or narrow line; and the breadth of the lino represents extent to which adjacent areas overlap. Si then, are the peripheral expansions of those nerves wl are liable to be acted on by external forces. Hero e monopolizes a relatively-great tract of the surface, and 1 an extremely minute one. Each is an independent} ugei: each is capable of having a ehaugo set up in it will changes being set up in its neighbours. Tlio skin is, i were, occupied all over with separate feelers, tliat are ] STRUCTURE OF THE NERVOUS SYSTEM. 27 widely scattered, hero clustered, and here crowded together MS closely as maintenance of their iiidi.vidimli.tios will allow.
From the nerve-plexus occupying one of these areas., there takes its rise the central fibre, or axis-cylinder, of a nervetube. Coated with its medulla and inclosing sheath., it takes its way from the surface inwards, and, proceeding without any bnmch or junction, eventually reaches a mass of grey matter with imbedded vesicles — a nerve-centre or ganglion. Into the substance of tin's the essential nervethread runs, becoming divested of its medullary sheath; and where the structure is least involved, the essential ncrvc-iibro frequently if not always ends in a norve- vesicle. Jn such, simple,, and what wo may call, typical, centres, there branches out from somo other part of tho nervo-vesicle, another nerve-fibre which, similarly inclosed in its double sheath, pursues an outward course, ordinarily along* the same general, route as tho first, until, reaching the same part of the body, it buries itself in. a bundle of muscular fibres amid which its ramifications cud. Minis wo have as ' the elements of what is cn.ll.ed a nervous arc- — 1, a peripheral expansion, placed where it is liable to bo disturbed by an external agent, and so formed as to be most easily disturbed; 2, a connected fibre capable of being readily affected by disturbances at this outer end, but shielded from I disturbances elsewhere; o, at, or near, the inner end of this j fibre, a corpuscle of unstably-arranged substance, apfc to i give out much molecular motion, when disturbed; 4, a second, libro diverging from the corpuscle, or its neighbourhood, and subject to disturbance from the molecular motion disengaged near its origin, but protected from other influences; 5, at the remote extremity of this second fibre, a subdivided termination amid a substance- that contracts greatly when disturbed, and which, in contracting, moves the part of the body in which tho first libro took its rise. Fig, 1 is a diagram representing these elements of a nervous arc: A being tho first, or, as it is called, afferent nerve, with its peripheral expansion a.; ~P> l>oing the norvo-eor~ pusclc or ganglion-cell;;uul 0 the) second, or oilbreiiD, iierve, with its termination c.
This arrangement of parts is perpetually repealed throughout the nervous system; and if we genondiy,e the eoneept-ion somewhat hy supposing that. the commencement ft, in nob necessarily external, but "may bo on an inner surface, or 'within an. organ, while the termination r is not necessarily" in a muscle but may be in a ghuul; wo shall have a coneop-•tion that is, in. a ci>rtain sonso, univorsnlly applicn,bh\.[ say in a certain sense, because, until nnoihor clement \3 added, the conception is iucomplt^tc*. Those coupled nerves, with the ganglion-coll acting as a direct:, or indirect; link between thorn, recurring* everywhere in snbsi.anl-in.lly the*. same relations, appear to form a compound structure out of which the nervous system is built...its unit of eomposi- "tion. But this is not so. By multiplication of such arcs •\vo may got a nmltitude of separate nervous agencies;., •but nob a nervous system. To produce a nervous system there needs an clement connecting on.eh such nervous arc with, the rest — there needs a third (ibre running from the ganglion-cell, or its neighbourhood, to some place \vhoro other com.munioatiug 'fibres come; and where, by direct or indirect junctions, actual or approximate, tho primary couples of nerves may bo brought into relation. That in, liho.ro requires what wo may call a rriif-rijtcfd.! nerve. "i; In, * The words Mn-kri.pfifal and twilrifti-flnl an* O(*«'asi<HnUy u^ul in ucrvt!-pliysioloj^y UB tho etiuivalonts ofc" (Ulmmt atid ciTcnniL Bui", as nflVivuti JUK! t'llVrent arti by far t.li« inosl; ^cuc'rally adopted, ami urn ulH«> tht* most di»smj)« tivo, it HtM'ins to inu that tho word cGnlripptttl may with ndvnnia^ci hiivo thtii moi*o Kpecial uicanin^ tjivoix to it; and cent.rifiit/(U tho corivlutivo Hieaulng-.
