One further fact which it remained for us to note, was that while the more important nervous structures are those which bring the parts that are acted upon by the outer world, into relation with the parts that react upon it, there are also nervous structures that bring all these into relation with the vital organs: so serving to unite the parts which expend, with the parts which accumulate and distribute.
CHAPTER THE FUNCTIONS OF TEE NERVOUS SYSTEAT.
§ 17. When, at the outset, we inquired wliat are the manifestations with which the nervous system, is associated, we necessarily, in drawing- a conclusion; asserted in general terms the part performed by the nervous system. And though in the chapter just ended the sole aim. has been to describe nerve-threads, nerve-cells, nerve-trunks, nerve- centres, and the ways in which they are put together; yet the ends subserved have unavoidably, from, time to time, come into view. Structure and function are in our thoughts so intimately related, that it is scarcely possible to give a rational account of the one without some tacit reference to the other. Here, however, function is to be our special topic. Having seen how the nervous system is constructed, we have now to see how it works.
The proposition with which the first chapter ended was that nervous evolution varies partly as the quantity of motion generated in the organism, and partly as the com- plexity of this motion. Here the initial inquiry must be, how the nervous system serves as at once the agent by which motions are liberated and the agent by which motions are co-ordinated. Three things have to be explained: — 1. What ? are the causes "which on appropriate occasions determine the nervous system to set up motion? 2. By what process T-TTE Fl'XCTIOSS OF THE NEKVOTTS SYSTEM. 47 doe? it liberate the insensible motion locked up in certain tissues, and cause its transformation into sensible motion? 3. How does it adjust sensible motions into those com- binations, simultaneous and successive,, needful for efficient. action on the external world? These questions cover the whole of its functions; or, at any rate, all those of its functions with which we are directly concerned. We have to interpret its passive function as a receiver of disturbances that set it going; its active function as a liberator of motion; and its active function as a distributor or appcr- tioner of the motion liberated.
Probably it will be thought that there is here introduced a function distinct from those before named. It seems that the receiving of disturbances, or SLiinuli, can be included neither under the head of disengaging motions nor under the head of co-ordinating motions. But on reducing the facts to their lowest terms,, and to those terms which Physiology proper can alone recognize, the difficulty disappears. For all nervous stimuli are motions, molar or molecular; and the function of co-ordinating motions comprehends not simply the combining and apportioning of the motions ex- pended., but also the combining of the motions received, and the adjustment of the one set into harmony with the other. A moment's thought justifies this proposition. The stimuli to the nerves of touch are sensible motions of the imbedding tissue, caused either by the impacts of external. movinsr bodies or by motions of the organism which brin^ it against external bodies, fixed and moving. The auditory nerve receives the motions conveyed to it from masses of matter that are vibrating*. Those minute agents that termi- nate the nerves of the retina are acted on by luniiniferous undulations — motions of the ethereal medium which produce motions among their molecules. So, too, the nerves excited by sapid and odorous substances, are, in fact, excited by the molecular movements these substances cause in their extre- mities by chemically changing them. Thus, speaking not 48 THE DATA OF PSYCHOLOGY.
figuratively "but literally, an afferent fibre of whatever kind is a recipient of motion given to its molecules: either by molar motion, as when a blow is received; or by the motion of other molecules, as when there is contact with a chemically-active body; or by those ethereal molecular motions which constitute radiant heat and light.
It will be well to consider more fully this sub-division of nervous functions, and the reasons for here proceeding upon it.
§ IS. Phvsiplogy is an objective science; and is limited to such data as can be reached by observations made on sensible objects. It cannot, therefore, properly appropriate subjective data; or data wholly inaccessible to external observations. Without questioning the truth of the as- sumed correlation between the changes which, physically considered, are disturbances of nerves, and those which, psychically considered, are feelings; it may be safely affirmed that Physiology, which is an interpretation of the physical processes that go on in organisms, in terms known to physical science, ceases to be Physiology when it imports into its interpretations a psychical factor — a factor which no physical research whatever can disclose, or identify, or get the remotest glimpse of, The relations between nerve- actions and mental states form a distinct subject, to be • dealt with presently. Here we are treating of nerve-acN tipns.. on.their physiological side, and. must ignore their Doing this, we have no alternative but to formulate them in terms of motion. And having recognized the primary division to be that between the liberation of motions and the co-ordination of motions, we find that this last division must be sub -divided. It includes, first, the co-ordination of the motions received with one another; and, second, the co-ordination of the motions expended with the motions received, and with one another,, Hence results a generalized TII5 FACTIONS OF THE XERVOU3 SYSTEM. 49 idea of nervous functions, as divisible into reciplo-'niotor, Ubero-iiiotai\ and dirigo-mofor.
