this conclusion was based an hypothesis

of conduction which is here submitted, not because there is not much

to be said against it—or, at any rate, many a hiatus in knowledge

to be filled—but because it happens to be the writer’s own. The

chrome-silver and methylene-blue methods which reveal the existence of

thorns do not stain neuro-fibrillæ. They colour the soft protoplasm

in which fibrils are embedded. By modifying the chrome-silver method

in every way which still allows a result to be obtained, it was found

that thorns sometimes appear as comparatively long slender filaments,

at others as shorter filaments ending in minute knobs, or as filaments

bearing two or three dots; or finally no filaments are visible, but

the dots are in the position which they would occupy if fibrils were

present, but not stained. From this it was argued that the soft

protoplasm which during life surrounds the filament as a continuous

film, either falls back towards the cell after death or is made to

shrink into the cell by reagents. This accounts for the appearance

of rod and knob. What is supposed to happen may be illustrated by

dipping a wire in treacle. At first, when the wire is withdrawn, it

is surrounded with a film. Then the film gathers into droplets. It

was suggested that the entrance of impulses into dendrites, their

conduction across the space which separates the end-twigs of axons

from the dendrites into which their impulses pass, is by means of

the thorns, although the thorns are not in themselves conductors.

Conduction occurs only when films of cytoplasm surround the thorns. The

first effect of impulses is to call out the films, in the same kind of

way that a current of electricity converts a row of falling drops into

a continuous stream. A succession of impulses, by adding to the number

of the filaments which are enveloped in cytoplasm, or by increasing the

amount of cytoplasm investing certain groups of filaments, increases

the openness of the path. Sleep is a condition in which all paths are

open. Hence no impulses are effective. Wakefulness, alertness, depends

upon the closing of all paths save those which are actually in use.

We may go further. The power of concentrating attention is the power

of limiting the spread of nerve-impulses in the brain. Alcohol opens

extra paths; the concentrated effort which was making progress with

a problem becomes more diffuse. The first effect appears in greater

brilliance of thought, gained at some sacrifice of cogency. Unexpected

analogies are discovered. Imagination takes a wider range. But as the

dose is increased, a condition akin to sleep is set up. Nerve-impulses

become ineffective because, many paths being open, they do not attain a

sufficient intensity in any set of paths. These few illustrations are

given for the sake of showing the need of a theory of the opening and

closing of paths. It is not suggested that they favour the particular

hypothesis here set forth as to the structural arrangement which

provides the paths and regulates their accessibility.

Recent discoveries in the finer structure of the central nervous

system have provided many problems which at present appear insoluble.

One of the discoveries most difficult to make use of in constructing

theory is the existence of extracellular or pericellular nets, which

have the appearance of extraordinarily delicate cases of wire-netting

immediately surrounding the nerve-cells. It is somewhat remarkable

that the spacing of the nets is often very similar to, if not

identical with, the spacing of thorns. While some anatomists look

upon the nets as nervous, others regard them as pertaining to the

connective tissue of the nervous system. At present it is not known

how impulses get across from the finest visible twigs of arborizing

axons to the dendrites of the neurones which they influence. The

wealth of structural detail which recent research has revealed is an

embarrassment to anyone who tries to devise a scheme. Not improbably,

pericellular nets are intermediate factors in the exchange; or, if

not the nets, the structures whose existence is indicated by the

appearance of the nets. In the case of many of the finer markings which

staining methods bring into view, it is impossible to say whether they

indicate the presence during life of the structure as it appears to

be, or whether the markings are due to coagulation of plasma or to

strain caused by shrinkage in coagulating agents. In a sense this is

not of much consequence. Coagulation in a uniform pattern would mean

the existence of an architectural substructure which determines the

pattern. We may be looking at the cake or at the tin the cake was baked

in.

There is a danger of seeing too much in a nerve-cell when examining

it under the highest powers of the microscope, and of endeavouring to

picture in too much detail the arrangements which regulate the flow

of impulses. Its markings are so complicated as to suggest to the

mind of the observer that it is itself a microcosm—a nervous system

in miniature. Neuro-fibrillæ appear to offer many alternative paths

within the cell. It is unlikely that such a way of looking at the unit

of structure is the right one. A certain motor cell of the spinal cord

is connected by its axon with thirty or forty separate muscle-fibres;

but there is no reason for thinking that the fibres ever contract save

as a single group. The axon consists of parallel fibrillæ, but these

do not appear to be needed as separate conductors; an impulse travels

down the fascicle. It does not appear to be necessary in the case of

a motor cell, and presumably the statement holds good for the large

cells of the cerebellum and cerebrum to picture any arrangement for the

simultaneous conduction in its axon of several impulses, or for the

conduction of one impulse along one of its fibrillæ and a different

one along another. What is necessary is that this particular efferent

path Z should be accessible from every other part of the nervous

system—from A to Y. If, merely for the sake of filling the space

which would otherwise be blank in the mental picture, we imagine a

pericellular net connected by thorns with the body and dendrites of

the nerve-cell Z, then the net is the meeting-ground of all the routes

through which Z is called into action. A nerve-wave from any of the

neurones A to Y, breaking upon this net, passes along the thorns into

the protoplasm of Z.

In size a granule of the cerebellum presents a marked contrast to a

motor cell of the spinal cord; yet it is formed on essentially the

same plan. From its minute round body (about 8 µ in diameter) four or

five slender dendritic processes are drawn out. Each dendrite ends in

a little bunch of twigs, resembling fingers curved over the palm. Its

single slender axon runs towards the surface of the cortex. As the

granules lie at a considerable depth, this course is, for those which

distribute to the most superficial layers, a long one. They pass from

the granular to the molecular layer between the big cells of Purkinje.

When the axon has reached a certain level in the molecular layer, it

divides into two threads which run for a great distance, right and left.

The granules of the cerebellum have a curious developmental history.

Every neurone in the body has a lifelong existence. Except for the

rare accident of its destruction by disease it occupies its station to

the hour of death. But at the time of birth many neurones are still

immature. Not all the granules of the cerebellum have yet assumed

their permanent form or situation. Beneath the pia mater there is

still a layer of minute undifferentiated cells. These, as they grow

into granules, elongate, in the first instance, into long spindles.

Subsequently they sink down through the molecular layer and between

the cells of Purkinje, leaving the poles of the spindle as the right

and left divisions of the axon (Fig. 21). It is interesting to learn

that such a migration is possible. It is also of interest to find

that a tiny granule of the cerebellum goes through the same stages in

attaining its adult form as one of the large cells of a spinal ganglion.

