Among the tacts of fundamental significance with which we are here concerned., one other may be named. This concerns the histological structures of nervous centres. In automatic ganglia, the direct union of nerve-fibres with nerveceils is habitual. Throughout the spinal cord the " axis-cylinders " may not unfrequently be traced running into the vesicles. But in the higher nerve-centres direct connections are much less readily made out; and it is questionable whether in the highest they occur at all. In the grey substance of the cerebrum, the delicate nerve-fibres which, divested of medullary sheaths, run among the imbedded corpuscles, do not- directly unite with them; or if it is too much to say that there are no such unions, we may say that they are rare. Such communications as exist are apparently between the branched terminations of the fibres and the ramified processes of the corpuscles. Thus at the one extreme, simple, clear, and complete connections are the rule; and at the other extreme, involved, vague, and incomplete connections.
§ 15. Some account must be given of certain remaining nervous structures, with which Psychology is indirectly concerned. Thus far we have dealt only with the fibres and centres that stand passively and actively related to the external world; but there are fibres and centres that stand related to those internal organs which make possible the continuance of relations to the external world.
4-2 THE DATA OF PSYCHOLOGY. « The first to claim attention arc the vase-motor nerves. Ban* dies of these issue from the spinal cord, and, joined by bundles of nerves arising from the sympathetic system, accompany the main arteries: dividing and subdividing wherever these do, so as to supply fibres to all their branches down to the most minute. The vase-motor nerves form, in fact, an additional series of efferent nerves. The nervous arc under its lowest form, consists of the afferent nerve with its peripheral extremity exposed to external actions, the ganglion-corpuscle to which its central extremity runs, and the efferent nerve thence issuing to end in some muscle. But as we have seen, the unit of composition of the developed nervous system, includes a centripetal fibre, running from the first or subordinate centre to a higher centre; and here we have to add, as an habitual element of this unit of composition in its complete form, a vaso-motor fibre, r inning to that part of the body in which the two ends of the nervous arc lie, and bringing the blood-vessels of that part into relation, with the other parts of the apparatus.
The cerebro- spinal nervous system, besides having these direct communications with the muscular walls of the tubes which bring blood, both to itself and to the muscles it sends fibres to, is also put into relation with other parts on which it is equally, though less immediately, dependent — the viscera. These have, indeed, a nervous system of their own, possessing apparently a considerable degree of independence— the sympathetic system; and one all-important viscus, the heart, has a nervous system that is demonstrably independent. The morphological interpretation of the visceral nervous system is not settled; but whether it has a separate origin, or belongs to the periphery of the cerebrospinal system, the undoubted fact is that the cerabro-spinal system, through the nerves running from it into the trunks of the sympathetic, communicates with all these vital organs; and that even the heart, complete as is its local TH^, STRUCTURE OF THE NERVOUS SYSTEM. 4-3 nervous system, is, by tlie vagus or pneuinogastric nerve, integrated with, the cerebro-spinal system.
A more particular account of these and sundry structures of the same class is not necessary here. The general fact of < significance for us,, is, that the brain and spinal cord which through their afferent nerves are put in relation with the actions of the external world, and which through their efferent nerves are put in relation with the structures that react on the external world, are also put in relation with the organs immediately or remotely instrumental in supplying them with nutriment, and removing the effete matters resulting from their activities.
