motions accurately adjusted in tlieir relative quantities and successions, and that are themselves compounded into courses of action directed to multiform objects. And with each such increment of complexity in the motor functions throughout the Vertcbrata, there goes an increment of nervous endowment.
This, then, is the secondary connection which traverses and complicates the primary connection. We saw that were there no other relation than that between quantity of nerve-tissue and quantity of motion generated, a Horse should have a fat larger nervous system than a Man, instead of having a smaller one. But finding that there is also a relation between quantity of nerve-tissue and complexity of motion, we are led to expect an exceptionally large nervous system in Man; and are enabled to understand why ho has a larger one than a Ho r so has. More obvious, because not involved with irrelevant differences, is the interpretation thus yielded of the general rule, already illustrated in the case of the Dogs, that in each natural group or order of Mammals, the nervous systems do not increase in the same ratio as the bodies. "We will glance at another illustration of this, supplied by the Primates: specially instructive because of the significant exception, it contains, and specially interesting because that exception, is furnished by mankind.
The small monkeys have relatively very large brains — larger relatively than the brains of their congeners, including even the highest. This connection, parallel to that presented in the spaniel and the newfoundland, lias a parallel.explanation. The movements of the little Capuchin monkey are approximately as varied and complex as those of tho great G-orilla; and hence, in so far as nervous evolution is related to heterogeneity of motion, the Capuchin should have a nervous system differing but little in size from that of the Gorilla. But since there is also a relation between quantity.of nerve and quantity of motion generated, the Gorilla's r r nervous system must be absolutely greater tliongli relatively smaller: which, we find it to be. Between the Gorilla and Man, however,, there exists a converse contrast. Heavier than a Man,, and moving about in the trees,, a Gorilla probably generates daily as much motion as a savage, or as a civilized labourer; and were it the sole function of nervetissue to originate motion,, should have at least as large a nervous system. But the nervous system of Man is twice as heavy. Here, therefore, all other relations being substantially the same, and the physiological processes being approximately alike in the two cases, the relative largeness of the human nervous system stands clearly related to the relatively-enormous complexity of human actions — a complexity shown partly in the more compound simultaneous movements, but mainly in the combination of successive movements,, simple and compound, directed to remote ends.
§ 6. This double relation must still be taken as approximate only. Seeing as we did at the outset that the genesis of motion depends on many physiological conditions, of which each is separately variable, it is manifest that the fundamental connections we have traced must have sundry minor irregularities. "Without treating of these in detail, it may be well to instance one — that due to difference of bodily temperature. Birds as a class are more active than Mammals as a class; and though many Mammals go through motions more heterogeneous than those of Birds, yet the inferior Mammals can scarcely be said to exceed Birds in the heterogeneity of their motions. Nevertheless, the nervous systems of Birds are relatively somewhat smaller than the nervous systems of Mammals. The explanation is that Birds have a higher blood-heat with its accompanying more active respiration — both implying a greater rate of molecular change. And a greater rate of molecular change enables a smaller nervous system to generate an * THE NERVOUS SYSTEM!. 13 amount of motion wliicli "would require a larger nervous system if the rate of molecular change were less.
A farther qualifying fact to be here named is that, all other things being equal, the power of a nervous system does not vary exactly as its mass. For reasons that will hereafter appear, its efficiency as a motor agent increases in a somewhat higher ratio than the quantity of matter it contains.
But after all modifying causes 'have been allowed for, there remain substantially intact the fundamental relations set forth — namely, that wherever much motion is evolved, a relatively-large nervous system exists; that wherever the motion evolved though not great in quantity is heterogeneous in kind, a relatively-large nervous system exists; and that wherever the evolved motion is both great in quantity and heterogeneous in kind, the largest nervous systems exist.
§ 7. It is with deliberate intention that I have set out '••• with this unfamiliar and, as many will think, somewhat strange presentation of the facts. My reasons for doing so are several.