[Illustration: FIG. 21.—THE GROWTH AND MIGRATION OF GRANULES OF THE

CEREBELLUM.

Half a dozen nuclei of as yet undeveloped granules

are seen lying beneath the pia mater. From this

level to the bottom of the drawing granules are

shown in successive stages of growth. These

developing granules, selected from various

preparations of the cortex of the cerebellum, were

drawn from nature.]

There are many different types of neurone. Any attempt to describe

them, or to give an account of the various details of structure which

recent improvements in technique have enabled anatomists to observe,

would fill a lengthy treatise; and would, moreover, be beside our aim,

which is limited to obtaining such an idea of the unit of the nervous

system as will enable us to form a conception, however crude, of the

way in which it works. From the brief account that has been given, it

will be evident that anatomists are approaching to an understanding

of the mechanism. It will also be evident that they have already more

information than they can apply. They are cognizant of many details of

structure which they cannot interpret in terms of function; and at the

same time are aware of wide gaps in their knowledge regarding facts

which are essential to the construction of any scheme. This much is

clear: A sense-cell on the surface or beneath it is touched (probably

entered) by the ultimate twig of the outer limb of a neurone whose

cell-body lies in a spinal ganglion, while its inner limb, as a fibre

of a posterior root, enters the spinal cord. In the spinal cord the

root-fibre splits into an ascending and a descending division which

rain branches into the grey matter over a considerable area above

its point of entrance, and a smaller area below it. The finest twigs

of these branches are to be seen in the vicinity of the cell-bodies

and dendrites of certain other neurones. The axons of these second

links arborize in a similar way in the vicinity of large motor cells,

whose axons in turn become fibres of anterior roots. (For simplicity’s

sake no reference is made to hosts of other neurones which link the

ganglion-cell and the motor cell to other cells higher in the cord or

brain.) An impulse generated in the sense-cell on the surface of the

body runs up the root neurone into the cord, where the ultimate twigs

of the posterior root-fibre offer it a wide choice of distribution.

Following the path of least resistance, it passes into neurone No.

2. Again, the arborization of No. 2 offers it alternative paths. It

makes a choice which lands it in No. 3. No. 3 passes the impulse on

to the muscle-fibres with which it is connected. Three points are

especially worthy of attention: (1) The impulse has a wide (literally,

an unlimited) choice of routes. The skin of the finger is touched. Any

muscle may respond, although resistance is so graded as to cause the

impulse to seek in the first instance the group of muscles which is

most often required to act in consequence of stimulation of the finger.

This means, we may suppose, that it follows the chain which, having the

smallest number of links, offers least resistance. If it cannot get

through to these muscles, owing to the fact that other impulses, acting

simultaneously, either increase the resistance in this particular path,

blocking its way, or reduce the resistance in an alternative path, it

spreads farther afield. (2) Owing to the ramification of the root-fibre

which conveys it to the cord, an impulse is not limited to a single

line of distribution. It reaches many secondary links. It may therefore

influence various effector neurones simultaneously. For example, a

stimulus which calls extensor muscles into action, at the same time

inhibits their flexor antagonists. (3) The path which it finally takes

is accessible to all other impulses. Its root neurone was peculiar to

itself. Link No. 2 was more or less a common path. Neurone No. 3 is

open to every impulse which traverses the nervous system.

Anatomy justifies the construction of the scheme just outlined. But

there are many points regarding structure upon which a physiologist

desires information, many details that he wants to see filled in.

How is the impulse passed from the arborization of axon No. 1 to

the dendrites of neurone No. 2? By what structural arrangement is

resistance introduced, and how is it regulated, if it varies? Supposing

the resistance to be higher in one path than in another, or supposing

that more force is needed to enable an impulse to invade a wider field,

how is additional energy supplied? To the first question no answer can

be given at present—the mechanism by which impulses are transferred

from one neurone to another is unknown; yet it is convenient to find

a name for the junction of axon-endings and dendrites. It is termed a

“synapse,” on the understanding that the word involves no hypothesis

as to its structural nature. It is generally held that resistance is

introduced into nerve-circuits at synapses; although this again is

a provisional statement. The phenomena for the explanation of which

the idea of synaptic resistance was introduced, may be accounted for

on a purely anatomical basis of distribution. The extent to which

one neurone influences another may depend upon the size of the brush

of fibrils with which its axon touches it. If a certain force is

needed to discharge a neurone, a nerve-current must either have a

sufficiently high potential when it reaches it, or it must act upon it

for a sufficient length of time. There is little to choose between the

arguments which place the resistance at the synapse and those which

transfer it to the nerve-cell body.

As a mechanism the nervous system is unthinkable, unless we picture

its units as independent, yet capable of forming associations; as

functionally discrete, yet entering into functional continuity. When

acting, they act as chains. Impulses run from link to link, from

the end-twigs of an axon of one cell to the dendrites of the next.

Neurones are so arranged as to make it impossible for impulses to

escape backwards out of dendrites into axon-twigs. In this respect the

system is valved. But there is no reason for thinking of the substance

of the neurone as polarized in any way. The physical accompaniment of

an impulse—the electric variation—travels with equal facility up and

down its axon.

There is no evidence of any specificity of neurones; on the contrary,

it is clear that impulses of every kind—that is to say, from every

source, for we recognize no specificity of impulses—can travel

equally well through neurones of all forms. At every junction, in

passing through each synapse, they are delayed. It takes at least 0·01

second (less if the knee-jerk be a true reflex action) for a message

delivered to the cord by a sensory root to reach a motor root. This

hundredth of a second—the sum of the delays entailed in fording two

or three synapses—is regarded as the minimum reflex time. To it

must be added, in considering any particular reflex action, the time

taken in travelling up sensory and down motor nerves. Delay indicates

resistance. If a sensory stimulus be not sufficiently pronounced to

provoke a reflex action, the reflex may be obtained on intensifying

it. Prolonging or repeating the stimulus—really the same thing, since

sensory impulses are rhythmic, not continuous—has a far more potent

effect than increasing its force. The resistance of synapses gives way

after a number of impulses have bombarded them. The desire of brushing

a fly from the skin, if resisted, becomes intolerably urgent after a

time. A persistent outflow of impulses produced by the irritation of

a spot in the cortex of the brain overwhelms the nerve-muscle system

in an epileptic fit. The following is an experiment illustrating the

spread of impulses from their customary path to another less often

used: A piece of blotting-paper, wet with vinegar, is placed on the

inner side of the thigh of a brainless frog. There is no use in trying

the experiment on a frog which retains its brain; the substitution

of one action for another would be an exhibition of the adaptation

of means to end—a demonstration of the animal’s right of choice.