It must be admitted that in their higher forms, these functions are so entangled that a tripartite division of them is difficult, if not impossible. To the simplest types of nervous structure, the classification is easily applied: each afferent nerve is a reeipio-motor agent; each ganglion is a Ubero-iiiotor agent; each efferent nerve is a diriyo-moior' agent. But in complex nervous systems, formed of inferior and superior centres connected by parts containing nerves that are centripetal,, centrifugal,, and commissural, there arise corresponding secondary functions which greatly obscure the primary functions. It remains true that all the afferent } nerves are receivers of motions, and that all the efferent! nerves are directors of motions; and it remains true that} the vesicles and portions of grey substance throughout the! centres are liberators of motions • but of the fibres largely! composing these centres we must say that their functions are both receptive and directive. Nevertheless, we shall be considerably helped by thinking of the afferent nerves as recipio-motor and the efferent nerves as dirigo-motor: while we think of the nervous centres as composed ot libero-motor elements along with elements that perform both the other functions.
This general conception has now to be made specific. In dealing with functions we will follow the same order as we did in dealing with structures — we will consider first the offices of the different kinds of nervous matter.
§ 19. The grey substance and the white substance — or, to speak more strictly, the nitrogenous matter in and around the vesicles and the nitrogenous matter occupying the centres of the nerve tubes— have not absolutely distinct duties. Certain simple animals yield evidence that in the rudimentary nervous system, there is no such structural differentiation and consequently no such functional differ- B 5ft THE DATA 01? PSYCHOLOGY. - eniiationi and there is proof that even in the highest animals the differentiation is incomplete.
On the one hand the vesicular substance, having for its chief office to give out molecular motion when disturbed, has also a considerable power of conveying or conducting molecular motion. When the fibrous parts of the spinal cord have been cut, it is found that if the central columns of grey matter remain uncut,, or if there remains even a narrow link to maintain the continuity of the grey matter, disturb- ance is still communicated through it to the brain: not,, in- deed, disturbance of any special kind, but disturbance of the most general kind. True, it does not follow that such dis- turbance passes along the grey matter from end to end. Throughout the whole length of the spinal cord, nerve-fibres divested of their medullary sheaths enter into and afterwards issue from the grey matter; and, again protected by their sheaths, proceed upwards to the brain in the surrounding white matter. Very likely these take up and convey molecular disturbances set up in the grey matter imbedding them. But even this implies that disturbances are propagated to some extent through the grey matter; and the argument requires no more.
Conversely, it is found that the matter forming the " axis- cylinder/^ or essential nerve-thread, can do something more than transmit molecular motion. It has a certain power of simultaneously giving out molecular motion: so sharing the property of the vesicular matter. When a nerve is irritated not far above its termination in a muscle, the effect is but small. If the irritation is at a point further removed from, the muscle, the effect is greater. And the effect increases as the length of nerve through which the disturbance is conveyed increases. From this we must infer that besides the molecular motion received and transferred, there is molecular motion liberated in the nerve-fibre itself. Not that this molecular motion, like that which the vesicular matter yields up, implies an equivalent decomposition. Pro- Tllf FUNCTIONS OF THE NERVOUS SYSTEM. f»l bttbly it is a concomitant of the isorneric transformation propagated through a disturbed nerve, and serving to con- vey the disturbance. Some such accompanying result is to be inferred, a priori, if the conduction is effected by isomeric transformation, or by any kind of molecular re-arrangement. When the molecules of a mass change from one form of combination to another, either absorption or liberation of motion is sure to occur. That there cannot in this case be absorption of motion is manifest; since that would involve a proportionate resistance to the transfer — the amount of force or motion received by the extremity of the nerve, would quickly be used up in transforming the adjacent part of the nerve, and the change would travel but a little way. Being thus obliged to infer that motion is liberated, we at once see whence nerve-fibre derives the power to increase the disturbance it conveys • since each portion, while passing on the wave of molecular motion, adds the molecular motion given out during its own transformation. This action may be rudely symbolized by the transfer of sensible motion along a row of bricks on end, so placed that each in falling knocks over its neighbour. For if instead of bricks which stand on tolerably broad ends and require some force to overturn them, we suppose bricks that are delicately balanced on narrow ends; and if we further suppose them so constituted that they do not dissipate motion by per- cussion or friction; we shall see that the motion transmitted will accumulate. Each brick, besides the motion it re- ceives, will pass on to the next the motion which it has itself gained in falling.