One of them is that we arc here primarily concerned with ' psychological phenomena as phenomena of Evolution; and, under their objective aspect, these, reduced to their lowest terms, are ^ incidents in the continuous re-distribution of Matter and Motion. Hence the first question respecting! the nervous system as studied from our point of view is — what are the leading facts it presents as expressed in terms of Matter and Motion?
Another reason is that, apart from any doctrine of Evolution, true conclusions respecting psychical phenomena must f| be based on the facts exhibited throughout organic nature; | f and that "the above statement does literally nothing else]\ than express these facts — expresses, too, all that direct induction can tell us respecting their essential relations.
14 THE DATA OF PSYCHOLOGY. * The actions of all organic beings, including thowe of our own species, arc known to us only as motions. Shut ling I | out our inferential interpretations, the leaps and doublings i;| of the escaping prey in common wiib the variously-adapted and rapidly-changed actions of the pursuer, an\ fo our perceptions, nothing but movements combined in particular ways; and so too are the changes of expression, tones of voice,, and verbal articulations of our fellow-beings, on which we put such hidden implications. As, then, science requires /;| \l us to distinguish, the (acts as actually presen^-d from the suppositions we ordinarily join with them, it is needful to I;; exhibit, in all its nakedness, this primordial ivlaiion between \t | the external motions and their internal originator.
ju ' Yet a fnrtlier reason for seltint!; out thn;, i; ihaf wo so escape from pre-coneept ions. Those who brimr \\ if h them I,1 ';' to the 'investigation of psychical phenomena, t ho h\ poi hoses, >, that have descended to us from the pa>t, are aim«. i Mire fo f» '!'; be more or less biassed thereby. While iutondiuLV {n avoid i |; assumptions they are, in great danger of Imviu««: ihrir etuitj' fl elusions vitiated, if not by some aucieni- or me»li;ev;d i«ie;» H':|- under its overt.form, yet by corollaries from it- \\n\i ha\e V!| unobtrusively embodied themselves in uu-.u-'pecied po-- ',;,'; tulates. As we shall pn\sent-Iy si»e, evt»n ph\-.-aol.»..;i...is have ;ijt' j,l been in sonu^ cases thus misltMl.
M;,, llcnce, then, without at all trailing in qne, lion th<« (ruth of those other and quito dill'erent interpretation^ of fiorvoui phenomena that) are tacitly expressed in ordinary l,in<n,aMV, it is proper fur us here to ignore them. Brforo Nludvi'ii" ' the facts from, a psychological point of view, we have first to study them from a physiological point of view. Tim i)ri. mary truth disclosed by the farts us so studied, 14 tin* universality of this relation, between the degree of H»TVO»,S ^I11;^011 an.cl tllc quantity and heti'rogcnett y of f|a, pro. d5^. motion. Wo now pass to tJio «ucuadary trutiw similarly disclosed.
THE STRUCTURE OF THE NERVOUS SYSTEM.
§ 8. An outline- of nervous structure must precede a detailed account of it; and the (Assent. iai farts to \H\ indicated in an outline may bo brought most. clearly into view by comparing1 with 011.0 another tho nervous systems possessed by different types, and by different grades of tho same type. We will limit our comparisons to tho throe supt^'ior subkingdoms of animals.
A iniimto nodule with div'cM'giiig iilvroadrtco.nstit.utos tlio rudimentary nervous system, as existing in tins lowest Mol.lu.sk. In tlie Lainellibranelis s(H;(iral svieli ininnto nodulos, or ganglia, arc distribntod, usually in pairs, in different parts of the body; an<l boyond tho irt^e iibros which they severally give oif to lUMghbonring orga.ns, there are fibres by which they are eotmeeted tog<vUuvr. (Sastoropods^ considerably liighcr in orgjini/«ttion and activity, hn.vo nervous centres among* which a considiTablis hottu'ogonoity is produced by the greater si/,o of NO mo tban oi" others. And besides a local integration of paired ganglia into single bilobcd go.ngl.ia, there is an advance in g(»iu^ral integration, shown by a clustering of the more important ganglia about the head. The Cephalopoda and especially the tlibran.chia.to division of them, in which tho molluscous typo roaches its highest, show ns, carried still further, that integration of the nervous system duo to simple growth, joined with that 16 THE DATA. OF rSVCMOLOOY. « integration due to concentration and eoal^seonre nf independent centres; and they also show us the difFrrenf iai -ions involved by their changes of size, form, and distribution.