Besides, the frog might choose not to act, and so the experiment would

fail. The brainless frog wipes off the blotting-paper with the foot of

the same side. This foot is then fixed so that the action cannot be

performed, and the blotting-paper replaced. After a longer interval the

frog removes it with its other foot. Evidently it is more difficult

for the impulses generated by the irritation which the vinegar causes

to get across the cord than it is for them to reach motor neurones on

the same side. Evidently, too, the continued irritation of the vinegar

adds to the travelling power of the impulses. They are strengthened

until they are capable of overcoming the resistance in the longer

path. “Resistance in conductors” and “potential of current” are terms

with which the study of electricity has rendered us familiar; but it

must be evident from the experiment just described that these terms

are not really applicable to nervous phenomena, convenient though they

may be for use in an allegorical sense. Holding the foot does not, by

any mechanism which we can recognize, switch off the shorter circuit,

yet the impulses abandon it for the longer path. There is no evidence

of a struggle to free the foot that has been fixed, coincident with

the spread of impulses, as they gather sufficient strength to reach

the nervous mechanism of the other leg. The right foot not being

available, the impulses _choose_ the route to the left foot. Any

attempt to explain this in terms of resistance and potential involves

the formulation of a number of subsidiary hypotheses; easy to devise,

no doubt, but stultifying to the explanation exactly in proportion as

they complicate it. Yet the hypothesis of lines of greater and of less

resistance (keeping as far away from electrical analogies as possible)

is essential to any explanation of nervous phenomena, and is, moreover,

justified by the evidence available. There are two causes in chief

upon which it depends: (1) The greater the number of neurones in a

linear chain, the greater is the number of synapses to be traversed.

If A, B, C are in the same circuit, the sum of their resistance has to

be overcome. (2) The greater the number of neurones amongst which a

nerve-current has to be subdivided, the smaller the charge available

for each of them. Imagine

B

A

C

so placed as to divide B and C, the charge delivered by A between. This

arrangement has, probably, an anatomical expression which accounts for

the relative ease or difficulty of a path, even on the supposition

that impulses do not open out as they advance—do not spread along

all the branches into which an axon divides—but keep to a given

line. The axon of neurone A divides, to branch about B, C, and D; but

its representation in the several pericellular nets (the expression

may pass for the sake of the simplicity which it introduces into the

picture) is unequal. In the vinegar experiment the impulses delivered

to the spinal cord by the root-ganglion neurone A pass to neurone B of

the posterior horn. B’s axon arborizes more freely about the cell-body

of neurone C in the anterior horn of the same side than it does about

neurone D in the anterior horn of the opposite side. Hence the impulses

generated by the vinegar stimulate C, sufficiently to discharge it, so

long as that road is open, more quickly than they stimulate D. That C

should be dischargeable only so long as the foot is free implies that

the activity of the neurone is in some way conditioned by its relation

with the muscles which it innervates. When the foot is held this

relation is interfered with, giving to the impulses generated by the

continued action of the vinegar time to overcome the resistance of D.

The simile of the opening up of paths is fairly applicable to the

results which follow the use of artificial stimuli. Neurones seem to

link up in series under the influence of the impulses which bombard

them, popping like fireworks united by a common fuse.

Experimental evidence points to the following conclusions: (1)

Resistance is offered at a synapse. This resistance must be overcome

before an impulse can get through from neurone 1 to neurone 2. (2)

The impulse does not, properly speaking, pass from 1, through 2. It

infects 2, causing it to discharge a fresh impulse. (3) Time is of the

essence of this process. Either the impulses head up at the synapse,

or, passing through into the neurone, they produce a cumulative effect

within it, which provokes it to discharge. (The latter hypothesis,

which is the less likely of the two, transfers the resistance from the

synapse to the neurone to be infected.) These conclusions are based

upon experiments of the following kind: The minimal stimulus which

will evoke a reflex action is determined. A stronger stimulus is then

applied. The reflex occurs more promptly, and is more pronounced. But

on further increasing the stimulus, it is found that the limit of

effectiveness is soon reached. The proportional relation of response

to stimulus is much less evident than it is when the experiment is

tried with a nerve-muscle. Choosing a reflex action easily provoked,

the afferent path is stimulated with an electric current interrupted

fifty times a second. The impulses which flow down the efferent path

to the muscle follow one another at the rate of about ten a second. A

column of nerve-fibres within the spinal cord is stimulated fifty times

a second. Again, the discharge into anterior roots has the natural

rhythm of about ten. The cortex of the “motor area” of the great brain

is stimulated with a rapidly interrupted current. The muscles which

it governs contract with their natural rhythm. The cortex is sliced

away, and the stimulus applied to the white matter beneath. A similar

result is obtained. Evidence such as this points to an independence of

action on the part of the neurones which one can express only in terms

of resistance and explosion. But there is another line of thought which

leads to the development of a picture of the working nervous system

which seems at first sight incompatible with the one that we have

sketched. The phenomenon of the knee-jerk (p. 274) reveals a nervous

system so intimately linked together, so homogeneous, so mobile,

that no event, however trivial, occurs in any part without sending a

vibration throughout the rest. Instead of a multitude of batteries

enveloped in a labyrinth of wires interrupted by myriads of switches

which are crackling on and off, the image of a sheet of water better

figures our conception—a material so frictionless that it is a-ripple

from side to side and end to end, from the most distant rivulet which

feeds it to the farthest trickle in which it drains away. It is a

fluid in a state of infinite commotion, the movements of its particles

varying in amplitude from tremulous quiverings which scarcely frost

the silver of its surface to waves which, breaking on the muscular

system, throw it up in heaps. The vinegar experiment seems to demand

a scheme of batteries and wires. The knee-jerk points to a continuous

conducting medium. Other phenomena suggest the superposition of the two

pictures; the conception of a nervous system consisting of a uniform

medium conducting, not indifferently in all directions, but with such

freedom that from our point of view the paths are infinite in number;

and within this conducting medium nerve-cell bodies and their processes

which collect and distribute groups of vibrations sufficiently strong

in combination to produce visible effects. In order that one of these

neurones may be stimulated to discharging-point, the medium by which it

is surrounded must be thrown into such a state of agitation as suffices

to infect it. The considerations which point to the formulation of

this double or superposed scheme are such as follow: The passage of

tone-impulses does not appear compatible with the ideas we have formed

on other evidence of synaptic resistance and neuronic discharge. They

are too feeble for such a mechanism. The short “reflex time” of the

knee-jerk points to the passage of the agitation up a sensory root to

the spinal cord, and through a non-resistant medium to the environment

of the motor cells which it discharges, missing the neurone or neurones

which intervene in the case of ordinary reflex actions. This is an

illustration of the way in which tone-impulses, which we imagine as

conducted by the non-resistant medium, pass over into discharges which

produce visible effects. Again, the phenomena of =inhibition= appear to

require the supposition of extra-neuronic conduction. Whenever a reflex

path is in use, all other paths in its neighbourhood are closed. The

passage of impulses leading to a particular reflex action is favoured

by the suppression of conduction in its vicinity. When A is talking

to D through the nerve-telephone, B and C are compelled to hold their

peace. Inhibition is a phenomenon of universal occurrence. In relation

to various actions, it is sufficiently pronounced to be visible in

the effects which it produces. A simple experiment will illustrate

this. Holding water in the mouth has no effect upon respiration, but

during the act of swallowing respiratory movements are suspended.