§ 16. In the foregoing description I have endeavoured to include all that Psychology needs. Many conspicuous traits of nervous structure which some will think ought to be set down, are really altogether irrelevant. That in the spinal cord the grey matter is placed internally, while in the cerebrum it forms an outside stratum, is a fact of moment in anatomy, but one v;hich throws no light on the science of mind. Knowledge of the truth that the posterior roots of the spinal nerves are afferent,, while the anterior are efferent, is all-important to the pathologist; but to the psychologist it is quite unimportant, since this arrangement might have been reversed without the principles of nervous structure being in the least changed • and it is with these principles only that the psychologist is concerned. The leading facts embodying these principles may be summed up thus: — The three great sub-kingdoms of animals in which the nervous system becomes considerably evolved, show us thai} along with the relatively -increased massiveness distinguishing the higher types of the nervous system, there goes that other kind of integration implied by increase of structural combination. There is multiplication and enlargement of the parts that unite local nervous centres with general nervous centres. Yery frequently there is an approach or 44 TITS DATA OF PSYCHOLOGY. * clustering of nervous centres that were previously far apart. And there is both a relative and an absolute increase in those centres which have the most multiplied relations with local centres, and through them with all parts of the body. j The nervous system is made up of threads inclosed in I sheaths, and corpuscles imbedded in protoplasm; of which,; the threads, united into bundles, constitute almost the whole of the peripheral parts, while the corpuscles with their matrix •are found chiefly in the central parts. Having at its outer extremity a plexus of highly-unstable matter, a nerve-thread, consisting as we conclude of less unstable matter but matter isomerically transformed with ease, runs inwards, surrounded by substances that shield it from lateral disturbance. Eventually it reaches a mass of highly unstable matter,, so conditioned as to undergo decomposition with the greatest facility; and from the place where this lies there run other like fibres to other masses of unstable matter, of the same kind, or of a different kind, or both — here to a portion of substance that contracts when disturbed, and here to a superior centre containing more of the easily- decomposed nerve-substance. These threads, afferent, efferent, and centripetal, with their connecting corpuscle or portion of grey matter, we regard as forming the unit of composition of the nervous system.
Such units are variously grouped and combined. Each local ganglion is a place where many afferent and many efferent nerves are connected by many portions of the unstable nerve-matter, capable of suddenly giving out much molecular motion. Each superior ganglion is a place where centripetal and centrifugal fibres from such local or inferior ganglia, are similarly connected by similar matter. And so with still higher ganglia in their relations to these. From which principle of combination it results that the possibilities of different compound relations increase as fast asm the centralization progresses.
We saw, however, that this establishment of more 7TT$ STRUCTURE OF THE NERVOUS SYSTEM. 45 numerous,, more involved., and more varied relations among the parts of the organism, implies not simply this grouping of fibres and this arrangement of centres; but also a multiplication of the nerve-corpuscles, or portions of grey matter, occupying their centres. And we found it to follow that, where the compound relations formed are among many points, or where the points are to be combined in many orders, or both, great accumulations of grey matter are needed: an important corollary being that the quantity of this matter capable of giving out much motion, increases in proportion as the combinations formed become large and heterogeneous.
Passing to the special nervous structure related to that special Psychology of chief importance to us, we saw that the^jginal^cord is a series of partly dependent, partly! independent, double nerve-centres; each concerned with 4 particular portion of the trunk or a particular limb, to the skin, muscles, and vessels of which it sends nerves^ The enlarged cephalic extremity of the spinal cord, the ^nedulla gblpiiQata, is a centre connected by centripetal fibres with these partially-differentiated inferior centres | and receiving, as it also does, directly or indirectly, nerves from the special sense-organs, the medulla oblong at a is a centre where the local centres concerned with nearly all part| of the body, are brought into communication. We saw., lastly! that the two great bi-lobed masses overlying the medulla oblong ata and the sensory ganglia, with which they are intimately connected, may be regarded as centres in which these compound connections are united into connections still more compound, still more various, and still more numerous.
One further fact which it remained for us to note, was that while the more important nervous structures are those which bring the parts that are acted upon by the outer world, into relation with the parts that react upon it, there are also nervous structures that bring all these into relation with the vital organs: so serving to unite the parts which expend, with the parts which accumulate and distribute.
CHAPTER CHAPTER THE FUNCTIONS OF TEE NERVOUS SYSTEAT.
§ 17. When, at the outset, we inquired wliat are the manifestations with which the nervous system, is associated, we necessarily, in drawing- a conclusion; asserted in general terms the part performed by the nervous system. And though in the chapter just ended the sole aim. has been to describe nerve-threads, nerve-cells, nerve-trunks, nervecentres, and the ways in which they are put together; yet the ends subserved have unavoidably, from, time to time, come into view. Structure and function are in our thoughts so intimately related, that it is scarcely possible to give a rational account of the one without some tacit reference to the other. Here, however, function is to be our special topic. Having seen how the nervous system is constructed, we have now to see how it works.