TTV2 STRUCTURE 0V THE KERVOCS SYSTEM". 29 Fig. 2 is shown, diagrammatical! y, the relation in which (,1ns stands to the others. A centripetal nerve being added, there results what we may fairly regard as the unit of composition of the nervous system. We shall have presently to recognize certain fibres which this conception does not include. But they are not essential; for a nervous system is possible without them. Let us, then, taking tin's as our unit of composition; consider the general method after which a nervous system is constructed.
§11. The fibres represented in the above diagrams,, do not ordinarily pursue their respective courses by themselves: they proceed in company, as shown in Fig. 3. The afferent nerves arising at a, in separate but adjacent areas on the skin, or in other organs recipient of external impressions, converge; and, while maintaining their separate individualities, become united into a bundle inclosed in a sheath.
\ \ Other sheathed bundles of fibres from other clustered areas in the same region, presently join them, and run along with them in a compound bundle, until they eventually reach the mass oi: imbedded nerve-vesicles constituting a ganglion or nervous centre B. Similarly the olforont nerves which have their roots in this ganglion,, issue from it as a bundle, which, commonly inclosed in the same general sheath, as the afferent nerves, goes back to the part of the body whence these arose; and secondary bundles of these efferent nerves, diverging and ro- diverging from one another as they enter this part, as at c, finally become lost in its various muscles.
In like manner the centripetal fibres J, originating in ganglion, take tlicir common course, joined perhaps byoi fibres originating elsewhere, towards a ganglion E, tha; larger and lias more numerous connections. Of course clustered lines and spotted circles in Fig. 'j, aro enfi diagrammatic — give no idea of the separate nerves bundles and ganglia as they actually exist; but merely of relations in which they stand to one another..It should ' ' added that the more central ganglion, to which convc other bundles of centripetal IHTVOS (together with s< afferent nerves that pass through, inferior ganglia wit I stopping) may itself bo subordinate to a still superior still more central; ganglion. To this it gives off what i be called superior centripetal nerves; and other nerve the same or of a lower order being brought to it, this higl ganglion becomes a place where there are establis communications among all the subordinate and snb-subo nate o*ani>'lia, with, their afferent and efferent iibres.
One further kind of connection exists. The inime majority of animals, have their parts symmetrically arran — sometimes radially but more frequently bi-latenilly. the corresponding parts there aro habitually eorrespoml ganglia; and the connections that remain to bo named those between these corresponding ganglia,, or ga.nglia wl belong to the same grade. Such connections consist what are called, commissural fibres. Thoyn.ro indicated ji where they transversely join the structure shown in del with the answering structure belonging to the other sid< the body. The word commissural is, indeed,, sometii used in a wider sense: including fibres that unite gang!!; different grades. But since the great majority of the fil called commissural are those which, join duplicate ganglia else ganglia that occupy like relations in the hierarehj will, I think, conduce to clearness to restrict its applioai to these: leaving the word centripetal for iibres which ( nect ganglia of lower orders with those of higher ord STRUCTURE OP THE NERVOUS SYSTEM. 31 The commissures tlius bringing into relation tlie in embers of each pair of centres, inferior or superior, and so linking the two halves of the nervous system, complete the nervous communications throughout the organism..
This description, purposely generalized with a view of exhibiting the principles of nervous organization., apart from any particular type, may be fitly supplemented by the description of a special structure that illustrates them. Each sucker on any arm of a cuttle-fish, has a ganglion seated beneath it. To tin's descend the afferent nerves that arc affected by touching the sucker; and from it ascend the efferent nerves distributed to the muscular fibres of the sucker. These form a local nervous system, that is experimentally proved to have a certain completeness in itself.