Whilst the swallowing reflex is occurring the respiratory reflex is

inhibited. This might be attributed to the volitional control of

respiration, and certainly when attention is being directed to the

process volition plays a large part. But if a finger is placed on the

pulse, it is possible to detect that, during the act of swallowing,

the pulse quickens, owing to the suppression of the slowing action of

the vagus upon the heart. Here is a case in which inhibition is in no

degree a voluntary action. Nor is it of any value as an adjunct to the

particular reflex with which it is associated. It is an illustration

of the universal rule that activity of any one spot in the nervous

system is the cause of the quieting of the surrounding area. Impulses

which reflexly check the heart cannot get through the medulla oblongata

whilst the swallowing impulses are traversing it. Inhibition has been

described as a drainage of nerve-force into the active area. On the

structural side it seems to require the conception of an extra-neuronic

substance which, agitated in the vicinity of the cells which are to

be discharged, is brought to rest around neighbouring cells. The

promulgation through the nervous system of the state which, when it

reaches the centres of consciousness, produces pain also seems to call

for an hypothesis of extra-neuronic conduction.

Any reference to =pain= in a work on physiology needs a few words

of preface, since popularly the term “pain” is used in various

senses. When I see pink geranium and nasturtiums growing in the same

flower-bed, I may exclaim: “It is positively painful.” The want of

harmony, and at the same time the insufficiency of contrast, of chalky

pink and translucent orange, jars my æsthetic sense. Dislikes, however

well founded, are ruled out in thinking of the physiology of pain.

Further, in defining pain, we must be careful to isolate the real

thing, and not to confuse it with sensations which seem to lead up to

it. If, putting my finger in a pair of pincers, I touch it as lightly

as possible, the first sensation is one of contact; a little harder,

and it becomes a sense of pressure; harder still, and all sense of

contact or pressure is lost in pain. It is usual to regard pain as

sensation carried to excess. But neither is this physiological. An

excessively bright light or an excessively loud sound is disagreeable.

It causes a sudden movement for the purpose of avoiding it—just such

a movement as one would make if one touched a red-hot poker—but it is

not, strictly speaking, painful. Not uncommonly in cases of accident

or disease of the spinal cord a sharp distinction is drawn between the

sense of touch and the capacity for experiencing pain. Below the injury

the patient retains his sense of touch undiminished in acuteness, but

no blow, or cut, or burn, causes him any pain. The pain caused by

squeezing the finger in a pair of pincers is not, therefore, an excess

of touch sensation. Pain begins to be experienced in the skin just when

the object applied to it is affecting it to an extent which might do

harm. If the point of a needle touches it, it causes pain as soon as

the pressure is a trifle less than that needed to pierce its surface. A

hot object begins to hurt when the temperature reaches 48° C.—almost

enough to coagulate the tissue fluids. Pain is not a discriminative

sensation. If I hold my arm out at right angles, I am conscious for

the first few minutes of its weight, and have, besides, some sense

of the traction exerted by the muscle of the shoulder. At the end of

ten minutes these sensations are merged in pain, and for some time

after lowering the arm the shoulder-muscle aches, much as it does

in rheumatism. Pain is an effect upon consciousness, which absorbs,

engulfs, and therefore obliterates sensation. To use an ancient phrase,

“It is less that I feel pain than that I am pain.” If we speak of the

capacity for pain as a sense, we may call it for the purpose of our

present argument the “sense of damage.” The nerves of the skin are

acutely affected by any agent which is likely to do harm. It is their

business to convey to the central nervous system an influence which

so affects it as to set up in consciousness the condition of pain.

Sensations of damage evoke reflex movements by means of which the part

of the body likely to be injured, or the whole body, is removed to a

safe distance. It being the duty of the skin to give this warning, a

service of nerves sensitive to noxious agents has been developed which

scouts in co-operation with the services devoted to the recognition

of physical contact and heat and cold (_cf._ p. 425). If, imagining

that the fire has not been lighted, I touch an almost red-hot stove, I

acquire quite a considerable amount of information of which I am able

to make use. I gain an accurate notion of the situation of the stove,

and I put the right part of my finger in my mouth. The skin sends to

the brain the ordinary sensations of touch and pressure before the

condition of pain is established. In seeking for a definition of pain,

we must eliminate the two attributes which have characterized all the

forms of stimulation which we have considered up to the present time:

(1) The tendency to provoke movement; (2) the supply of information.

If I am suffering from a whitlow, the last thing that I am disposed

to do is to jerk my finger about. Although it enhances the urgency

of skin-reflexes, pain, in general, inhibits movement instead of

provoking it. This is well illustrated in pleurisy. So long as a man is

healthy he is quite unconscious of the fact that at each respiration

the lower part of the lung slides on the lining of the chest-wall;

but commencing inflammation on the surface of one of the lungs causes

intense susceptibility to friction, and the pain produces an effect

which the man is quite unable to produce by an effort of will; it stops

the movements of the chest on the damaged side. Pain is inhibitory, not

stimulant. It is not, properly speaking, a sensation. Frequently being

mixed with sensational elements, it conveys topographical information;

but pure pain approaches in quality the nebulous sense of distress of a

patient who, when asked where he felt it, replied: “Nowhere; but there

is a deal of it in the room.”

Sufferers describe pain in figurative language, as “burning,”

“stabbing,” “throbbing,” “aching,” and so forth. Two persons afflicted

with the same lesion, the same source of pain, use approximately the

same terms. Hence we cannot say that pains do not differ in character.