The proposition with which the first chapter ended was that nervous evolution varies partly as the quantity of motion generated in the organism, and partly as the complexity of this motion. Here the initial inquiry must be, how the nervous system serves as at once the agent by which motions are liberated and the agent by which motions are co-ordinated. Three things have to be explained: — 1. What? are the causes "which on appropriate occasions determine the nervous system to set up motion? 2. By what process T-TTE Fl'XCTIOSS OF THE NEKVOTTS SYSTEM. 47 doe? it liberate the insensible motion locked up in certain tissues, and cause its transformation into sensible motion? 3. How does it adjust sensible motions into those combinations, simultaneous and successive,, needful for efficient. action on the external world? These questions cover the whole of its functions; or, at any rate, all those of its functions with which we are directly concerned. We have to interpret its passive function as a receiver of disturbances that set it going; its active function as a liberator of motion; and its active function as a distributor or appcrtioner of the motion liberated.
Probably it will be thought that there is here introduced a function distinct from those before named. It seems that the receiving of disturbances, or SLiinuli, can be included neither under the head of disengaging motions nor under the head of co-ordinating motions. But on reducing the facts to their lowest terms,, and to those terms which Physiology proper can alone recognize, the difficulty disappears. For all nervous stimuli are motions, molar or molecular; and the function of co-ordinating motions comprehends not simply the combining and apportioning of the motions expended., but also the combining of the motions received, and the adjustment of the one set into harmony with the other. A moment's thought justifies this proposition. The stimuli to the nerves of touch are sensible motions of the imbedding tissue, caused either by the impacts of external. movinsr bodies or by motions of the organism which brin^ it against external bodies, fixed and moving. The auditory nerve receives the motions conveyed to it from masses of matter that are vibrating*. Those minute agents that terminate the nerves of the retina are acted on by luniiniferous undulations — motions of the ethereal medium which produce motions among their molecules. So, too, the nerves excited by sapid and odorous substances, are, in fact, excited by the molecular movements these substances cause in their extremities by chemically changing them. Thus, speaking not figuratively "but literally, an afferent fibre of whatever kind is a recipient of motion given to its molecules: either by molar motion, as when a blow is received; or by the motion of other molecules, as when there is contact with a chemically-active body; or by those ethereal molecular motions which constitute radiant heat and light.
It will be well to consider more fully this sub-division of nervous functions, and the reasons for here proceeding upon it.
§ IS. Phvsiplogy is an objective science; and is limited to such data as can be reached by observations made on sensible objects. It cannot, therefore, properly appropriate subjective data; or data wholly inaccessible to external observations. Without questioning the truth of the assumed correlation between the changes which, physically considered, are disturbances of nerves, and those which, psychically considered, are feelings; it may be safely affirmed that Physiology, which is an interpretation of the physical processes that go on in organisms, in terms known to physical science, ceases to be Physiology when it imports into its interpretations a psychical factor — a factor which no physical research whatever can disclose, or identify, or get the remotest glimpse of, The relations between nerveactions and mental states form a distinct subject, to be • dealt with presently. Here we are treating of nerve-acN tipns.. on.their physiological side, and. must ignore their Doing this, we have no alternative but to formulate them in terms of motion. And having recognized the primary division to be that between the liberation of motions and the co-ordination of motions, we find that this last division must be sub -divided. It includes, first, the co-ordination of the motions received with one another; and, second, the co-ordination of the motions expended with the motions received, and with one another,, Hence results a generalized TII5 FACTIONS OF THE XERVOU3 SYSTEM. 49 idea of nervous functions, as divisible into reciplo-'niotor, Ubero-iiiotai\ and dirigo-mofor.