The general truths to be carried with us are, that in its primordial undifferentiated state, nerve-matter unites the properties of giving out molecular motion and convey- ing molecular motion; but that with the advance of evolution, it becomes specialized into two kinds, of which the one, collected together in masses, lias mainly the function of giving out motion, though it can still to some extent con- is 2 52 THE DATA OF PSYCHOLOGY.
duct it, -while the other, collected together in threads, l:ns mainly the function of conducting motion,, though it can stiiJ to some extent give it out.
5 20. The co-operation of these differentiated kinds of nerve-substance, having differentiated functions, is seen in its simplest form where they are combined into what was Wore described as the unit of composition of the nervous system. An afferent nerve, changed by a touch at its outer end, and traversed by a wave of isomeric transformation that gathers strength as it goes, communicates this wave to the comparatively large mass of unstable matter connected with its inner end. The shock of molecular disturbance, im- mensely increased by the decomposition set up in this unstable matter constituting a ganglion-corpuscle or its matrix, diffuses itself around, but takes mainly the shape of a relatively-powerful wave of isomeric transformation along the efferent nerve. And the efferent nerve being distributed a tits other end among the fibres of a muscle, this powerful wave sets up in them an isomeric transformation of another kind, resulting in contraction (P-rinciples of Biology, § 303).
The belief that these are the offices of the respective parts, is borne cut by those peculiarities of structure which were described as occurring in the afferent fibres of certain special sense-organs. We saw that the outer ends of the optic nerve, the auditory nerve, and the olfactory nerve, are alike characterized by the presence of vesicular matter; and that while in this they differ from the outer ends of the nerves of touch, they also differ in being excessively sensitive. If grey matter, or the matter of vesicles, has the function of immensely multiplying any molecular motion it receives, and passing on the augmented wave of change along connected fibres, we at once have a satisfactory explanation of these peculiar peripheral structures. Take as an example the retina. One of the minute cones in its sensitive layer, measuring not ^th of an inch in diameter., has its com- TITS FUNCTIONS OP THE NERVOUS SYSTEM. 53 ponent matter changed by the etlierial vibrations emanating from a candle in a cottage-window at a great distance. The infinitesimal impact received from so faint a ray, may well be supposed insufficient to send through a considerable length of afferent nerve,, an adequately-rapid wave of molecular change; but this wave, after passing through an extremely delicate fibril less than ^ of an inch in length, conies to a layer of ganglion-corpuscles, with one of which we may presume that it unites, in this the minute disturbance sets up destructive molecular change — unlocks a considerable amount of molecular motion; and thus greatly augmented., the wave of transformation traverses the remainder of the afferent nerve without that loss of time that would result had it to gain strength by a series of increments, starting from an infinitesimal first term.
i±ow such appliances for multiplying action co-operate in these cases where the initial action is excessively minute, may be illustrated by certain artificial appli- ances that co-operate in an analogous manner. A man with a hair-trigger pistol in his hand, puts its muzzle to the end of a train that runs to a powder-magazine. The slightest pressure on the trigger liberates a spring, and this drives down the hammer. Here is something like the external multiplier which, as we have seen, habitually intensifies the action that falls on the end of an afferent nerve. The propelled hammer explodes the unstable detonating powder in the cap; thus playing a part com- parable to that of the concentrated pencil of light, which causes decomposition in one of the minute sensitive rods or cones of the retina. The explosion of the cap explodes the powder in the pistol: a change that may symbolize the setting up of decomposition in an adjacent ganglion-cell by a disturbed retinal element. The flash from the mouth of the pistol fires the train, which, carrying the flame onwards, blows np the magazine; and this serves to illustrate the action of the partially-decomposed ganglion-cell which pro- ^ THE DATA OF PSYCHOLOGY. * pagates a shock through the afferent nerve to a large de- posit of unstable matter in the optic centre, where an immense amount of molecular motion is thereupon disengaged.