A delicate cord, running from cud (u end of the body, IHK! giving off lateral fibres in pairs, constitute.-, the nervou •, system in tlio lower Aiin.ul<wt>. \\ in'ii 1'nnn limblo; s Annelids wo pass to the Articulate types, cmnpn-'ed nf •;««»/-incnts bearing limbs, \ve find flu* nervous sysiom furiued <•{' a series of cenlres, each sending1 fiiH'os i«» tin* dill'm-ut <»r;.ran; of its own segmeni'., an<l all nf Ihcin united by a, ihirk cord of fibres wilJi a, fused clusirr (»f.Minilar rrnirr; in the head. In the hig'her Arf!i'ulttftt. t!nT«' i; an HUTC;!,rd relative size of tin; nervous ivnirrs as ctmiparrd \\iili their coimeeting" sfcruclvnn^s; a.u actual,Mp|»r««afh ni" tbo chii-l' nervous cent-res to ono au«»ilnM', bnilj lont-'iludiunilv and laterally; and a (inal coa.lesci'nrc oi'flirni, Thi •: iuti-jn'at i«>n disclosed. l>y comparisons ol' lower and bi^'lu-r t\|»i''» mav also be observed in j)rogr<iss during' the drvrl-iptncnf «>{' thr individual insect) or the individual. rrusla'vau. And:d«»utjr with advancing* g*rowt-h, consolidaliou, and rombinat inn of H,ervoiLS sinu* Lures, there may lu^ (raced an umva.,iii*j" uulikeness, both among tlie erniral ma; •*"-5 tlh'tn rlvr,--;, among their cunneci.ing <<ords, and anion;:; i hrir divt4r^;rui. fibres.
Such traits of evoluiion are exhibited nnd'-r annHh -r i'»rm in the vertobratio »s lib-kin g'dom. Jt.s l«»w«*st. kn<>\vn nn-mher. the AM>I 'thw.KUs, luis a. simple erani«)-.s|)inal a\i', i in* ant*-riur extremity of which is not made appreciably dillereut. frotn the rest by development of <list,ini'.|» cerebral "-an?!•!»;», and which gives oil' LUcraJ, nerves iha-t IIM\-I» but, minor disfiirnilariti.es. Tho cyelosi.onu^ Fishes, pus/cssed uf eet'et.rat ganglia that are tolerably manifest, lead u.-.j It? the ordinary fishes, in which these ganglia,, indiv idna My tuiu'h larger, form a cluster of ma,sses, or rudimentary bmin. Here, however, though in contact, they preMerve n serial arrangement: their aggregation in little muro tiiiJi STRUCTURE OF THE NERVOUS SYSTEM. 17 tliat of close linear succession. But in the highest fishes certain of them which have greatly increased,, overlap the others; and tend so to form a more compact,, as well as a larger, aggregate. Superior Reptiles and Birds display this relative increase of certain of the clustered ganglia, and consequent obscuration of the rest, in a greater degree. It is carried still further in the inferior Mammals. From tlieni upwards, the leading change of nervous structure is an augmentation of the two largest pairs of these aggregated nervous centres. In Man one pair has "become so enormous that the others are most of them hidden by it, and nearly merged in it. Along with this direct integration there goes on the indirect integration constituted by more intimate and multiplied connections. Tliese are both longitudinal and transverse. While in the Ampliioxus, the cranio-spinal axis contains but a small proportion of the nerve-fibres which, running longitudinally, serve to unite its different parts; in a superior vertebrate animal, such uniting nerve-fibres are among the chief components of the cranio-spinal axis. And, similarly, while the lateral halves of the cerebrum are but slightly connected, in Birds, and have connections that are relatively deficient in the inferior Mammals, they become, in the highest Mammals, joined together by a thick mass formed of innumerable fibres. Meanwhile there have been arising differentiations no less conspicuous. Beyond that general one due to development of the anterior end of the craniospinal axis into cerebral ganglia; and the further