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 com- position of nerve centres— -not to represent any actual com- position. 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 iiorve- veHicles, 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 com- bined. 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 multi- plicity and heterogeneity of nervous connections, there must go increasing massiveness of the nervous centres, or accu- mulations 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 con- necting 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 com- plexity, 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 distur- THE DATA OP PSYCHOLOGY.
hancos. 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 Mam- mals, 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, pass- ing 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 un- sheathed 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 ol- factorv nerve, is infinitesimal. Such Inminiferous undula- tions 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 in- expressible by our ilgures. Similarly with those atmo- spheric 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 con- tinuous 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 inde- pendence; 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 relatively- inferior 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 THE DATA OF PSYCHOLOGY.
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 communica- tino' 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 r- siologists 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 intermedia- tion 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.
Among the tacts of fundamental significance with which we are here concerned., one other may be named. This concerns the histological structures of nervous centres. In automatic ganglia, the direct union of nerve-fibres with nerve- ceils is habitual. Throughout the spinal cord the " axis-cylin- ders " may not unfrequently be traced running into the vesicles. But in the higher nerve-centres direct connections are much less readily made out; and it is question- able whether in the highest they occur at all. In the grey substance of the cerebrum, the delicate nerve-fibres which, divested of medullary sheaths, run among the im- bedded corpuscles, do not- directly unite with them; or if it is too much to say that there are no such unions, we may say that they are rare. Such communications as exist are apparently between the branched terminations of the fibres and the ramified processes of the corpuscles. Thus at the one extreme, simple, clear, and complete connections are the rule; and at the other extreme, involved, vague, and incomplete connections.
§ 15. Some account must be given of certain remaining nervous structures, with which Psychology is indirectly concerned. Thus far we have dealt only with the fibres and centres that stand passively and actively related to the ex- ternal world; but there are fibres and centres that stand related to those internal organs which make possible the continuance of relations to the external world.
4-2 THE DATA OF PSYCHOLOGY. « The first to claim attention arc the vase-motor nerves. Ban* dies of these issue from the spinal cord, and, joined by bundles of nerves arising from the sympathetic system, accompany the main arteries: dividing and subdividing wherever these do, so as to supply fibres to all their branches down to the most minute. The vase-motor nerves form, in fact, an additional series of efferent nerves. The nervous arc under its lowest form, consists of the afferent nerve with its peripheral ex- tremity exposed to external actions, the ganglion-corpuscle to which its central extremity runs, and the efferent nerve thence issuing to end in some muscle. But as we have seen, the unit of composition of the developed nervous system, includes a centripetal fibre, running from the first or subordinate centre to a higher centre; and here we have to add, as an habitual element of this unit of composition in its complete form, a vaso-motor fibre, r inning to that part of the body in which the two ends of the nervous arc lie, and bringing the blood-vessels of that part into relation, with the other parts of the apparatus.
The cerebro- spinal nervous system, besides having these direct communications with the muscular walls of the tubes which bring blood, both to itself and to the muscles it sends fibres to, is also put into relation with other parts on which it is equally, though less immediately, dependent — the viscera. These have, indeed, a nervous system of their own, possessing apparently a considerable degree of inde- pendence— the sympathetic system; and one all-important viscus, the heart, has a nervous system that is demonstrably independent. The morphological interpretation of the vis- ceral nervous system is not settled; but whether it has a separate origin, or belongs to the periphery of the cerebro- spinal system, the undoubted fact is that the cerabro-spinal system, through the nerves running from it into the trunks of the sympathetic, communicates with all these vital organs; and that even the heart, complete as is its local TH^, STRUCTURE OF THE NERVOUS SYSTEM. 4-3 nervous system, is, by tlie vagus or pneuinogastric nerve, integrated with, the cerebro-spinal system.
A more particular account of these and sundry structures of the same class is not necessary here. The general fact of < significance for us,, is, that the brain and spinal cord which through their afferent nerves are put in relation with the actions of the external world, and which through their efferent nerves are put in relation with the structures that react on the external world, are also put in relation with the organs immediately or remotely instrumental in supply- ing them with nutriment, and removing the effete matters resulting from their activities.
