§ 254. Thus far the evolution of nervous structures, and of their functions, has been considered without reference tojhe varying physiological conditions that affect the process from moment to moment, as well as from generation to generation. We have spoken as though the physical actions ijy which nervous channels are opened and made more permeable, equally with those by which the discharges along established channels produce their respective effects, are actions always alike in kind and degree if the stimuli and the structures are alike in kind and degree. But this is not the case. Here we must take note of several circumstances, general and local, which modify the influence of the same outer agent on the same inner part; and observe the correspondence between the variations of physical effect and the aceompanying variations of psychical effect.
The better to follow in thought the production of theso variations, let us carry further a comparison before made. When describing how discharges of moleeular*motion go along lines of least resistance/ and by recurring render them lines of less resistance, it was pointed out that in this respect there is an analogy between the flow of molecular m»tion and the flow of a liquid; for a stream, in proportion as it is strong and continued, cuts for itself a large and definite channel ( § 224). That the transfer of molecular motion may be properly thus paralleled, we see in tlie case of those forms of molecular motion known as heat and electricity; the conduction of which through solid bodies, is compared to a flow that takes place more easily through some substances than through others, and which, in bodies having polarized molecules, passes with less resistance in some directions than in others.* Reverting, then, to the common hypothesis of a nervous ef fluid " which moves in nerve-" currents " — admitting that though the molecular motion which works nervous effects is not a fluid, and its transfer not a current, they may be conveniently dealt with as though they were; let us consider what variations of special results will arise from incidental variations in the genesis and escape of the nervous fluid. Let us consider the nervous system as an immensely involved set of channels, some wide and allowing an easy flow, some narrow and little permeable — some communicating with one another by large openings and others by openings through which nothing passes except under high pressure; but all of them more or less permeable and more or less connected. Let us suppose the aggregate of channels so constituted, to have multitudinous places through which its contents escape and multitudinous places through which there come gushes that increase its contents; and that these places of subtraction and addition are opened in various localities, in various numbers, and in various degrees— sometimes the subtractions being in excess and sometimes the additions. Let us hence infer a considerable variability of pressure in the fluid filling these ramifying channels — some excessive outflow having now greatly reduced its pressure, and the large inflows having now raised its pressure above the usual height. And lastly, let us draw the necessary corollary that at one time its ebbing streams pass only along the fully open and the more permeable channels; while at a time of high tide its streams find their * See Prof. Tyndall's Heat as a Mode of Motion, Chapter VII EVIDENCE FEOM FORMAL VARIATIONS. 587 ways into less permeable channels,, and, aided by local disturbances, escape even through the least permeable channels. Tims symbolizing the physical actions to which the nervous system as a whole is exposed, let us ask how its functions will be affected by changes of physiological conditions; and how the accompanying subjective states will be modified.
§ 255. Take first the general variations which are seen on contrasting certain mental traits of youth and age.
While waste and repair are rapid, the ramifying set of channels forming the nervous system, receives such a large and perpetual influx at multitudinous points, that it remains well filled notwithstanding the large efflux continuously going on at multitudinous points. The ingoing waves of molecular motion caused by peripheral impressions, liberate from moment to moment, in the sensory centres, larger waves,, or, as we may here call them, quick gushes of the ee nervous fluid;" and from the massive plexuses of the higher centres, aroused by combinations of disturbances thns arising, there are added to the contents of the.nervous system still more powerful and continuous gushes. « Considering first the physiological results, we see that the channels of tbe automatic nervous system are filled t© overflowing. The heart pulsates powerfully; the alimentary canal works vigorously; the lungs are well inflated; and every glandular organ receives that continuous discharge which keeps up the peculiar molecular changes carried on in it. Meanwhile the voluntary muscles, receiving their share of this abundant efflux, are all in states of partial tension, so producing attitudes characteristic of vigour; and they are severally ready to contract with great force, and to keep up their contractions for long periods. Among accompanying psychical results, we see that the feelings of both orders are vivid: the" sensations are distinct and the emotions lively. We see 58S PHYSICAL SYNTHESIS.
also — and tliis it is which more especially concerns us hero — that the establishment of relations between feelings is easy, and that the relations when established are relatively permanent. Along whatever lines of nervous- communication are opened, discharges pass that are strong because the pressure is great; whence results a great amount of molecular re-arrangement along each line taken by a discharge. A subsequent like discharge passes with comparative facility; making the antecedent state easily produce the consequent state — the terms of the relation are rendered coherent — the memory of it is good.