The joint action of an afferent fibre, its centrally-seated ganglion- corpuscle, and the connected efferent fibre, is com- monly known as a reflex action. The name indicates the general truth that the disturbance in travelling from its place of origin to the place where its effect is seen, passes through a point at which its course is bent or reflected; and in so far as it describes this very general trait the term is a good one. But if the foregoing interpretation be correct, the term is in other respects objectionable. On the one hand, it implies as essential what is non-essential. That the wave of disturbance makes a sudden turn at one part of its course, is a fact of no intrinsic moment — is merely a con- comitant of the fact that the nerves it traverses have to be put in communication with other nerves, and that points of junction imply angles. On the other hand, it leaves out of sight the fact that one of these points of junction from which the wave of disturbance is said to be reflected, is a place at which it is greatly augmented; and that this augmentation of the wave is the all-important office of the matter lying at the point of junction.
§ 21. "Remembering that bundles of such afferent nerves are joined to bundles of such efferent nerves, by clusters of such corpuscles imbedded in the grey matter of a ganglion, and that bundles of centripetal nerves proceed thence to higher ganglia; we have next to consider the functions of these structures as wholes.
A nervous centre, even of an inferior order, is not simply a place where afferent nerves are severally linked -with their corresponding efferent nerves, by corpuscles or portions of grey matter that multiply and pass on disturbances; nor is the only further office it serves that of sending to higher ganglia, portions of these disturbances; but it is also a TITJ: FUNCTIONS OF THE NERVOUS SYSTEZI. 55 place where more involved communications are effected. For in all ganglia save, perhaps., the very simplest, the corpuscles or vesicles give off processes more or less numerous,, and usually more or less branched; and these "branched processes, spreading through the matrix of grey matter, may be assumed to propagate in various directions, and various degrees, the disturbance set up in the corpuscle. This diffusion of liberated molecular motion has two im- plications. First, the number and complexity of the cor- related changes produced by the original change, increase with the multiplication and variety of these processes and their connexions. And, second, along with increase in the number of correlated changes, there goes increase in the total quantity of molecular motion given out, directly or indirectly.
Fully to understand the importance of this last implica- tion, it is needful to refer back to Fig. 4, and to the accom- panying description of the way in which a nervous centre that serves to establish the various possible relations among different points in an organism, must contain a large accu- mulation of these connecting and multiplying links; and where it was shown how immense must become the accumu- lation of vesicular matter in a centre that has the office of establishing relations among these many parts in various orders. For it will be seen that as fast as the connexions become numerous and complex, so fast will enlarge the crowds of these connecting corpuscles and multipliers of disturbance which simultaneously come into action. And hence the quantity of molecular motion evolved in the nervous centres will become great in proportion as the nervous relations increase in integration and heterogeneity.
When we see how the arrangements for liberating and multiplying motion, described under their simple form in the last section, are thus compounded — when, recurring to our simile, we see how the first central magazine of force exploded, communicates with other larger magazines, and 55 THE DATA OF PSYCHOLOGY. r these again with still larger, which are subsequently ex- ploded; we shall be at no loss to understand how the slightest impression on one of the recipio -motor nerves, may evoke from the lilero-motor centres a relatively-in- commensurable amount of force,, which, discharged along the dingo-motor nerves, may generate violent muscular contractions. So that, to take a case, a slight sound may produce a convulsive start of the whole body; or an un- expected motion of some adjacent object, infinitesimal as is the modification it produces in the retina, may neverthe- less cause an involuntary jump and scream.
§ 22. In treating nervous functions in general, I have unawares ended with illustrations from the nervous func- tions of human beings: so coming to the division of the subject on which we have next to enter. For the brief account given in the last chapter of the special nervous structures with which we are most concerned, must here be supplemented by a brief account of their special functions.