It must be admitted that in their higher forms, these functions are so entangled that a tripartite division of them is difficult, if not impossible. To the simplest types of nervous structure, the classification is easily applied: each afferent nerve is a reeipio-motor agent; each ganglion is a Ubero-iiiotor agent; each efferent nerve is a diriyo-moior' agent. But in complex nervous systems, formed of inferior and superior centres connected by parts containing nerves that are centripetal,, centrifugal,, and commissural, there arise corresponding secondary functions which greatly obscure the primary functions. It remains true that all the afferent } nerves are receivers of motions, and that all the efferent! nerves are directors of motions; and it remains true that} the vesicles and portions of grey substance throughout the! centres are liberators of motions • but of the fibres largely! composing these centres we must say that their functions are both receptive and directive. Nevertheless, we shall be considerably helped by thinking of the afferent nerves as recipio-motor and the efferent nerves as dirigo-motor: while we think of the nervous centres as composed ot libero-motor elements along with elements that perform both the other functions.
This general conception has now to be made specific. In dealing with functions we will follow the same order as we did in dealing with structures — we will consider first the offices of the different kinds of nervous matter.
§ 19. The grey substance and the white substance — or, to speak more strictly, the nitrogenous matter in and around the vesicles and the nitrogenous matter occupying the centres of the nerve tubes— have not absolutely distinct duties. Certain simple animals yield evidence that in the rudimentary nervous system, there is no such structural differentiation and consequently no such functional differ- B 5ft THE DATA 01? PSYCHOLOGY. - eniiationi and there is proof that even in the highest animals the differentiation is incomplete.
On the one hand the vesicular substance, having for its chief office to give out molecular motion when disturbed, has also a considerable power of conveying or conducting molecular motion. When the fibrous parts of the spinal cord have been cut, it is found that if the central columns of grey matter remain uncut,, or if there remains even a narrow link to maintain the continuity of the grey matter, disturbance is still communicated through it to the brain: not,, indeed, disturbance of any special kind, but disturbance of the most general kind. True, it does not follow that such disturbance passes along the grey matter from end to end. Throughout the whole length of the spinal cord, nerve-fibres divested of their medullary sheaths enter into and afterwards issue from the grey matter; and, again protected by their sheaths, proceed upwards to the brain in the surrounding white matter. Very likely these take up and convey molecular disturbances set up in the grey matter imbedding them. But even this implies that disturbances are propagated to some extent through the grey matter; and the argument requires no more.
Conversely, it is found that the matter forming the " axiscylinder/^ or essential nerve-thread, can do something more than transmit molecular motion. It has a certain power of simultaneously giving out molecular motion: so sharing the property of the vesicular matter. When a nerve is irritated not far above its termination in a muscle, the effect is but small. If the irritation is at a point further removed from, the muscle, the effect is greater. And the effect increases as the length of nerve through which the disturbance is conveyed increases. From this we must infer that besides the molecular motion received and transferred, there is molecular motion liberated in the nerve-fibre itself. Not that this molecular motion, like that which the vesicular matter yields up, implies an equivalent decomposition. Pro- Tllf FUNCTIONS OF THE NERVOUS SYSTEM. f»l bttbly it is a concomitant of the isorneric transformation propagated through a disturbed nerve, and serving to convey the disturbance. Some such accompanying result is to be inferred, a priori, if the conduction is effected by isomeric transformation, or by any kind of molecular re-arrangement. When the molecules of a mass change from one form of combination to another, either absorption or liberation of motion is sure to occur. That there cannot in this case be absorption of motion is manifest; since that would involve a proportionate resistance to the transfer — the amount of force or motion received by the extremity of the nerve, would quickly be used up in transforming the adjacent part of the nerve, and the change would travel but a little way.
Being thus obliged to infer that motion is liberated, we at once see whence nerve-fibre derives the power to increase the disturbance it conveys • since each portion, while passing on the wave of molecular motion, adds the molecular motion given out during its own transformation. This action may be rudely symbolized by the transfer of sensible motion along a row of bricks on end, so placed that each in falling knocks over its neighbour. For if instead of bricks which stand on tolerably broad ends and require some force to overturn them, we suppose bricks that are delicately balanced on narrow ends; and if we further suppose them so constituted that they do not dissipate motion by percussion or friction; we shall see that the motion transmitted will accumulate. Each brick, besides the motion it receives, will pass on to the next the motion which it has itself gained in falling.