The converse connexion of phenomena in advanced life, brings out more clearly, by contrast, the law we are contemplating. In common with the body at large, the nervous system is supplied with poorer blood circulated more slowly, and hence when wasted it is less rapidly repaired. Its channels, therefore, receive from moment to moment feebler gushes of nervous fluid; the general pressure is diminished; and all the various overflowing gushes become less. The physiological results are that the actions of the viscera go on more slowly. Digestion if not positively difficult is a sensible tax; and the propulsion of blood to the surface is no longer active enough to contend with any great loss of heat. Throughout the muscular system, too, the failing nervous discharge is seen; alike in the chronic relaxation of attitude and in the quick following of fatigue upon exertion. Psychically, this state is one in which the feelings aroused are less vivid and the relations formed between them less coherent. For reading a strong light is required, taste and smell become less keen, hearing grows dull, and there is apathy in presence of circumstances which yield strong pleasurable emotions to the young. At the same time the comparative want 'of cohesion between impressions is shown in the inability to recollect the names of persons, the times of occurrences, &c. And if we trace out the successive stages of failing memory, we see EVIDENCE 1EOM NORMAL VARIATIONS. 589 fcliafc they follow the order inferable from the hypothesis. Earliest among the related impressions which no longer so cohere that one recalls another, are those made by daily trivialities — those represented in the nervous system by lines through which feeble discharges have but once passed, Interesting statements that are read, and passing events of considerable importance, presently cease to be recallable; though like statements and events which date back to early life are still recallable: the reason being that the channels of nervous communication long ago made by the strong gushes answering to the vivid feelings of youth, remain more permeable than those lately made by the feebler gushes answering to the fainter feelings of age. Passing over many gradations, we conie to incoherences of thought in which the place now inhabited is confounded with places inhabited long ago, and the business of middle life is referred to as though transacted yesterday — incoherences implying that comparatively permeable channels are now so far deserted that the discharges along them do not arouse the elements of these familiar ideas in their proper relations. And eventually we reach the extreme state, similarly explicable, in which even members of the family, who have been companions through life, cease to be recognized.
§ 256. Let us consider next certain general psychical variations that accompany differences of bodily constitution. Some of these, of the same natures as the foregoing, we will glance at before passing to some of another nature.
In jjonsequeiLce of specialities of inheritance, specialities of education, and specialities of mode of life,, higfL mental manifestations of certain kinds may go along with wejaJbajess of /body. But classing such cases as^abn(^rmaX^^iations from that constitutional balance which is needful for sifrvival through future generations; and limiting our attention to cases where no monstrosity has been produced by undue forcing of the individual or his ancestors; we shall, I think, trace a connexion between abounding physical vigour and power of thinking and feeling, as well as between sluggishness of constitution and comparative inertness, intellectual and emotional. On the one hand we have a type of man overflowing with muscular energy that gives superiority in sports, games, and feats of strength; who is keenly alive to all orders of gratifications, sensational and emotional; who acquires knowledge easily, and retains it tenaciously; and who, after leaving the academic life throughout which he was marked by these united traits, gains distinction partly because of his mental activity (not necessarily of a high order), and partly because of the strength of constitution which enables him to bear intense and prolonged application. On the other hand we have a type of man whose bodily functions are slow; who from boyhood upwards cares little for active exercises; who even in youth is indifferent to pleasures which others enjoy greatly; who all along finds learning laborious; and who, in after life, lapses into apathetic idleness.
