its fitness for again propagating a wave of transformation.
Much as there is here of hypothesis, the indirect evidence 22 T1IK DATA OF PSYCHOLOGY.
makes it probable that if tin's is not the triio interpretation, the true interpretation is amilon'ous to it. That tho matter contained in the vesieles is the scat; of desirnet ivo moleenhu changes, with, accompanying disengagement, of motion, while tlic matter contained in the lubes is the seat oi cliangcs •vvliic.'.li, of wli.:vlcv(»r special nature, <lo not involve iiiucli destructive decomposition and disengagement oi ;m>tion, are beliefs for which wo lia.vo several warrants;— among others^ the following. The grey tissue contains fa,r more water than tho wliiic t -issue: the propor- tion of solids to waiter IIIMIIL^' a-boulr 12 per cent, in the1 <j»Toy tissue, while in tli(i whi((x tissiu^ it is sonio 2~» per centi. Now abundance of water (a.ciliiaies iuol(»cular change, n.nd ha-bitna-lly (^in.ra.cteri/ies parts in which Iho ra-ic* of mohuuilai cliange is higli..H.c»nco tho implica(i<>n is that, t.lu^ grc»y matter undergoes mota.worphosis \vilh much grealer rapidity than tho white. Stronger evidence is ajlbrded by tlu fact that the grey or vesicular substance ha,s a vasr-ularity imnionsoly excetMling tliat of (lu^ whili* or iil)rous snb- stance. On coni])a.ring l.-lie neii- works of blood vessels thai permeate the two, the difference is conspicuous; and it!.*• much greater than a,t first n-ppejirs. An estimate based or measurements, proves that a- given bulk of tho one contaim about five times as many capillaries as an equal bulk of the other.* Now since those minute ea.naJs that:, bring and take * The drawing cm which this ostimatu is bnwM'l, is rontjiin«»d in th« Mtmmt of Hnnum J/iNtn/iH/y, by A. Kullikc.r: translated and odilrd by <3cor^c. liusk F. K.S., and ThoniaH lluxloy, K. K.S. Tlw I'stinmtn is oasily iuud<». / nuiuhisr of («]iii-diHtant parallel linos hc.ing (innvn transversely through the. t\v\ net- works, the number of places ut which onn of these lines crosses blood-v<\sHel within a given length {say an inch) is counted, and the like being doim will an equnl length of each of thii other ]iurallel lines traversing the same m\l work, thtjre, is obtained, by taking an average, the ntunher of vessels usuulb met with iix n, s}»e«;ilied distunes. The likt^ pr<x:»'ss is then *<;onei through will lines of the name length traversing the other net-work. These averages d< not, however, truly express tho comparative, numbws of sur.h iuterst'ctions ii tho two net-works; since the mashes of tho one are, unlike those of the othe in shape. Ilcuce it is needful to draw an equal number of parallel lougitu TJJE STRUCTURE OF THE NERVOUS SYSTEM. 23 away materials,, must be numerous in proportion as com- position and decomposition are quick; we may infer a great difference between the rates of destructive change in the two tissues. Another contrast supports this conclusion no less strongly. The unstable granular protoplasm contained in the corpuscles, is shielded from adjacent dis- turbing forces by a membrane which, even where thickest, is so delicate that its existence can be demonstrated only by the help of re-agents; and which in many corpuscles cannot be made visible at all. Hence between the matter contained in these corpuscles, or vesicles, and the streams of blood that run among them so abundantly, are interposed little else than the delicate walls of the capillary blood-vessels; and thus the disturbing substances brought by each capillary, can pass with the least possible hindrance into the unstably- arranged contents of the neighbouring vesicles. Quite otherwise is it with the relations of the blood to the contents of nerve-tubes. The wall of each nerve-tube is thick enough to make it easily demonstrated* and between it and the central thread of essential matter, conies the coat of nerve-medulla. Through these barriers the disturbing agents, carried among the nerve-tubes by sparingly-distributed capillaries, cannot readily pass; and the essential nerve-thread is prevented from having molecular changes set up in it at places between its two extremes. This protection suffices so long as the disturbing agents remain normal in their amounts; but when they become excessive, as they do if the blood-vessels become congested, local changes in the nerve-threads are caused: whence one kind of neuralgia. It should be added that by dinal lines; and to repeat with them this process of averaging. By taking the means between the resulting numbers and the previous numbers, we get a cor- rect representation of the relative frequencies with which the vessels occur ir space of one dimension. To ascertain their relative frequencies in space of three dimensions, or in solid tissue, it is of course needful simply to cube the two numbers so arrived at.