The general truths to be carried with us are, that in its primordial undifferentiated state, nerve-matter unites the properties of giving out molecular motion and conveying molecular motion; but that with the advance of evolution, it becomes specialized into two kinds, of which the one, collected together in masses, lias mainly the function of giving out motion, though it can still to some extent conis 2 duct it, -while the other, collected together in threads, l:ns mainly the function of conducting motion,, though it can stiiJ to some extent give it out.
5 20. The co-operation of these differentiated kinds of nerve-substance, having differentiated functions, is seen in its simplest form where they are combined into what was Wore described as the unit of composition of the nervous system. An afferent nerve, changed by a touch at its outer end, and traversed by a wave of isomeric transformation that gathers strength as it goes, communicates this wave to the comparatively large mass of unstable matter connected with its inner end. The shock of molecular disturbance, immensely increased by the decomposition set up in this unstable matter constituting a ganglion-corpuscle or its matrix, diffuses itself around, but takes mainly the shape of a relatively-powerful wave of isomeric transformation along the efferent nerve. And the efferent nerve being distributed a tits other end among the fibres of a muscle, this powerful wave sets up in them an isomeric transformation of another kind, resulting in contraction (P-rinciples of Biology, § 303).
The belief that these are the offices of the respective parts, is borne cut by those peculiarities of structure which were described as occurring in the afferent fibres of certain special sense-organs. We saw that the outer ends of the optic nerve, the auditory nerve, and the olfactory nerve, are alike characterized by the presence of vesicular matter; and that while in this they differ from the outer ends of the nerves of touch, they also differ in being excessively sensitive. If grey matter, or the matter of vesicles, has the function of immensely multiplying any molecular motion it receives, and passing on the augmented wave of change along connected fibres, we at once have a satisfactory explanation of these peculiar peripheral structures. Take as an example the retina. One of the minute cones in its sensitive layer, measuring not ^th of an inch in diameter., has its com- TITS FUNCTIONS OP THE NERVOUS SYSTEM. 53 ponent matter changed by the etlierial vibrations emanating from a candle in a cottage-window at a great distance. The infinitesimal impact received from so faint a ray, may well be supposed insufficient to send through a considerable length of afferent nerve,, an adequately-rapid wave of molecular change; but this wave, after passing through an extremely delicate fibril less than ^ of an inch in length, conies to a layer of ganglion-corpuscles, with one of which we may presume that it unites, in this the minute disturbance sets up destructive molecular change — unlocks a considerable amount of molecular motion; and thus greatly augmented., the wave of transformation traverses the remainder of the afferent nerve without that loss of time that would result had it to gain strength by a series of increments, starting from an infinitesimal first term.
i±ow such appliances for multiplying action co-operate in these cases where the initial action is excessively minute, may be illustrated by certain artificial appliances that co-operate in an analogous manner. A man with a hair-trigger pistol in his hand, puts its muzzle to the end of a train that runs to a powder-magazine. The slightest pressure on the trigger liberates a spring, and this drives down the hammer. Here is something like the external multiplier which, as we have seen, habitually intensifies the action that falls on the end of an afferent nerve. The propelled hammer explodes the unstable detonating powder in the cap; thus playing a part comparable to that of the concentrated pencil of light, which causes decomposition in one of the minute sensitive rods or cones of the retina. The explosion of the cap explodes the powder in the pistol: a change that may symbolize the setting up of decomposition in an adjacent ganglion-cell by a disturbed retinal element. The flash from the mouth of the pistol fires the train, which, carrying the flame onwards, blows np the magazine; and this serves to illustrate the action of the partially-decomposed ganglion-cell which pro- ^ THE DATA OF PSYCHOLOGY. * pagates a shock through the afferent nerve to a large deposit of unstable matter in the optic centre, where an immense amount of molecular motion is thereupon disengaged.
The joint action of an afferent fibre, its centrally-seated ganglion- corpuscle, and the connected efferent fibre, is commonly known as a reflex action. The name indicates the general truth that the disturbance in travelling from its place of origin to the place where its effect is seen, passes through a point at which its course is bent or reflected; and in so far as it describes this very general trait the term is a good one. But if the foregoing interpretation be correct, the term is in other respects objectionable. On the one hand, it implies as essential what is non-essential. That the wave of disturbance makes a sudden turn at one part of its course, is a fact of no intrinsic moment — is merely a concomitant of the fact that the nerves it traverses have to be put in communication with other nerves, and that points of junction imply angles. On the other hand, it leaves out of sight the fact that one of these points of junction from which the wave of disturbance is said to be reflected, is a place at which it is greatly augmented; and that this augmentation of the wave is the all-important office of the matter lying at the point of junction.