But this is not a sufficient reason for assigning any specific quality

to pain. It varies in severity, in continuity or intermittence, in

suddenness of onset, in the sensations which accompany it, in the

emotional tone to which the disturbance of the organ from which it

proceeds gives rise, in the tenseness of the part affected and its

consequent sensitiveness to a throbbing pulse. All these things make

a complex of pain plus sensation, which causes toothache to differ

from headache, and both from the pain of burned skin. But they do

not give specific qualities to different varieties of pain. This

being the case, there is no need to presume the existence of special

nerve-endings for the reception of pain, or of a special region of the

cortex of the brain for its reception. On the contrary, the evidence

is conclusive that the nerve-fibres which serve the more highly

specialized senses, which have well-defined connections in the cortex

of the brain, do not convey the influence which enters consciousness as

pain. It is the innumerable nerves which have no specialized receptors

that take up pain. The afferent nerves of the viscera—the vagus and

sympathetic—convey no impulses which enter consciousness, so long as

the tissues which they supply are healthy. They have no representation

in the cortex. The organs with which they are connected (with trivial

exceptions, easily accounted for) are absolutely insensitive to

injury. Before the virtues of chloroform were known—in the days when,

however severe the operation, the patient had to nerve himself to

bear it without an anæsthetic—surgeons proved that the liver or the

intestines, or practically any other viscus, may be cut or cauterized

without the patient being aware that it is being touched. The same is

equally true of the brain itself. But if damage in a viscus is set up

gradually, its nerves convey to the central system an agitation which

has the most pronounced results upon consciousness, and on the way

profoundly affects the reflex actions which the spinal cord can carry

out, and also its capacity as a conductor. Once in his life, perhaps,

a man passes a gall-stone; for generations such a thing may not have

happened in his family. Yet the man finds that he is provided with a

nervous apparatus which conveys to consciousness intensest pain.

It is difficult to think of pain as travelling along nerves in the form

of rhythmic impulses, similar to those which produce in consciousness

the effects which we have distinguished as sensations. A few lines

above we stated that no impulses which affect consciousness normally

travel up the vagus or the sympathetic nerve, limiting the term

“impulse,” perhaps unjustifiably. The vagus conveys an influence which

enters our experience, as hunger. Probably other states of feeling

for which we have no names, which resemble pain and hunger and their

opposites, are set up through the agency of visceral nerves.

Fifty years ago attention was called to the difficulty of finding

pain-paths amongst the white tracts (nerve-fibres) of the spinal cord.

It is as difficult to point them out now as it was then; but the

inference that pain travels up the grey matter has given way to the

“neurone theory”; under a misapprehension as the writer holds. Pain

travels slowly. If one happens to notice a person who unsuspiciously

touches a hot surface, one observes that an interval elapses between

contact of his finger with the iron and the exclamation with which he

“relieves his feelings.” It amounts to more than a second—if the iron

is not very hot, to several seconds—whereas the “reaction-time” for

touch is only one-seventh of a second. The slowness of movement of

pain through the nervous system can on the neurone theory be explained

only on the hypothesis that it travels from link to link along a very

long chain of very short neurones. That pain is a state of the grey

matter rather than a succession of impulses, and that (within the

cerebro-spinal axis) the state is transmitted through an extra-neuronic

medium, seems a simpler explanation.

The state set up in the segment of the cord in which afferent fibres,

conveying pain from viscera, embouch affects its conductivity. It

subdues reflex action through the segment, and at the same time

facilitates or reinforces the transmission of sensory impulses towards

the seat of consciousness. This shows itself in the apparent increased

sensitiveness of the skin of the area of the surface supplied by

the posterior root which joins the segment of the spinal cord into

which the pain influence is also being poured. For example, afferent

sympathetic nerves from the cardiac end of the stomach join the sixth

and seventh thoracic spinal nerves. Other afferent fibres run up the

vagus to the medulla oblongata. When the cardiac end of the stomach

is diseased, pain is referred to the skin area supplied by the sixth

and seventh dorsal roots. The ordinary inevitable stimuli acting upon

this area cause pain. Experimental stimuli which elsewhere would be

felt as touch or warmth are painful. The impulses to which they give

rise pass through pain-agitated segments of the spinal cord. The vagus

nerve carries its pain influence to the medulla oblongata. Now, it

happens that the sensory nerve of the face—the fifth—spreads for a

considerable distance up and down the axis of the brain. The fifth

nerve in consequence pours its sensory impulses into a region which is

pain-agitated by those fibres of the vagus which come from the cardiac

end of the stomach. Hence disease of that organ gives rise also to an

“illusion” of pain—pains and illusions of pain are philosophically

indistinguishable—on the surface of the head. The viscera, having no

direct access to consciousness, appear by deputy. When the stomach

is distressed, it makes its appeal to the whole body politic for

considerate treatment through certain nerves which have the privilege

of appearing at Court. The message is misread as coming from the front

of the chest—“heart-burn”—or from the shoulder, or from the scalp, or

from the other skin areas which these nerves serve. When the liver is

in trouble, consciousness, having no knowledge of its whereabouts—is

it the business of hand and eye to explore another man’s liver, or

incumbent upon the mind to accept their findings?—infers that the cry

comes from the shoulder. Nor have the tissues beneath the root of the

nail, or the muscle of the shoulder, or the pulp of a tooth, any direct

representation in consciousness; but since the pain-condition in the

grey matter converts it into a microphone, messages from neighbouring

structures which otherwise would fail to arouse attention, after

traversing the pain-segments of the nervous system, ring out clearly,

and hence the mind locates approximately the “pain” of the whitlow, the

muscle-ache, the decayed tooth. Sufferers from toothache are familiar

with the phenomenon of the spread of pain from a definite spot to the

whole jaw or the whole side of the head, dependent upon the spread

of the pain-agitation from the segment of the axis of the brain in

which the dental nerve ends to neighbouring segments. Our ability or

inability to localize a pain does not depend upon the presence or

absence of pain-nerves, but upon the existence or non-existence of

nerves coming from the same organ, or from its neighbourhood, and

capable of conveying impulses to the seat of consciousness. In passing

through the part of the spinal cord or of the axis of the brain which

is disturbed by the influence exercised by a damaged organ, silent

impulses acquire force sufficient to render them audible, and combine

with the pain to produce a feeling which consciousness can analyse, to

a certain extent. Informed as to its whereabouts by these accentuated

sensations, consciousness recognizes a sense of pain limited in its

topographical extension.

Sneezing when a bright light falls upon the eye is a curious

illustration of the exaggeration of the effectiveness of sensory

impulses when they happen to be poured into an agitated segment of

grey matter. About one person in every three is affected in this way.