24 THE DATA OF PSYCHOLOGY.
this sheathing of nerve-medulla, the essential nerve- threads., "besides being shielded against disturbances from neighbour- ing currents of blood, are shielded against disturbances from nerve-threads in the same bundle. Were "axis-cylinders *y lying in lateral contact not thus coated., a molecular change propagated through one would set up molecular changes in its neighbours; as, in fact, it does in an early stage of ataxy, characterized by loss of the medullary sheaths. Hence, too, the explanation of that normal absence of me- dullary sheaths which sundry nervous structures show us. For among the Invert ebrata, in which this normal absence) occurs, the fibres contained in the same bundle have nothing like those many and varied distinctions which they have in the higher animals: they have termini of which the structures and functions are much less differentiated. Similarly with those bundles of grey or non-moduli ate tl fibres, contained in the sympathetic system of vertebrate animals; for these bundles, serving to establish relations among the viscera, each of which is much less divided into parts that act independently, there needs no such perfect in- sulation of the nerve-fibres. And the like holds even in cer- tain portions of the peripheral cerebro-spinal system; as the olfactory expansion, which consists of an extensive plexus of non-medullatecl fibres, and which has the peculiarity that different parts of its area are not acted upon sepa- rately.
The evidences, direct and indirect, thus justify us in concluding that the nervous system consists of one kind of matter under different forms and conditions. * In the grey tissue this matter exists in masses containing cor- puscles, which are soft and have granules dispersed, through them, and which, besides being thus unstably composed, are placed so as to be liable to disturbance in the greatest possible degree. In the white tissue this matter is collected together in extremely slender threads, that are denser, that are uniform, in texture, and that are shielded iu an uuusual TEE STRUCTURE OF THE NERVOUS SYSTEI1. -x> manner from disturbing forces, except at tlicir two extre- mities. And the implication on/which, we liencefortli pro- ceed is, that the masses., unstably constituted and con- ditioned, are seats of destructive molecular changes, and disengagement of motion; while the stably constituted and conditioned threads, are the seats of molecular changes that are not destructive, and are probably isomeric.
§ 10. Nerve-tubes with their contained protein-threads, and nerve-cells with their contained and surrounding masses of changing protein-substance, are the lustologic elements of which the nervous system is built up; and we have now to ask in what way they arc put together. We will begin with the peripheral termina- tions of the nerve-tubes; or rather, with those of them which lie on the outer surface.
Suppose the skin, including those introverted portions of it which form the receptive areas of the special senses, to be marked all over in such a way as to form a net-work. Suppose the meshes of this net-work to vary extremely in their sizes; so that while in some places they are as large as those of a fishing-net, they are in other places not large enough to admit the point of a needle. Or, to speak specifically, suppose that on the middle of the back the meshes are some 2i inches in diameter, and that being equally large over the middles of the fore-arms, and the middles of the thighs, they diminish to 2 inches and loss over the neck and breast, to 1£ inches at the extremities of the legs, to 1£ inches on the backs of the handy, to less than an inch on the forehead, to less than half-an-iuch over the cheeks and over the palms of the hands, to a quarter of an inch and less over the lingers, to a twelfth of an -inch at the inner tips of the fingers, and at the tip of the tunguo to one twenty-fourth of au inch in diameter; and sup- pose, further, that over the back of that dermal sac which forms the eye, these meshes are so small that a microscope 20 TIl'E DATA OF PSYCHOLOGY.
is required to distinguish them. Having imagined sad net-work of which the meshes, irregularly polygonal their outlines., are thus wide over parts of the surface tl have but little variety of converse) with the external wor and become smaller in proportion as the surfaces ]u multiplied and variable contacts with things; we shall h; gained an approximate idea of the relations among 1 separate local areas in which there arise independent uerv To complete the conception, however, something else in be supposed. The large meshes we must represent marked out by very broad lines—say a quarter of an ii broad where the meshes are largest. We must imag them narrowing as the mesh.es become smaller; mi when wo come to the meshes over the surface of retina, the dividing lines have dwindled to the tin ness of a gossamer thread. And now let us conceive t within each of these areas, large or small, as it may happ there exists a plexus of fibres, formed of the essential ne substance, that are continuous with one another, but h no connection with the iibres occupying adjacent arc Not, indeed, that we must conceive any sharp limitatioi the space occupied by each plexus. We must assume t the line separating two areas, here very broad and lie re. v narrow, covers a space into which iibres from both theai run, without joining one another. Hence the area belo ing to each independent plexus, is the internal area of mesh, plus the space occupied by its circumscribing br 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 nerve- tube. 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 nerve- thread 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 peri- pheral 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 neigh- bourhood, 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 through- out the nervous system; and if we genondiy,e the eoneep- t-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 com- position 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 indivi- dualities, 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.
30 THE DATA OF PSYCHOLOGY.
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 ex- perimentally 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. an- nular 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 qualifica- tions, not essential to such a conception as concerns us here wo thus sec that in. nervous structure there is a centraliza- tion 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 ar- rangement among connecting structures, there goes somo 82 THE DATA OF PSYCHOLOGY.
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 inde- pendent of the rest, there must be at least 247 connecting links at the centre F. Without pursuing the calcu- lation, it will be manifest that as these points in the or- ganism 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 com- bined, not in this succession only, but in all possible suc- cessions, thero will require 120 different links of connec- tion 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 sup- posing 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