But now from, the ganglion underneath each, sucker, iibres rnn along the arm., in company with fibres from all similar ganglia in the arm; and this bundle of centripetal iibres eventually roaches a ganglion, at the base of the arm. Each airn, similarly constructed, thus has a chief nervous centre in which the Iibres from, all its minor nervous centres arc brought into cominmiieation. Further, all round the ring formed by the united base of tho arms, there runs an. annular commissure connecting these superior ganglia. And then from each of them is given off a humlle of fibres that proceed eentripetally to a still higher centre — the cephalic ganglion; where, consequently,, nerves from all the arms tiro brought into direct communication with, one another, and also into communication with nerves arriving* from ganglia in other parts of tho body. Omitting details and qualifications, not essential to such a conception as concerns us here wo thus sec that in. nervous structure there is a centralization and re-contralitfation, that is carried far in proportion as tho organization is high.
§ 12. We may bo sure that along with a principle of arrangement among connecting structures, there goes somo principle of composition in the centres that arc cormoctecl which a-ro not simply places for the in outing1 of fibres,, l>u places in which there exist agents liable to Lo acted on 1>, the in-coining1 111.) res and capable of acting on. the otit-^oiiij librcs. Inspecting the principle of composition, our reason ings must be mainly hypothetical; but they will, I think prove of some worth, by leading us to conclusions that lull inonizo with observation, so far as this carries us.
in ascending from the lowest to the highest types of th nervous systcjm, we see that the distribution and combi nation, of nerve-fibres are so modified, as to make possible a increasing multiplicity, variety, and complexity of relation among different parts of the organism. What kind of me dification. does this necessitate at places where the nerve fibres are put in coninn.niicfiti.oii? With out ass run. ing* tlui two fibres wliich bring* two parts of the organism into rola lion, are always united at their central extremities by a intermediate nerve-corpuscle, it maybe safely assumed flu continuity between their contra! extremities must be of Tec to either by a nerve-corpuscle or by some less-defined portio of grey substance; and it is clear that in proportion to tb number of different connections to be established amon<L»* th nerves coining to any ganglion, must be the number of th more or loss independent portions of grey substance re quired to establish thorn. Let us consider the implication: Suppose that a, and /;, l^ig. 4, are two points in the 03 ganisni. To join the nerves proceeding from them, tlici needs only the single ganglion-cell A. Similarly, to "brin into nervous relation the points r, and ^, tho single gansjflior cell J> su diet's. So long as A and.B remain unconiJiu;toc th(;s(j two simple relations are the only possible ones ainon the points (t,, by c, <L But now assume that from A and th ere run fibres to the centre C — not a single libro froi each, but two fibres, one of which in each case proceeds froi tt< or />, and from c. or d. This being so, thoro may be forine ub U, eleven simple and compound relations: these l;oa TTTE STRUCTURE OF THE NERVOUS SYSTEM. tfjj points cm) be arranged into six groups of two, ci> 7>, a* '% fi d, b c, b d, c d; into four groups of three, /> a c, b a d, a c d, c b d; and into one group of four, a b c d. Hence, supposing the centre C to be made up of the independent cells, or portions of grey substance, severally serving to link the members of a group into a separate combination, thero must be at least eleven such. If, again, from this centre C, we assume that there ^un adequately numerous fibres to the higher centre F, and that this is also duly connected through the centres D and B, with, the points e, /, f/, h; then the possible number of groups, simple and compound, that may be formed at E, will amount to 247; and to unite the members of each group so that it may be independent of the rest, there must be at least 247 connecting links at the centre F. Without pursuing the calculation, it will be manifest that as these points in the organism increase in iiuml)er, and as the clusters of thorn that are to be brought into relation become larger and more various, the central elements through which their relations arc established must grow multitudinous, A;n inadequate conception, however, is thus reached; for wo have considered only the requisites for forming among these points, the greatest mmiber of different groups, simple and compound; ignoring the different orders in which the v THE DATA OF PSYCHOLOGY.