A friend of the writer, who was particularly sensitive, rising in the

night because he heard his child cry, three times lighted a candle

and three times sneezed it out before he could watch the application

of match to wick without suffering from a nerve-storm. Some nervous

dogs—especially fox-terriers—are very liable to this neurosis. Many

persons who do not sneeze feel, when the sunshine stimulates their

retinæ, a tickling in the nose. Again the illusion is to be traced to

the door of the fifth nerve—the sensory nerve of the whole of the

face. The nose is the true tip of the body. Morphologically it is

anterior to the eyes. Just as the fifth nerve extends its distribution

to the nose, so also its root-fibres extend their connection within

the axis of the brain forwards, until they traverse the mid-brain,

the primary centre of the optic nerve. A bright light, by stimulating

the optic nerve, sets up a commotion in the mid-brain. The ordinary

every-moment impulses from the nose, carried by the fifth nerve to

this region, ought not to appear in consciousness at all; but owing

to the excited condition in which they find the grey matter they

assume an importance which does not belong to them, and discharge

the reflex action of sneezing, just as they would do had one taken

snuff. Several lessons are to be learned from this phenomenon—as, for

example, one which cannot be too often impressed, that the impulses

which appear in consciousness (or, more accurately, the impulses to

which attention is directed) are but a most insignificant fraction of

those delivered by sense-organs to the central nervous system. The

impulses which give rise to the sensation of tickling in the nose

are not exceptional impulses which happened to be started when the

light fell on the eye. They were reaching the brain in a steady flow

before the agitation of the mid-brain gave to them exceptional force.

No consideration regarding the working of the nervous system has a

more important bearing than this. We cannot picture to ourselves the

activity of the sensory nervous system. Our experience is limited

to the scattered sensations which we _perceive_. Are the sensory

nerve-endings incessantly responding to external forces, throwing

an almost continuous procession of impulses up each of the millions

of nerve-fibres which connect them with the central system? Such a

conception is probably nearer to the truth than the conception which

we should develop if we trusted to experience. Yet even experience

tells us that an infinity of messages is delivered to the brain, of

which consciousness takes no account. Changing trains at a roadside

station in France, my attention was attracted by an electric bell

on the platform, which was ringing continuously. “Why does the bell

ring?” I asked the station-master. “To make known that everything

goes well,” was the response. “If it stops, something is wrong.” “But

do you not become so accustomed to it that you cease to hear it?”

“Yes, truly; it rings day and night. One does not pay attention to it

until it has stopped.” Sensory impulses generated by the contact of

my skin with the chair that I am sitting on are incessantly ringing

the bell of consciousness. I should notice them immediately if they

stopped. As it is, they do not attract my attention until they ring a

little louder than usual, or until some particular group, owing to

unrelieved pressure, produces a cumulative effect. Another lesson; that

the condition of the nervous system, and therefore its conductivity,

is determined at any given moment by the sensory impulses which are

reaching it. We cannot describe the effect of a bright light as pain,

yet it agitates the grey matter, altering its state, in the same way as

the nerve-inflow which we recognize as pain. A wet rag on the forehead

does not assuage a headache by cooling the brain (_cf._ p. 106). The

headache is “in the scalp.” The cool wet rag diminishes the dilation

of the bloodvessels of the forehead, and quiets the impulses from the

skin which are pouring into a tract of grey matter pain-agitated by the

influences ascending a visceral nerve—usually the vagus.

It is necessary to warn the reader that a reversion to the old idea of

“conduction through grey matter”—_i.e._, otherwise than by a chain

of neurones—is unorthodox. It is set forth here because it seems to

the writer that the various phenomena which have to be accounted for

fit in best with the hypothesis of a double path. If evidence of the

anatomical possibility of extra-neuronic conduction is asked for, it

may be pointed out that the chrome-silver and methylene-blue methods,

upon which our knowledge of neurones is based, do not, in the very

nature of the case, show that grey matter consists only of neurones

and their obvious branches. As they select particular elements of

structure, we can never by their use alone know what they fail to

show. Attention may also be called to the fact that the same staining

process which reveals pericellular nets (p. 301) shows also a structure

resembling a network in the substance which intervenes between them.

Truly the method is a rough one. It may well be thought that the

nitric acid used to fix the tissue may cause strange coagulations with

solution of uncoagulated substance; but, as was remarked with regard

to the pericellular nets, regular patterns indicate architectural

differentiation. But whether these nets do or do not give hints as

to the nature of the conducting medium, there is no difficulty in

finding sufficient material, after all the substance entering into the

formation of the conducting neurones, as we imagine them, has been

accounted for. _Ex hypothesi_, the conducting material is provided

by the fibrils of the sensory nerves in their extensions beyond

the limits to which the deposit of subchromate of silver extends,

when the chrome-silver method of displaying neurones has been used.

Sensation-impulses enter neuronic chains. The condition which, when it

affects the seat of consciousness, is known as pain, progresses up the

vertebrate neuropil.

Energy is developed within the nervous system. The =force of impulses=

is adjusted to the resistance which they have to overcome. Stimulation

of the millions of twigs of the vagus nerve in the lungs brings about

the gentle movements of ribs and diaphragm which constitute peaceful

respiration. A crumb of bread touching the mucous membrane of the

larynx stimulates a few of the endings of the same vagus nerve. Like

an avalanche, the impulses gather head as they advance, causing, not

the diaphragm and intercostal muscles alone to do their utmost, but

calling into action half a dozen accessory muscles of respiration. It

is difficult to account for this reverberation of the messages which

clamour for the ejection of the crumb of bread without figuring them as

spreading from neurone to neurone, urging each in turn to deliver its

maximal discharge.

Neurones are provided with material which serves as a store of energy.

In their cell-bodies, including their dendrites, are to be seen coarse

granules of nucleo-protein, which, being fitted in between groups of

neuro-fibrillæ, assume an angular form. They are known as Nissl’s

corpuscles, or are termed “tigroids,” owing to the spotted appearance

which they give to the substance of a cell. If the nerve-cells of birds

be examined just after they have alighted from a migratory flight, the

granules are found to be few and small. In a bee returning to the hive

at evening with its last load of pollen, they are smaller than they

were when it commenced its morning’s work. They disappear in certain

pathological conditions, and under the influence of various drugs; and

since their presence is revealed by staining, their disappearance is

spoken of as “chromatolysis.”

[Illustration: FIG. 22.—THE BODY OF A MOTOR NEURONE.

In its centre is a large clear spherical nucleus,

with a nucleolus. The body-substance is prolonged

into five dendrites and an axon. Neuro-fibrillæ

are seen in dendrites and axon. They traverse the

body of the cell in all directions, in little

bundles which are separated by angular granules of

stainable substance (tigroids).]