one of like nature which results from the relatively - enormous growth of some of these; other differentiations have been constituted by the local unlikenesses of structure simultaneously established. As they enlarge, the greater ganglia are rendered externally dissimilar from the rest by the formation of folds or convolutions; and their internal parts severally acquire distinctive characters. The same thing holds of the peripheral nervous system. Pairs of o 18 THE DATA OP PSYCHOLOGY, nerves that wero originally almost uniform, are rendered multiform by tlio much greater growth of some than ot others, and by the inner differences that accompany these outer di f it T cue es, This cursory survey of the nervous system under the various forms it presents throughout the animal kingdom, suffices to allow how its evolution conforms to the laws of ': evolution iu general. We are also shown by it what hew more immediately concerns us — that while the rudimentary nervous system, consisting of a few threads and imuuio centres, is very much scattered, its increase of relative si/o and increase of complexity, go hand, in hand with increased I concentration and increased multiplicity and variety of eonlujctions. Carrying with us this general conception, let us now study its structure more closely: considering, at first, not any particular forms of it but its universal form.
§ 9. The nervous system is composed of two tissues, which •both diller considerably from those composing the- rest, of (he organism. They are usually distinguished from o:no another by their colours as grey and white, and by their minuto strurtures as vesicular and Jibrons. Chemical analyses have not, at present thrown more than a flickering light on the constitution of nerve-matter in general, or on the constitution of one kind of nerve-matter as contrasted with, the other. All that can be asserted with safety is, that each kind coniains phosphatie fats and protein-substances^ but that (.hose components are both differently distributed and in diifert'tii. states in the two tissues. Lot us see what wo are fold about them by the microscope, aided by chemical ro-ngonls.
Whore their evolution can be traced, the vesicles or rorpuseles of the grey tissue appear to take their rise out; of a nitrogenous protoplasm, full of granules and containing nuclei. Hound these nuclei the protoplasm, aggregates into spheroidal masses, which, becoming severally inclosed in delicate membranes (in many cases inferred rather than seen) TKE STRUCTURE or THE NERVOUS SYSTEM. 19 are so made into nerve-cells. The protein-substance,, thus forming alike the chief contents of the nerve-cells and the chief part of their matrix,, is, though coagulated, soft. The granules imbedded in it, both within and without the cells, consist of fatty matter. And on comparing together nervecells in different stages, there are seen differences in the colours of the granules, indicating a progressive metamorphosis. To complete a general idea of the grey tissue, it must be added that the more developed of these nucleated cells, or nerve- corpuscles, give off processes, usually branched, that vary in number and degree of ramification; that among the corpuscles and their branches are distributed the terminations of nerve-fibres; and that while in some nervous centres it is common for these fibres to run directly into the cells or to be continuous with certain of the processes, in other nervous centres the connections between fibres and cells are rarely if ever direct, but where they exist, are made through the remote sub-divisions of branches given off by both.