These contrasts between men whose nervous systems work under high pressure and under low pressure respectively, I draw less for the purpose of showing their analogy to the contrasts between the young and the old, than for the purpose of drawing attention to accompanying contrasts of another kind. We have seen that when the pressure throughout the nervous system is high, so that any disturbance which facilitates escape along certain channels is followed by a strong gush along those channels, even though they are not very permeable; there is an easy revival of old connexions of ideas and a formation of new connexions that are very coherent. But there is much more than this. Interwoven as the higher nervous plexuses are in such intricate ways, in correspondence with the intricate interweaving of phenomena, it necessarily happens that a wave of nervous fluid let into one of EVIDENCE PROM NORMAL VARIATIONS. 591 them, though it escapes most largely along certain most permeable channels,, escapes in part along other channels that are less permeable. The stronger the wave the greater the number of these supplementary discharges; and the further do all the discharges,, larger and smaller, make themselves felt — rushing out into more multitudinous and remote ramifications of the plexuses they enter. Answering to this physical result the psychical result is the production of ideas that are more numerous, and more distinct, and more discursive. The area of consciousness simultaneously widens and brightens as the pressure of the nervous fluid increases; so that while its near and central elements become clearer,, elements more distant from the centre come into view. Between the two types of constitution above described,, we see such mental differences as are hence to be inferred. The man whose nervous system works under high pressure shows us an abundance of ideas. He has always something to say; and instantly finds words fit for the occasion. All the proximate bearings of a situation or an event quickly occur to him; and out of the several courses which almost simultaneously suggest themselves,, he takes the appropriate one. He thus shows what we call "presence of mind;" and habitually trusting1 with success to the fertility of his resources, he has courage in facing difficulties. In the man whose nervous system works at low pressure, thoughts come slowly in single file instead of rapidly in a column formed of many almost. abreast. The various possible causes and consequences of each act dawn upon him gradually one by one, and some of them not at all; so that the occasion has passed before he has had time to adjust himself to it. Finding *chat he is consequently unable to cope with men who have " their wits about them/' he leaves the crowded thoroughfares of life and takes to its quiet byeways. * § 257. That general physical causes entail such general psychical differences, we see not only on contrasting the minds of the young and old as well as those of the constitutionally vivacious and the constitutionally sluggish, but also on contrasting the exalted and depressed constitutional states of the same individual.
Most persons have had experience of a general prostration during which pleasures are accepted apathetically, while thinking is a fatigue and the effort to recollect unfamiliar things repugnant; and along with this diminished cohesion of ideas there is a diminished number of them — instead of coming in a continuous crowd they come as a train of stragglers. Conversely, there is an exceptional invigoration, often traceable to some favourable combination of conditions, physical and social (as an excursion along with intimate friends) in which the mental manifestations are unusually vivid and abundant. Every thought is clearly and quickly seized; apt expressions come to the mouth without hesitation; illustrations are ready on the instant; long-forgotten anecdotes recur; and out of the flood of ideas, now so broad and swift, there are readily formed those complex combinations of likeness and difference which constitute wit, even by those who ordinarily are not witty.
Clearly these opposite deviations from the mean state, are, like the others, interpretable as caused by relatively low pressure, and relatively high pressure, throughout the nervous system.
§ 258. One other variation of constitutional state, occurring daily, presents us with a series of similar effects similarly produced.
The diminution of nervous efflux which, reaching a certain point, shows itself in an increasing quietude, lapsing into sleep, is accompanied by a descending series of psychical activities conforming to the general principle set forth. When drowsiness begins, there is first a failure of the EVIDENCE FRCB1 NORMAL VARIATIONS.
feebler and more complex connexions of ideas, as well as a decreased quantity of ideas. Thought leaves its remoter and less-beaten tracks, and confines its excursions to the more familiar tracks — common-place remarks and allusions take the place of wit and speculation. Gradually becoming limited to a still narrower range, consciousness is by-and-bye made up of little else than those almost automatic interpretations of the impressions received from things around which constitute recognitions of them. And at length when the circulation has fallen to the requisite degree, and low tide of the nervous efflux has been reached, even the place and the persons are no longer known.
The dreams that occur during the ensuing sleep present traits of like meaning. For sleep-consciousness, while differing from waking consciousness mainly in being independent of, and uncorrected by, impressions received through the senses, differs also as the consciousness of the old does from that of the young, or that of the inert from that of the vivacious. Its elements are less coherent and less abundant. An ordinary dream is so faint that it is not recallable unless thought of just after waking; and then only a few of its closing scenes are recallable. Even these are not coherent over any considerable space; but through some accidental associations each new act or occurrence leads off into quite another series of acts and occurrences — there is a perpetual wandering away from what was just before thought or intended. Meanwhile, the narrowing of the area of consciousness is shown in the absence of those multitudinous collateral thoughts which the successive scenes are fitted to arouse, and in the consequent acceptance* of these scenes without any sense of their absurdity. To dream of flying and not to suspect any illusion, implies that thought is limited to a narrow train of simple ideas; and tha^ there aro not aroused any remembrances of those antagonistic experiences and of those general conceptions framed on them, which are implied by scepticism and disbelief.
A verification meets us when we compare tlie dr earns accompanying quiet circulation of the blood,, with the dreams accompanying excited circulation — either through the system as a whole or through the brain only. For under conditions implying a higher rate of molecular change,, and consequently of nervous discharge, than is usual during sleep, the dreams become both more vivid and more rational. Many acts are performed in succession with a view to some desired end; and the earlier members of the series do not wholly disappear from consciousness as the later arise. At the same time the things done, the means used, the difficulties overcome, are less incongruous with waking experience; because of the greater excursiveness of thought, and the consequent accompaniment of criticism on the main current of ideas.