members of cadi group may be combined. Two tilings can bo arranged in succession in only 2 different; ways; three tilings can be arranged in 6 different ways; four tilings in 24 ways; iivo things in 120 ways; six things in 720 ways; seven things in 5,0-10 ways; and so on in a progression increasing with enormous rapidity. Assuming, then, that at the centre F, certain points, a} b, c, d, c, are to bo combined, not in this succession only, but in all possible successions, thero will require 120 different links of connection for this ono group of iivo points only. These links, whether separate vesicles or less-differentiated portions of grey matter, must occupy a considerable space; and supposing they arc aggregated near those pre-existing cells or links which they have to re-combine in various orders, thero may result a protuberance from the centre.fc1, as shown at G. IF wo suppose that instead of a group of five, a group of six is to have its inombers thus variously combined; or if instead of ono group to bo so dealt with, there are many; this lateral outgrowth may become relatively very large*. And since its vesicles, or portions of grey matter, will be much more bulky than the fibres running from them to the members of groups which they combine', there may be expected to arise, a,s at II, a Literal centre attached to the original centre, V, by a pedicle of fibres.
Of course these diagrams and numbers are intended to convey nothing but a general idea of the principle of composition of nerve centres— -not to represent any actual composition. It would bo au absurd assumption that among* a number of points in the body, there have to bo formed as many unliko groups as axe theoretically possible; and it is not to bo supposed that the members of any group need over to bo combined in as many different orders as they might bo combined. But while, on the ono hand, the above description greatly over-states the accumulation of iiorveveHicles, or their equivalents, implied by such, correlations as aro actually required among a given number of points m the ThE STRUCTURE OF THE NERVOUS SYSTEM. 85 organism; it immensely under-statos the number of points to be so correlated, as well as the number, and variety, and complexity, of the groups into which they are to be combined. The places from which afferent nerves proceed, as well as the places to which efferent nerves proceed, are multitudinous. Very large groups of such places havo their members put in simultaneous communication. The- different groups so formed, are innumerable. And extremely varied relations of succession are established among members of the same group; as well as among' different groups. Hence we are safe in asserting that along with an increasing multiplicity and heterogeneity of nervous connections, there must go increasing massiveness of the nervous centres, or accumulations of vesicular matter.
One further corollary deserves noting. Each vesicle, or each portion of grey matter that establishes a continuity between the central termini of fibres, is not merely a connecting link: it is also a reservoir of molecular motion, which it gives out when disturbed. Hence, if the composition of nerve-centres is determined as above indicated, it follows that in proportion to the number, cxtcnsiveness, and complexity, of the relations, siinultaneous and successive, that are formed among different parts of the organ ism, will bo. the quantity of molecular motion which the nerve- centres arc capable of disengaging.
§ 13. As a datum for Psychology of Iho most general kind, the foregoing description of nervous structure might suffice. But having to deal eh icily with, that more special Psychology distinguished as human, it will be proper to add some account of the human nervous system. A low facts of moment respecting its poriphorul parts, may bo sot down before we study its contra!, parts.
At the surface of the body, where the extremities of nerve-fibres are so placed as to be most easily disturbed, wo generally find what may be called multipliers of disturhancos. Sundry appliances which appear to "have nothing1 in common, liavo the common function of concentrating, on the ends of nerves, the actions of external agents. That this is the effect produced by the lenses of the eyes, is a familiar %ct. It is a less familiar fact that certain otolites and minute rods or fibres, immersed in a liquid contained in the internal ear,, serve to transform the less sensible vibrations communicated to this liquid, into the more sensible vibrations of solid masses,, and to bring these directly to bear on the nerve-terminations. So, too, is it over the integument; or, at any rate, over the parts of it subject to many and varied contacts. Though men have not, like many inferior Mammals, the well- developed tactual multipliers called vibrisscfo (known in a cat as ""whiskers"), each of which is a leveir that intensifies a slight touch at the outer end into a stronghf pressure of the imbedded end upon an adjacent nerve-fibre; yet every one of the short hairs 023. a man's skin acts in th |3 same way. And then, in addition to these, there are, a ft places where the contacts with objects are perpetual, an d where hairs do not grow, certain multipliers below the sui face — small dense bodies named corpuseula tactiis, round eac" i of which a nerve-fibre ramifies, and each of which, whe moved by the touch of a foreign body, gives to its attachei 1: nerve-fibre a greater pressure than this would receive wen -3 the surrounding substance homogeneous: a fact which will be understood on remembering the effect of pressure on tib skin when some small hard body, as a thorn, is imbedde in it.