If we leave out such afferent and efferent fibres as pass through the s^jnaJjOjQrd to and from the encephalon, and also those centripetal and centrifugal fibres which connect its various parts with the encephalon, we may regard the partly dependent and partly independent centres composing the spinal cord, as being co-ordinators of the actions performed by the skin and muscles of the trunk and limbs. A large proportion of these actions, including many of considerable complexity, the spinal cord is able to co-ordinate without aid from the higher centres; and some of the partially- differentiated centres composing the spinal cord, are able to effect simple co-ordinations without aid from the rest. We will glance at these simple co-ordinations first. If a patient paralyzed by some injury of the spinal cord that has left the lumbar enlargement intact, has his foot touched, the leg is quickly withdrawn; not only without a cerebral act, but even without his brain being in any way affected,, Tlffi FUXCTTONS OF THE NERVOUS SYSTEM. 57 unless indirectly by the shaking of the bed. Thus the branched corpuscles and fibres contained at that point in the cord with which the afferent and efferent nerves of the leg are connected, have at once the function of giving out, when the disturbance is communicated to them, the requisite quantity of molecular motion, and of so directing this to the respective muscles of the leg, as to cause the appropriate movement. More involved co-ordinations are effected by the co-operation of several such centres, or portions of the grey substance, contained in aljacentparts of the spinal cord. In the human subject demonstration of this is not easy; but it is shown by ex- periments on inferior Vertebrata. A decapitated frog that has its side irritated, will bring the hind foot of that side to the spot, and move it so as to displace the irritating object. Even something further is done. If a scalpel be applied to the skin between the hind legs, these act jointly in such a manner as to push away the scalpel. The explana- tion is that by commissnral fibres, transverse and longi- tudinal, the disturbances conveyed to particular centres, are communicated to sundry adjacent centres; and through their efferent nerves these direct and appor- tion the multiplied disturbances among a great variety of muscles. How such definite co-ordinations as these are effected by such an apparatus, we shall better understand on remembering that the relations between positions on the skin and the movements needed to bring the extremities to touch them, are tolerably constant. A frog's hind foot can reach a given point on the frog's side, only by one particular muscular adjustment; or, at any rate, by a muscular adjustment that varies within narrow limits And since in all frogs, generation after generation, the pro- portions of parts, and therefore the relations of muscular adjustments to given positions, remain practically the same; it becomes comprehensible how, through the organized nervous connections that arise, a touch at any point may 53 THE DATA OF PSYCHOLOGY. f cause tlie combined contractions needful to bring tlie end of tlie limb to that point. It should be observed here,, that the conception of these acts of the spinal cord as co-ordinations of motions, is incomplete so long as the only motions contemplated are those of the muscles. Under the head of motions must be included the disturbances con- veyed along the afferent nerves; for the muscular motions are so adjusted that their joint results have special rela- tions to these received disturbances. The co-ordination is between the recipio-motor acts and the dingo-motor acts. AVe may, then, regard the spinal cord as a centre of co-ordinations which, though some of them have considerable complexity, are yet relatively simple — simple, inasmuch as the disturbances received from the skin are much alike from all parts; simple, inasmuch as each muscular adjustment is mainly of a fixed or invariable kind; and simple, inasmuch as the component acts of the co- ordinated group are practically simultaneous.
That enlarged and differentiated part of the spinal cord called the medulla, oblong at a 9 including the root-portion of the pons Varolii, adnate with it and structurally so entangled that the two cannot be demarcated, we may roughly distinguish as a centre of compound co-ordination. It receives directly the auditory impressions, the impressions of taste, and, indirectly through the corpora quadrigemina, is affected by visual impressions: meanwhile sending impulses to the various muscles of the eyes, the face, the jaws, and the mouth. By it the movements of all four limbs are com- bined in joint acts; and by simultaneously regulating them, it makes the head and jaws co-operate with the limbs. The various impressions and muscular motions implied by the act of swallowing, it brings into due relation. Receiving the respiratory stimulus, it emits the stimuli to those muscles which enlarge and diminish the thoracic cavity, so causing inspiration and expiration; and, as a consequence, it is the centre which, disturbed by the more violent irritations TIIJ* FUNCTIONS OF THE NERVOUS SYSTE3T. f>9 of the respiratory surface, sends out to tlie respiratory muscles those more Violent impulses which, cause coughing and sneezing: to which may be added,, as actions belonging to tlie same class, crying and yawning. Lastly, through the pneumogastric nerve, it controls the action of the heart, and the actions of other viscera. Thus it is a centre to which come, in some cases directly but in most cases in- directly, impressions from all parts of the external surface, as well as from the mucous lining of the mouth, oesophagus, and lungs; and to which there also come, directly or indirectly, impressions received through the higher senses. At the same time the minor centres severally commanding groups of muscles, are by it put in relation with one another; and their respective simple actions so combined as to constitute compound actions. In short it lias redpio -motor relations with all the parts that hold con- verse with the external world, while it has dirigo-motor relations with all the parts that react on the external world; and its function is that of adjusting the complex movements in obedience to the complex stimuli. This is not all. Being the centre which initiates and directs involved and extensive