§ 259. Another class of facts offer a kindred problem which admits of a kindred solution. I refer to the psychical variations that accompany variations not in the state of the organism as a whole but in the states of its different parts.
Given a nervous system in any constitutional condition, what will happen to the rest if one portion of it is greatly excited? Supposing large demands to be made on the general supply of nervous fluid by a powerful discharge in one direction, what will be the effects on discharges in other directions? The question is not by any means simple; for ordinarily a nervous action is accompanied by an invigorated pulse and a raised respiration, whence it results that being better supplied with materials, the nervous system generates more nervous fluid. Up to a certain point, therefore, the efflux in the performance of some one kind of function, has the effgct of increasing instead of diminishing the general efflux. This is especially the case with those modes of nervous expenditure which bring with them increased excitements of the sensations and emotions. Nevertheless, EVIDENCE FKQ1I NOEMAL VARIATIONS. 595 there is reason to suspect that some effects such as the hypothesis implies are produced.
When muscular effort is suddenly pushed: to excess, sayby running a long way at full speed or by climbing' a mountain till forced to desist by want of breath., the power of thinking is appreciably diminished. Though it remains easy to unite ideas in simple combinations, it becomes difficult to unite them in complex combinations — a metaphysical question demands a greater mental effort than can be made. The emotions undergo- a like enfeeblement — a temporary apathy ensues. That is to say, an excessive abstraction of nervous fluid diminishes so much the general pressure throughout the nervous system, that no discharges take place along the less permeable channels. It is true that the aeration of the blood falls in arrear, and that diminished genesis of nervous fluid thus becomes a partcause of these effects; but we shall find evidence that it is only a part-cause. For the alleged connexion of phenomena is quite clearly shown on passing to those nervou's discharges which have not increased excitements of feelings as their concomitants, Wheii the muscles and glands of the alimentary canal are at work", the heart and lungs have their actions raised; and the evolution of nervous energy is thereby favoured. But their activity brings no such increased evolution of nervous energy as does that of the locomotive organs; since their activity neither yields direct sensations, nor incidentally entails more vivid and varied perceptions and ideas, with the feelings immediate and remote which they imply.
Consequently, the abstraction of nervous fluid by the stomach when food has been put into it, is an almost uncompensated deduction from the general supply of nervous fluid. In youth the mental effect is not much feltf but in middle life and after, we see that the digestion of a heavy meal (at least in the absence of social excitements) entail*-such a diminution of pressure throughout the nervoiv f"f)g PHYSICAL SYNTHESIS.
svsvm, that only the simple and coherent relations of ideas are termed with facility. Processes.of thought which imply discharges through involved sets of channels that are not \vrv permeable, are performed with.difficulty. There is a disinclination to mental work as well as to bodily work; and not uncommonly, the overflow so far fails that even the simpler relations of ideas becoming faint and confused, there presently follows the unconsciousness of sleep.
§ 260. More special antagonisms, akin to these in their natures and effects, may be traced. A very strong emotion makes such a draught on the supply of nervous fluid as to incapacitate the intellect throughout much of its higher sphere. Conceptions that come in the lines of production and discharge of the emotion, may be formed with facility and vividness (though in some persons even these fall into confusion); but conceptions unconnected with the occasion, especially of kinds that are abstract or involved, become for the time impossible. There seems some reason to think that, conversely, great expenditure of energy in intense intellectual action is accompanied by a temporary diminution of emotional sensibility. It may be suspected, too, that long-continued intellectual absorption, of a kind which has little or no emotional excitement for its accompaniment, leads to a permanent enfeeblement of the emotions. Indeed, there is an antagonism of different faculties that appears to necessitate this— competing with one another as they do for supplies of energy and materials from the same general stock.