The wasting of tigroids during functional activity proves clearly that

nerve-cells do work, in the physical sense. Energy is expended in

transmitting messages from receptor to effector, from sensory cell to

muscles, from recipient nerve-ending to glands. Have nerve-cells any

privileges or duties? Their functions, so far as we have considered

them hitherto, are automatic, from a mechanician’s point of view.

Their situation and connections determine the direction in which they

conduct, and the degree in which they reinforce stimuli impressed upon

the nervous system by the environment, including what may be termed

the internal environment, food in the alimentary canal, secretions

in ducts, and so forth. Have the cells any directive or executive

functions? There is no evidence that they have; nor, it must be added,

is there any line of reasoning which leads inevitably to the conclusion

that they have not. Remembering that, until recently, it was the

custom to solve all obscure problems and to shelve all difficulties

by conferring human attributes upon nerve-cells and collections of

nerve-cells, termed “centres,” a physiologist admits the negative with

reluctance. The unconscious argument in the past used to run somewhat

thus: “I decide to act or to abstain from action. The nerve-cell is

the mechanism by means of which I decide. Therefore the nerve-cell

decides.” (In the past a distinction was drawn between the cell-body

and its processes, but that, we now see, was absurd.) It is very

difficult to relinquish completely this attitude of mind. I feel, I

remember, I will. There must be a _something_ which feels, remembers,

wills. But a physiologist finds in the nervous system no evidence

of a capacity for any function other than that of conduction, with

adjustment of the force of current. He can no more discover feeling,

memory, or will in a chain of neurones than he can find music in a

violin. He hears the strings singing in the breeze. He can twang them

with an electric shock. But he has no vision of ghostly performers,

no glimpse of the conductor’s baton. Yet he knows, as every sane man

knows, that the neurones are the instruments played in the orchestra of

mind. He knows that, while all are sounding, some are muted, in order

that the others may produce a dominant effect. He knows, too, whenever

he decides to continue writing or to close his notebook, that the

conductor is raising the baton or allowing it to sink by his side.

A neurone or nerve-cell is a transmitting link. It is scarce a thing

to wonder at that physiologists, having wrestled successfully with the

superstition of the “pontifical nerve-cell,” are unwilling to reinstate

it even as doorkeeper in a free church. It may be that it exercises

some discretion in admitting impulses, but until its authority as a

guardian of the path which stretches behind it has been established,

it is better to regard it merely as a door which swings open whenever

pressed with sufficient force.

Is it possible to classify neurones according to their function?

They can be classified according to size, and, with some degree of

completeness, according to form. But if, as we believe to be the

case, size and form are governed by purely physical requirements, the

divisions into which the cells fall have no physiological significance.

The motor cells of the spinal cord and axis of the brain are large

and irregular in shape. Their dimensions are clearly dependent upon

the size, thickness rather than length, of the nerve-fibres which are

drawn out from them. They discharge impulses to groups of voluntary

muscle-fibres at a considerable distance. Small cells could not do the

work. Precisely similar reasons can be given for the large size of the

cells of Purkinje in the cerebellum, which transmit the elaborated

product, as we may term it, of this organ to the great brain; and

for the dimensions of the large pyramids of the great brain, which

convey its decisions to the spinal cord. The small pyramids of the

cortex of the great brain distribute the first crude impressions of

sensations to neighbouring (association) areas of the cortex. A cell of

Purkinje (Fig. 23) has a more complicated, and at the same time a more

regular, form than any other nerve-cell. It resembles an exceedingly

richly branched espalier pear-tree, set at right angles to the narrow

convolutions of the cerebellum; a disposition easily accounted for,

when the structure of the cortex of this organ is considered. Its

outer layer in which the espalier processes ramify is traversed

longitudinally by an infinity of nerve-threads, the bifurcated axons of

granules. These granules are small neurones which take up impulses from

afferent (“mossy”) fibres, and distribute them to the dendrites of the

Purkinje cells—each collecting from a few fibrils only of the sensory

channels. (The word “sensory” is used to indicate that sense-organs

are their provenance, and not that their messages become sensations.)

The numerous spreading branches of a Purkinje cell, disposed in a

transverse plane, are obviously arranged to hold up and keep apart

these myriads of longitudinal threads. A cerebral pyramid is shaped

like a fir-tree. It is placed in a definitely stratified layer. By

its branches it collects impulses from the superficial strata, which

it transmits through its stem to the white matter beneath the cortex.

The various parts of the central nervous system have work of different

kinds to do, and we find interposed in the circuits which compose the

several parts cells of various types. We speak of the large cells as

“motor,” the granules as “sensory,” the small pyramids as “association”

cells—such terms indicating the positions which they occupy in the

arcs, but not defining their functions. Of specialization of function

the physiologist cannot obtain a hint. He cannot classify nerve-cells

in groups concerned in reflex action, in feeling, in remembering, in

willing, in thought. On the contrary, he can assert with confidence

that such distinctions are not to be drawn.

In various situations in the central nervous system a certain type

of cell is found for which, in the present state of knowledge, it

is impossible to account. We mention these cells lest it should be

inferred, from what has been said above, that all neurones can be

fitted into a simple scheme of conducting arcs. In the spinal ganglia

there are neurones whose axons divide to form “baskets” around other

ganglion-cells. In the cerebellum there are similar cells, the axons of

which divide into branches, which break up to encase Purkinje-cells.

Cells of the same kind are found in a few other situations. In some

cases the end-branches which enter into the formation of the baskets

are few in number, and thick and clumsy. They grasp the body of the

cell which they surround, with gouty fingers, as it were. In other

cases the basket is a tangle of fine threads. It is difficult to see

what rôle cells of this kind can play in conduction. From the olfactory

and optic centres nerve-fibres extend outwards to the olfactory bulb

and retina. Here again is an arrangement which does not fit in with any

scheme. We might multiply examples. But enough has been said, perhaps,

to convey the impression which we wish to leave, that, although

experiment abundantly proves that the nervous system consists of an

association of sensori-motor conducting arcs, and although anatomical

investigation demonstrates the existence of chains of neurones which

take part in the formation of such arcs, it is impossible to reduce

the system to schemata or to prepare diagrams in which all structural

elements are, even hypothetically, fitted into place.