§ 21. "Remembering that bundles of such afferent nerves are joined to bundles of such efferent nerves, by clusters of such corpuscles imbedded in the grey matter of a ganglion, and that bundles of centripetal nerves proceed thence to higher ganglia; we have next to consider the functions of these structures as wholes.
A nervous centre, even of an inferior order, is not simply a place where afferent nerves are severally linked -with their corresponding efferent nerves, by corpuscles or portions of grey matter that multiply and pass on disturbances; nor is the only further office it serves that of sending to higher ganglia, portions of these disturbances; but it is also a TITJ: FUNCTIONS OF THE NERVOUS SYSTEZI. 55 place where more involved communications are effected. For in all ganglia save, perhaps., the very simplest, the corpuscles or vesicles give off processes more or less numerous,, and usually more or less branched; and these "branched processes, spreading through the matrix of grey matter, may be assumed to propagate in various directions, and various degrees, the disturbance set up in the corpuscle. This diffusion of liberated molecular motion has two implications. First, the number and complexity of the correlated changes produced by the original change, increase with the multiplication and variety of these processes and their connexions. And, second, along with increase in the number of correlated changes, there goes increase in the total quantity of molecular motion given out, directly or indirectly.
Fully to understand the importance of this last implication, it is needful to refer back to Fig. 4, and to the accompanying description of the way in which a nervous centre that serves to establish the various possible relations among different points in an organism, must contain a large accumulation of these connecting and multiplying links; and where it was shown how immense must become the accumulation of vesicular matter in a centre that has the office of establishing relations among these many parts in various orders. For it will be seen that as fast as the connexions become numerous and complex, so fast will enlarge the crowds of these connecting corpuscles and multipliers of disturbance which simultaneously come into action. And hence the quantity of molecular motion evolved in the nervous centres will become great in proportion as the nervous relations increase in integration and heterogeneity.
When we see how the arrangements for liberating and multiplying motion, described under their simple form in the last section, are thus compounded — when, recurring to our simile, we see how the first central magazine of force exploded, communicates with other larger magazines, and 55 THE DATA OF PSYCHOLOGY. r these again with still larger, which are subsequently exploded; we shall be at no loss to understand how the slightest impression on one of the recipio -motor nerves, may evoke from the lilero-motor centres a relatively-incommensurable amount of force,, which, discharged along the dingo-motor nerves, may generate violent muscular contractions. So that, to take a case, a slight sound may produce a convulsive start of the whole body; or an unexpected motion of some adjacent object, infinitesimal as is the modification it produces in the retina, may nevertheless cause an involuntary jump and scream.
§ 22. In treating nervous functions in general, I have unawares ended with illustrations from the nervous functions of human beings: so coming to the division of the subject on which we have next to enter. For the brief account given in the last chapter of the special nervous structures with which we are most concerned, must here be supplemented by a brief account of their special functions.
If we leave out such afferent and efferent fibres as pass through the s^jnaJjOjQrd to and from the encephalon, and also those centripetal and centrifugal fibres which connect its various parts with the encephalon, we may regard the partly dependent and partly independent centres composing the spinal cord, as being co-ordinators of the actions performed by the skin and muscles of the trunk and limbs. A large proportion of these actions, including many of considerable complexity, the spinal cord is able to co-ordinate without aid from the higher centres; and some of the partiallydifferentiated centres composing the spinal cord, are able to effect simple co-ordinations without aid from the rest. We will glance at these simple co-ordinations first. If a patient paralyzed by some injury of the spinal cord that has left the lumbar enlargement intact, has his foot touched, the leg is quickly withdrawn; not only without a cerebral act, but even without his brain being in any way affected,, Tlffi FUXCTTONS OF THE NERVOUS SYSTEM. 57