There arc special cases which illustrate this relation. T may name one — the case of the Porpoise. A. Porpoise's brain exceeds greatly in. size the brains of other Mammals that have bodies commensurate with its own, except. that of Man and, perhaps, that of the. (iorilla. Such n, structure in a creature, leading so simple a life, is a serious difficulty in the way of current interpretations; but. is quite* in harmony with the interpretation here given. Porpoises accompanying a steam -vessel, gambolling and making ex- cursions on either side without apparent, effort, prove, by keeping up so high a velocity through so dense a medium, that their motor energies arc enormous.
§ 4. A closer examination of the facts soon reveals the insufficiency of the foregoing generalisation. Deep as is the connection between nervous development' and locomotive activity, further comparisons show that, it is complicated with some other connection scarcely less radical..If, other things being equal, the quantity of motion generated varied, directly as the quantity of nerve-tissue, then, in creatures constitutionally alike or but little dissimilar, a. tolerably constant ratio would exist between the mass of the nervous system and, the mass of the body: supposing- the body, whether largo or small, to he carried from phice* to place with, equal velocity. The ratio is far from eoustn.nl, however.
A. horse gallops much faster than a man runs; and a horse in ordinary work daily moves his body through a space greater than that through which a man moves his body, or greater than that transposition of his body which a man's daily labour is equivalent to. Hence were there a simple relation between amount of nerve-tissue and amount of mo- tion evolved, ahorse, which weighs some seven times as much THE NERVOUS SYSTEM. 9 as a man, should have a nervous system at least, seven times as heavy. Instead of this it has a lighter nervous system. Its brain weighs but ono pound seven ounces; and were its spinal cord added, the total weight would probably not. exceed two pounds. But a man's brain and spinal cord, weigh, between three and four pounds. Thus tho horse's corohro-spinal axis is but one-tenth of what it should bo, were this relation the only one. Still dearer is the proof thai, there is some other relation.,, when, wo avoid modifying causes, by comparing animals of the sa.me genus, or species, but of different sixes. The varieties of dogs supply good illustra- tions. A ncwfoundla.nd. and a spaniel are alike in organisa- tion, food., temperature, respiration, &c.; and they are approximately alike- in. their powers of locomotion: the advantage being on the sides of the larger of the two. Wero genesis of motion measured by <pianti(y of nerve- tissue, a nowfoundland's cer^bro-spina! axis should, there- fore, exceed in si/,o that of a spaniel as much as a. newfound- land's body exceeds in. sr/e that of a spaniel..Hut it by no means does so. While considerably larger absolutely, it is much smaller relatively.
Consequently, wo must say that though the nervous system is the initiator of motion, and. though there is evidently some* relation between degree of nervous develop- ment and degree of motor energy] yet. this relation is involved with, a.nd obscured by, another..Let us ro-oxaiuiuo i the facts in search of it.
§ 5. In what other way than in relative feebleness, do tho motions of inferior creatures differ from those of superior creatures? They differ in rela.tivo simplicity. Animals that aro but little evolved perform actions which, besides; being slow, aro fbw in. kind, and severally uniform in coin- f position. Animals th«,1i aro much evolved, perform actions* which, besides being rapid, aro numerous in kind and sevo- rally involved in, composition. Tho movements in the one 10 THE DATA OP PSYCHOLOGY.
r case are small and homogeneous, and in tlio other case great and heterogeneous. Each sub-kingdom of animals exemplifies this second general relation, as much an it doo« the first.
Humble Mollusks, like the fixed Tnmeatn,, display scarcely any energies beyond those required to contract their bodies when disturbed and afterwards to unfold them. But, in the highly-organized Cuttle-fishes, besides tho rapid, quickly- varied, and well-adjusted movements exhibited in. the pur- snit and capture of prey, we have the IWUUTOUH and com- bined movements of the suck oral arms, it sod not; only for prehension but occasionally for travelling over solid surfaces.
The Annulosa, including with them, the Atninltritht, supply a like general contrast. Between tho uniform, littlo-variod motions of a Nomertino worm, and tho multiform, variously- combined motions of the Crab or the Spider, the difference is paralleled by the difference fa. nervous evolution. And a like structural contrast accompanies tho contrast between the few simple actions of the caterpillar and tho numerous complex actions of the butterfly, But that heterogeneity of movement incrrasoB along witlt relative size of the nervous system, is best shown by com- parisons among vertebrate animals. Progressing by alter- nate contractions of its lateral muselea, and opening itn jaws to take in food and water, the Pi»h addn fco thasa little else but those undulations of tho fins and tail that to balance and tarn it. A Reptile, using its limbs in tho water or on land or both, performs muscular actions considerably more varied and more combined j bat still, actions that urci directed to comparatively few ends. An ordinary Mammal exhibits in the chase and destruction of prey, in tho making of burrows, in the rearing of young, in tho laying up <if food, a greater variety of actions tint are severally moro compound. On arriving at the higher Mammals, ending with Man, we meet with motions that aro almost iti their kinds., that are severally composed of THE NERVOUS SYSTEM. 11 • 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 excep- tionally 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 in- crease 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, in- cluding 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 12 THE DATA OF PSYCHOLOGY.
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 pro- bably generates daily as much motion as a savage, or as a civilized labourer; and were it the sole function of nerve- tissue 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 sub- stantially 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 com- plexity 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 ap- proximate 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 differ- ence 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 explana- tion is that Birds have a higher blood-heat with its accom- panying 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 rela- tions 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 Evolu- tion, 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 per- ceptions, 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.
CHAPTER II.
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\ indi- cated 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 sub- kingdoms 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. (Sastoro- pods^ 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 bi- lobcd 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 indepen- dent 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" uu- likeness, 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 dis- fiirnilariti.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 con- sequent 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 com- ponents 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 cranio- spinal axis into cerebral ganglia; and the further one of like nature which results from the relatively - enor- mous 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 strur- tures as vesicular and Jibrons. Chemical analyses have not, at present thrown more than a flickering light on the consti- tution 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 com- ponents 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 ror- puseles 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 nerve- cells in different stages, there are seen differences in the colours of the granules, indicating a progressive meta- morphosis. To complete a general idea of the grey tissue, it must be added that the more developed of these nucle- ated 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 dis- tributed 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 con- tinuous, instead of having its continuity broken by fat granules. That this central thread of protein-substance is 20 THE DATA OF PSYCHOLOGY.
the essential nerve, to whirl i the sheath <>{' medullary matter with its suiTomidmg membranous sheath are hut acces- sories, there tiro several proofs. One is that in the lower animals, as well as in the embryos of I lie higher, no me- dullary 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 ter- minations of nerves, even in superior animals, the medullary sheath commonly, if not always, slops shor!; while tin* con- tral 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 nxis- eylinder 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 ap- pearing to be mainly that., in the nerve-vesicle, {-he proteinsubstance contains more water, is mingled with fat- granules, 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 dis- sociated and distributed through surrounding space, eif her apart or in new combinations; si, ml one in which the com- ponents, instead of being dissociated, are merely re-ar- ranged, 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 transforma- tion. 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, resump- tion 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 mole- cular 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 com- municates 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