It may be convenient at this point to call attention to the differences

which distinguish the =sympathetic system=—the ganglia and nerves

of the viscera and bloodvessels—from the system devoted to bringing

sense-organs into connection with the skeletal musculature which we

have chiefly considered hitherto. The fibres of the posterior root

of a spinal nerve which convey impulses from the skin and muscular

sense-organs, and the fibres of its anterior root which convey impulses

to skeletal muscles, have a similar diameter of about 15 µ. In addition

to these, the roots contain fibres which carry impulses from and take

them to the viscera. Those which bring impulses from the viscera vary

greatly in thickness, some being as large as the other sensory nerves

of the posterior root. The diameter of the fibres which go to the

viscera is not more than one-fifth as great as that of the other fibres

of an anterior root. Similar slender fibres are found in the vagus

nerve. If all organs are removed from an animal’s chest and abdomen, a

string of small pearl-like ganglia, united by a longitudinal cord, is

seen lying on either side of the bodies of the vertebræ, one ganglion

for each segment. This string of ganglia is termed the “sympathetic

chain” (_cf._ p. 243). The small medullated fibres of the anterior

spinal roots join these ganglia. Some of them arborize about their

cells; some pass by them to arborize in ganglia which lie farther

afield, on the course of the great bloodvessels and within the viscera.

The axons of neurones whose cell-bodies are within a ganglion break up

into bunches of non-medullated fibres. In this way the fibres of the

sympathetic system are increased in number. Each of its neurones is a

multiplying and distributing station. There is no evidence that it in

any way serves as a “centre,” takes part in reflex action, or otherwise

usurps the functions of the grey matter of the spinal cord. Nerve-cells

are thickly strewn between the mucous membrane and the muscular coat,

and again between the two layers of the muscular coat of the alimentary

canal. It is not so certain that this system has no “central”

functions. The remarkable degree in which the wall of the intestines

retains its capacity for co-ordinated movement, after all nerves which

reach it from the ganglia and through the vagus have been cut, suggests

that the plexus of nerves within it does act to some extent as a reflex

centre. If we leave the case of the intrinsic nervous system of the

alimentary canal open, awaiting further proof, there is no reason for

looking upon the sympathetic system as in any degree independent of

the spinal cord and brain. It does its work on a large scale, and its

work is of a low order. Nature does not need to connect up the viscera

and bloodvessels with the central nervous system by means of fibres

as thick as those used for skeletal muscles. It is more convenient to

provide for the multiplication of the nerves—which must be extremely

numerous, owing to the relatively minute size of the muscle-fibres for

which they are destined—outside the central system than it would be to

include the necessary distributive cells within it. Again, we find that

a nerve-cell, when we see it at close quarters, shows no evidence of

administrative capacity. Although of a different shape, a ganglion-cell

of the sympathetic system is as large and as complex in form and

structure as a pyramidal cell of the cortex of the brain; yet the work

which it does is of a purely mechanical order. It receives, reinforces,

transmits impulses which reach it from the central nervous system.

The often-repeated statement that a nerve-fibre is a drawn out process

of a nerve-cell body has prepared the reader to anticipate that it

dies when cut off from its central connection. When the axon is dead,

the sheath which invests it rapidly loses its tubular character. If

the situation of the cell-bodies of a nerve be known, it can be at

once foretold on which side of the cut =degeneration= will occur.

Suppose that the median nerve has been severed at the wrist. All

nerve-fibres on the distal side of the wound must atrophy, whereas none

of the fibres on the proximal side will be affected. The motor fibres

have their cell-bodies in the spinal cord, the sensory in the spinal

ganglia. Degenerations following lesions in the central nervous system

have taught pathologists more about the course of the fibres in the

white matter than any other class of observations. Degeneration above

the lesion is spoken of as ascending, below as descending—not that it

progresses upwards or downwards. It occurs throughout all the stretch

of the fibre which has been isolated from its cell-body at the same

time, or nearly so. The thought that impulses can no longer ascend or

can no longer descend, as the case may be, has given sanction to the

expressions “ascending” and “descending” degeneration.

Restoration to functional activity of tracts of fibres which have

degenerated in the brain or spinal cord never occurs, but severed

peripheral nerves =regenerate=. Not that fibres join cut end to cut

end, however clean the wound. A wound in the wrist which has divided

the median nerve may heal in a few days “by first intention,” so far

as other tissues are concerned; but the patient does not for two or

three months recover the power of using the muscles of the hand which

the nerve supplied or the sense of touch in the area of skin to which

it was distributed. The ends of the axons on the proximal side of the

wound have to grow downwards to establish new connections in the

muscles and in the skin. The interval which elapses between the healing

of the wound in the wrist and the restoration of sensation and power of

movement is occupied in their downgrowth.

The re-connection of regenerated nerves with their terminal apparatus

presents to the mind a curious problem. There is no evidence that as

function is re-established the brain has to re-learn the situation

of the sensory spots on the skin, or to re-acquire skill in using

the muscles which again come under its control. From the moment that

the outgrowing nerves have recovered their terminal connections skin

and muscles have their right representation in the brain, however

much the two cut ends may have been twisted in their relation one to

another. It seems inconceivable that each nerve-fibre can find its

way to its original station; but if it does not, our conception of

the mode of working of the nervous system still needs much refining

from the telephone-exchange analogy by which we naturally help out our

explanations. If a telephone cable has been severed, it can be made

useful again only in one of two ways. Either the two segments of every

wire that has been cut must be reunited, or the subscribers’ numbers

must be redistributed.

The experiment of uniting the proximal segment of one nerve with the

distal segment of another of a quite different function gives results

which have an even more disconcerting effect upon our theory of the

nervous system. The sympathetic cord of the neck and the vagus nerve

lie very close together, alongside the carotid artery. The vagus is

both afferent and efferent. The sympathetic is wholly efferent—_i.e._,

it conducts impulses, which enter the sympathetic chain within the

thorax, in the direction of the head. If both nerves are cut, and

the end of the vagus turned round, so that it is in apposition with

the upper end of the sympathetic, its regenerating fibres make their

way along the sympathetic cord, headwards, to the superior cervical

ganglion. They arborize about the bodies of its ganglion-cells, just

as the sympathetic fibres used to do. The vagus is a nerve of many

functions. Amongst others, it inhibits the contraction of the heart,

constricts the bronchi of the lungs, dilates the bloodvessels of the

intestines, and helps in regulating the movements of these viscera.

After it has taken the place of the upper segment of the sympathetic

it dilates the pupil, constricts the bloodvessels of the ear, erects

the hairs of the head, as if to the manner born. To take another

example, in a monkey the two nerves supplying respectively certain

flexor and certain extensor muscles of the forearm were cut, and

their ends crossed, so that flexor nerve-fibres grew down to extensor

muscles, and extensor fibres to flexor muscles. There was no bungling

of reflex actions or of voluntary actions when the new roads were first

used. The monkey did not jerk its hand open when it tried to scratch or

to grasp a nut.