When we pass to the white or fibrous tissue, we meet with matters that at first sight appear as distinct from the others in nature as in mode of arrangement. The fibres prove to be minute tubes. Within the extremely delicate membrane of which each tube is formed, there is a medullary substance or pulp, which is viscid like oil, has a pearly lustre, and consists of albuminous and fatty substances. But unlike as the contents of the nerve-tubes and the nerve-cells thus appear to be, a careful scrutiny discloses between them an essential kinship. For imbedded in the pulp which fills th<5 tube or sheath, there lies a delicate fibre, or " axis-cylinder/^ which is composed of a protein-substance. Though chemically similar to the protein-substance contained in the cells of the vesicles, this is physically different; since, besides being comparatively firm or solid, it is uniform and continuous, instead of having its continuity broken by fat granules. That this central thread of protein-substance is the essential nerve, to whirl i the sheath <>{' medullary matter with its suiTomidmg membranous sheath are hut accessories, there tiro several proofs. One is that in the lower animals, as well as in the embryos of I lie higher, no medullary sheaths exist: the nerve eonsisis of the axis-cylinder and its protecting' membrane, will unit, any pulp lying between them. Another proof is Urn! at tlie peripheral terminations of nerves, even in superior animals, the medullary sheath commonly, if not always, slops shor!; while tin* contral thread,, covered, by the outermost; membrane., eontinnes further, and ends in delicate ranti lien! ions not inclosed in distinguishable sheaths. And a further proof is that, where a nerve-fibre unites with a nerve-cell, the* medullary sheaih ceases before arriving1 at/ the 'place of union; while the nxiseylinder joins the content's of the cell, and its pro tooling membrane becomes continuous wit h the cell-wall, where (his exists. Hence concluding, as we are warranted in doing, that the axis-cylinder is its essential part, we see that, the matter of nerve-fibre has much in common with the matter of nerve-vesicle: the differences between them appearing to be mainly that., in the nerve-vesicle, {-he proteinsubstance contains more water, is mingled with fatgranules, and forms part of an obviously unstable; mass; whereas in the nerve-tube the protein-substance is denser, and is distinctly marked oil' from the fatty compounds thai, surround it: so presenting an arrangement, that is relatively stable.
What is the moaning of this diiferonco? Before seeking an answer wo ninst remember that, compound substances undergo two fundamentally different kinds of metamorphosis — one in which the components are some or all of them dissociated and distributed through surrounding space, eif her apart or in new combinations; si, ml one in which the components, instead of being dissociated, are merely re-arranged, so as to alter the perceptible proportion of the/ mass without destroying its physical continuity. The iirsc THE STRUCTURE OF THE NERVOUS SYSTEil. 21 we call decomposition; the second isomeric transformation. These forms of change are further distinguished in this, that the one is usually accompanied by a great dissipation of motion, whereas the motion given out or taken up along with the other is relatively insignificant. There is yet a third contrast. After decomposition the separated components cannot be readily made to resume their previous relations: often it is impossible to combine them again; and in most other cases it is difficult to do this. But in many instances of isomeric transformation, resumption of the original form may be produced by a very moderate change of conditions.
Now the two kinds of molecular change thus strongly contrasted, are the two kinds of molecular change which we have reason to suspect are undergone by the two forms of nervous matter. While the protein-substance mingled with fat-granules in the vesicles, is habitually decomposed; the protein-substance forming the axes of the nerve-fibres is habitually changed from one of its isomeric states to another. Such, at least, isjtlie assumption here made, in conformity with the conclusion drawn in the Principles of Biology (§ 302); where it was argued that the propagation of molecular disturbances from one place in an organism to another, tends so to modify the mingled colloidal substances as to produce, between the two places, a form of colloid that undergoes isomeric transformation when disturbed, and communicates the disturbance in undergoing the transformation; and where it was argued that this easily-transformable colloid, having had such a change set up at one end of it and passed on to the other, giving out in the process some molecular motion and consequently falling in temperature, immediately re-absorbs from the adjacent tissues permeated by blood, an amount of molecular motion equal to that which was lost: thereupon resuming its previous isomeric state, and 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 proportion 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 snbstance. 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 composition 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 disturbing 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 correct 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.
this sheathing of nerve-medulla, the essential nerve- threads., "besides being shielded against disturbances from neighbouring 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 medullary 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 insulation of the nerve-fibres. And the like holds even in certain 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 separately.
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 corpuscles, 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 extremities. And the implication on/which, we liencefortli proceed is, that the masses., unstably constituted and conditioned, 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 terminations 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 suppose, 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