But the^nost interesting and instructive solution belono-- ing to this group, is that afforded of the aberrations which emotions produce among intellectual processes. When we rem*mVer that the plexuses co-operating in any involved act, are made up of multitudinous channels of various of permeability, we shall see that the mental act can be properly performed only when the discharges through EVIDENCE FROM NORMAL VARIATIONS. 597 the co-operating plexuses take place under the normal pressure. As before shown, the least permeable plexuses are the first to become functionally inactive as the pressure diminishes; and here it is to be observed that for the same reason-, the least permeable parts of each plexus will as the pressure diminishes have their discharges appreciably enfeebled before the more permeable parts. But right mental adjustments, implying accurate nervous co-ordinations,, depend on the maintenance of due proportions among the strengths of the discharges j and anything that alters these proportions interferes with the adjustments. Necessarily, then, a strong emotion disturbs the intellectual balance. Both derangements of simple perceptions and derangements of complex judgments show us this. Among derangements of perceptions, I may refer in passing to those which great fear produces— the misinterpretation of visual impressions being in this state of mind very marked. But examples that are better, because the effects are numerically measurable, occur among those who play games of skill. If when about to make a stroke at billiards arty emotion has been raised, by the presence of spectators or otherwise, failure is very apt to result; and this though the heart's action and the muscular tone are not appreciably affected. The cause is clear. Success presupposes great exactness in the ratios among the many combined contractions, and in the adaptation of them all to the many combined impressions; the ratios among which have also to be exactly appreciated. But when a great draught of nervous fluid to the parts of the nervous system occupied in an emotion, has diminished the pressure under which these sensory and motor discharges are made through the co-operating plexuses, the ratios among the actions of their parts are so far changed that the co-ordination becomes imperfect. That among those higher intellectual actions we class as judgments, a like disturbance leads to a like derangement, is obvious.
Take a case. To decide which of several results will most likely follow some step,, saj in a negociation, implies representations of them as caused by complex motives and circumstances. These several results rise in consciousness with different degrees of vividness and pertinacity; and to believe that one of them will qccur, is to feel that this one persists in consciousness more decidedly than the others — the greater persistence being determined by some preponderance of kindred experiences. But due proportion among the tendencies of these several representations to arise and continue, depends on the maintenance of the normal pressure of nervous fluid. This is interfered with both locally and generally by strong emotions. In the first place, the particular emotions excited in reference to the question at issue, perturb the judgment by increasing the discharge along those lines of representation that further their own excitement. In the second place, these particular emotions, or any other emotions, perturb the judgment by affecting the general supply of nervous fluid. Under the high tide which extreme elation implies, the nervous discharges pass easily along the less permeable channels, and the feebler representations are raised more nearly to a level with stronger ones, so that discrimination becomes less easy; whence it happens that improbable results of a desired kind are thought probable. While under a state of depressed spirits, judgment fails because the proportions among the nervous discharges are interfered with in an opposite way.
§ 261. Fully to explain these last derangements of judgment, however, we must take into account one further class of variations among the mental activities. In entering on this class, I find tlgte opportunity of redeeming a promise made in § 128; where, after propounding an hypothesis respecting the natures of pleasures and pains, it was hinted that some verification would be furnished at a later stage of the argument. We saw reason to think " that while Plea- EVIDENCE FROM NORMAL VASIATJONS.
sures and Pains are partly constituted of those local and conspicuous elements of feeling directly aroused by special stimuli, they are largely, if not mainly, composed of secondary elements of feeling aroused indirectly by diffused stimulation of the nervous system/' Here we have to consider what further reasons for thinking this are now apparent; and what further solutions they introduce us to.
That every special pleasure or pain, peripheral or central, does produce a diffused effect is clear. I do not mean simply that this is a corollary from the foregoing argument: I mean that it is shown experimentally. Beyond the familiar fact that each strong sensation or emotion affects the action of the heart, we have the fact that the accompanying gush of nervous fluid, spreading along all the vaso-motor nerves, changes the state of the arteries throughout the whole body. Much more then does it spread through those more directlyrelated parts of the nervous system which are seats of conscious actions. What remains here to inquire, then, is how far the diffusion is specialized-according to the nature of the feeling.
In tracing out the genesis of emotions, we hare seen that the plexuses which co-ordinate certain clustered impressions received from without, with the combined actions appropriate to them, are necessarily entangled with kindred plexuses that perform kindred co-ordinations. We have inferred that when a particular plexus is excited, it immediately excites the mass of kindred plexuses with which it is organized — the result being that the feelings proper to this mass of excited plexuses are aroused, and in their multitudinous but vague aggregate, constitute the accompanying emotion. But the pi*)ees£ does not end here. This mass of plexuses thus excited has to discharge itself; and the question now to be asked is — what general directions will its discharge take, and what*will be the general nature of the produced feelings? The answer is this. Any excited mass /jf plexuses will discharge itself into the masses of plexuses with which it has most in common, and these into others similarly related to them. Xow the plexuses in which one kind of pleasurable emotion is seated,, must have much in common with the plexuses in which some other kinds of pleasurable emotions are seated;