§ 16. In the foregoing description I have endeavoured to include all that Psychology needs. Many conspicuous traits of nervous structure which some will think ought to be set down, are really altogether irrelevant. That in the spinal cord the grey matter is placed internally, while in the cerebrum it forms an outside stratum, is a fact of moment in anatomy, but one v;hich throws no light on the science of mind. Knowledge of the truth that the posterior roots of the spinal nerves are afferent,, while the anterior are efferent, is all-important to the pathologist; but to the psychologist it is quite unimportant, since this arrangement might have been reversed without the principles of nervous structure being in the least changed • and it is with these principles only that the psychologist is concerned. The leading facts embodying these principles may be summed up thus: — The three great sub-kingdoms of animals in which the nervous system becomes considerably evolved, show us thai} along with the relatively -increased massiveness distinguish- ing the higher types of the nervous system, there goes that other kind of integration implied by increase of structural combination. There is multiplication and enlargement of the parts that unite local nervous centres with general nervous centres. Yery frequently there is an approach or 44 TITS DATA OF PSYCHOLOGY. * clustering of nervous centres that were previously far apart. And there is both a relative and an absolute increase in those centres which have the most multiplied relations with local centres, and through them with all parts of the body. j The nervous system is made up of threads inclosed in I sheaths, and corpuscles imbedded in protoplasm; of which ,; the threads, united into bundles, constitute almost the whole of the peripheral parts, while the corpuscles with their matrix •are found chiefly in the central parts. Having at its outer ex- tremity a plexus of highly-unstable matter, a nerve-thread, consisting as we conclude of less unstable matter but matter isomerically transformed with ease, runs inwards, surrounded by substances that shield it from lateral distur- bance. Eventually it reaches a mass of highly unstable matter,, so conditioned as to undergo decomposition with the greatest facility; and from the place where this lies there run other like fibres to other masses of unstable matter, of the same kind, or of a different kind, or both — here to a portion of substance that contracts when disturbed, and here to a superior centre containing more of the easily- de- composed nerve-substance. These threads, afferent, efferent, and centripetal, with their connecting corpuscle or portion of grey matter, we regard as forming the unit of composition of the nervous system.
Such units are variously grouped and combined. Each local ganglion is a place where many afferent and many efferent nerves are connected by many portions of the un- stable nerve-matter, capable of suddenly giving out much molecular motion. Each superior ganglion is a place where centripetal and centrifugal fibres from such local or inferior ganglia, are similarly connected by similar matter. And so with still higher ganglia in their relations to these. From which principle of combination it results that the possibili- ties of different compound relations increase as fast asm the centralization progresses.
We saw, however, that this establishment of more 7TT$ STRUCTURE OF THE NERVOUS SYSTEM. 45 numerous,, more involved., and more varied relations among the parts of the organism, implies not simply this grouping of fibres and this arrangement of centres; but also a mul- tiplication of the nerve-corpuscles, or portions of grey matter, occupying their centres. And we found it to follow that, where the compound relations formed are among many points, or where the points are to be combined in many orders, or both, great accumulations of grey matter are needed: an important corollary being that the quantity of this matter capable of giving out much motion, increases in proportion as the combinations formed become large and heterogeneous.
Passing to the special nervous structure related to that special Psychology of chief importance to us, we saw that the^jginal^cord is a series of partly dependent, partly! independent, double nerve-centres; each concerned with 4 particular portion of the trunk or a particular limb, to the skin, muscles, and vessels of which it sends nerves^ The enlarged cephalic extremity of the spinal cord, the ^nedulla gblpiiQata, is a centre connected by centripetal fibres with these partially-differentiated inferior centres | and receiving, as it also does, directly or indirectly, nerves from the special sense-organs, the medulla oblong at a is a centre where the local centres concerned with nearly all part| of the body, are brought into communication. We saw., lastly! that the two great bi-lobed masses overlying the medulla oblong ata and the sensory ganglia, with which they are in- timately connected, may be regarded as centres in which these compound connections are united into connections still more compound, still more various, and still more numerous.