So much for the instruments that are external to th, peripheral expansions of the nerves, and serve to exag gerate the effects of incident forces. We may now corj template these peripheral expansions themselves, as bein adapted to receive these exaggerated incident force? In the first place, the ultimate nerve-fibrillae, ramifyin where they are most exposed to disturbances, consist nerve-protoplasm unprotected by medullary sheaths and no TE& STRUCTURE Off THE NERVOUS SYSTEM. 87 even covered, by membranous sheaths. In fact), they appear to consist of matter like that contained in nerve-vesicles, but without the fat-granules; and may be regarded as, like it, more unstable than the matter composing the central fibres of the fully-differentiated nerve- tubes. To tin's general character of the nerve-terminations, have to be added the more special characters of the terminations exposed to special forces. The delicate pale fibres which form a layer on the surface of the retina, are not directly atFeeted by the, rays of light concentrated upon them; but theso rays, passing through them, fall on a layer of closely packed, but quite separate, little bodies which are the true sensitive structuros; and then the minute nerve -fibrilke that run. from these to the stratum of retinal nerve-fibres, pass on their way into a layer of nerve-vesicles, with which we may presume they have connections. That is to say, this peripheral expansion of the nerve on which visual iinngos fall, contains numerous small portions of the highly-uust.abk' nerve-matter, ready to change, and ready to give; out molecular motion in changing. It is thus, too, wiili those terminal ramifications of the auditory nerve, on which sonorous vibrations are concentrated. And there is au analogous peculiarity in the immensely-expanded extremity of the olfactory nerve. Here, over a largo tract/ covered by mucous membrane, is a thick plexus of the grey unsheathed fibres; and among them arc distributed both nerve-vesicles and granular grey substance, such as that out 5 of which the vesicles arise in the nervous centres.
i The significance of these structural peculiarities wo shall see hereafter. For the present wo need only note the distrit bution of them. Over the skin, which is conversant with i forces of a relatively-considerable intensity— mechanical \ impacts, pressures, tensions, — we do not Hud that the nerve, terminations contain deposits of the peculiarly unstabl nerve-substance. But we find such deposits when* tin incident forces are extremely feeble, or fall on excessive!
.-mall areas., or both. The quantity of matter which, floating a? faint odour through the air, reaches the end of the olfactorv nerve, is infinitesimal. Such Inminiferous undulations as are allowed, during a momentary glance, to fall on one of the minute areas of the retina, are equivalent to a mechanical force inappreciable by our measures, if not inexpressible by our ilgures. Similarly with those atmospheric waves which, produced by the church-bell a mile awav, and weakening as they spread in all directions, are conveyed to the minute otolites and rods of the inner ear, to be by them impressed on the auditory nerves. And in these places it is that we find peripheral deposits of t secially-unstable nerve-substance.
§ 14. Arising from these variously-specialized periphery structures, the afferent nerves, collected into their bundles and compound bundles, run inwards to the spinal cord; out of which issue the corresponding bundles of efferent nerves. In one sense the spinal cord may be regarded as a continuous nervous centre; and. in another sense, as a series o. partially-independent nervous centres. Each pair of trunk nerves with its segment of the spinal cord, has a certaii decree of individuality: and those segments into whicl enter the pairs of massive nerves from the limbs, have. individualities considerably pronounced; since it is experi mentally proved that when severed from the rest they are not incapacitated. The tract of grey matter in the spinal: cord to which the afferent nerves of a limb come, and from which the efferent nerves issue, is practically the ganglionic centre of that limb, having very much of automatic independence; and being' joined by coniniissural fibres to s like centre belonging to the fellow limb, it forms with this an automatic pair. So that, remembering how the entire cranio-spinal axis is originally one and continuous, and that its anterior part lias been differentiated and developed into quite distinct centres, we may say that its posterior part, the THlT STRUCTURE OF THE XERVOrS SYSTEM. 80 spinal cord, lias also been so differentiated, though to a much, smaller extent-. To this conception two additions must be made. Beyond tlie internal tracts of grey or vesicular matter, and tlie bundles of nerve-fibres that enter into and issue from them laterally; and beyond the traiiverse coinmissural fibres which connect the corresponding lateral portions of grey matter or partially-differentiated pairs of nervous centres; there are longitudinal eommissural fibres, joining these successive pairs of nervous centres with one another, and serving to integrate the series of pairs in the same way that the members of each pair are integrated. And then, along with these fibres that unite nervous centres of the same order, there are what we found it desirable to distinguish as centripetal fibres, running from the relativelyinferior nervous centres to the relatively-superior ones • with centrifugal fibres running back.
. Of these relatively-superior nervous centres, we have first to notice the medulla oblong at a; including those parts of the pons Yarolii which are woven into it, and similarly arise out of the fourth ventricle. This is the enlarged r •?rmination of the spinal cord, lying within the skull. tistinguished as it is from lower parts of the spinal cord by 1s greater niassiveness, it is much more distinguished by the multiplicity and variety of its peripheral connections. *»rhile the successive segments of the spinal cord proper, -ave pairs of afferent and efferent nerves which are limited their distributions to particular regions of the body; and wXhiie even such an entire group of these segments as occupy the lumbar region, have relations only with the legs and the lower part of the body; the 'medulla ohlongata, by the intermediation of centripetal fibres, is brought into relation not only with the lower part of the body and its limbs, but with the upper part of the body and its limbs; and not only with these, but also with sundry of the parts which we know as the organs of the special senses j and not only with these., but also with, tie more important viscera. 'The auditory nerves and tlie nerves of taste go directly into it, and tlicaigli tlie optic nerves do not,, yet from the centres to which, they run there are fibres communicatino' with it; from its laterally-appended parts arise the nerves of the eye-muscles and the facial nerves; and the pneuniogastric nerves, given off from its posterior part, put it in communication with the larynx, the lungs, the heart, the liver, and the stomach. Eespecting its connections, direct and indirect, much remains to be ascertained; but what is known justifies the conclusion that the medulla ollongaf a," including tlie structures that are adnate, is a portion! of the originally-uniform cerebro-spinal axis, which has| been differentiated into a centre of a higher order than! those behind it, or those at the base of the mass in front! of it — higher in the sense that it has become that portion! of the axis in which centripetal fibres running from th|3 posterior ganglia, and from some, if not all, of the anterior ganglia, called by some sensory, are brought into relaticln with one another — a centre through which.these loelal centres are united into one system. I Passing over with a mere recognition the anterior ganglia just named, the exact relations of which are ill-understoopP but some of which comparative morphology proves to 1 portions of the front end of the cerebro-spinal axis th have become differentiated into ganglia of the first orde receiving those special external stimuli to which the froi end of the body is exposed; there remain only to be noticed the two great bi-lobed ganglia, which in Man form the chi<?f mass of the brain — the cerebellum and the cerebrum. Pin rsiologists and anatomists are agreed in regarding these a>s centres of a still higher order. Anatomical proof of their superiority, as being the seats of still higher centralizatioia, is very incomplete; for the difficulty of tracing the courses of all the nerve-fibres that enter into and issue from them^ has hitherto been insuperable. But their connections witty the subjacent minor centres and with the medulla oblongataj TriE STirCCTUHE OP THE NERVOUS SYSTEM. 41 arc such as to make it certain that through -lie intermediation of these, tlicv communicate with the whole peripheral from centres of both the iirst and second order.'-;, joined, possibly with some simply afferent fibres, are brought into various relations: relations, however, that most likelv differ in their natures from those established in inferior centres — diner, perhaps, as those supposed to be formed in the centre H, Fig. 4, ditfer from those formed in the centre F.