* I am indebted for this fact to Dr. Bastian, who observed it in oiie of his own patients.
72 THE DATA OF PSYCHOLOGY.
an annual recurrence of this relation of cause and effect; ^cl - mammals that do not hybernate, as in ourselves, it ^V holds that prolonged exposure to extreme cold de- JT« 4 nervous action, causing strong tendency to sleep, a^l that- death results if the bodily temperature is allowed ~~ Tiiar local loss of heat when carried far, is followed by l^eaHBaetioii of the nerves, is shown by the fact that parts of the body that have been greatly cooled down, naturally or artificially, may be pricked or cut or pinched without any of Vhe usual disturbances being conveyed to the nerve- eenrres. It is true that where the refrigeration is extreme, there is usually a partial deprivation of blood; but there is evidence that when this is not the case— when, indeed, the blood-vessels are congested, as in red hands on a winter's dav. loss of heat entails decrease of nerve-function. That the like holds of the respective centres, is shown by the use of cold as a therapeutic agent: ice to the head being pre- scribed when there is excessive cerebral action, and ice to the spine being a means of diminishing reflex excitability.
It- is worth remarking that this dependence of nervous action on supply of heat, yields indirect support to the views set forth in foregoing chapters. If, as was inferred, the disturbance conveyed along a nerve-thread is an isomeric change, during which some molecular motion is yielded up bv each molecule as it passes on the accumulated wave to its neighbour — if resumption of the previous isomeric state implies an equivalent absorption of molecular motion from surrounding matter; then, in proportion to the heat of surrounding matter, will be the rapidity with which the nerve-fibre, resuming this previous isomeric state, becomes fit to transmit another wave of molecular change.
§ 28. That nerves and nerve-centres act only so long as they are furnished with those materials which the blood- vessels bring them, is a familiar truth. The quantity of THE CONDITIONS ESSENTIAL TO NERVOUS ACTION. 73 blood present in any part, and the rapidity •with which fresh blood is propelled to the part, both affect the degree of nervous activity in the part.
General depletion is a cause of nervous inaction: if the total quantity of blood in the body is much diminished, the great nervous centres are the first organs to feel the change. Temporary loss of blood produces fainting,, or sudden arrest of cerebral action; and permanent deficiency of blood is accompanied by debility, which implies a decreased nervous discharge. Supposing that no blood has been lost,, in- sensibility nevertheless instantly results if the heart ceases to supply the brain with fresh blood in place of the blood it contains. Or if there is chronic feebleness of the heart's action, there is proportionate diminution of nervous power. Where the total quantity of blood is adequate and the heart is not in fault, local nervous function may still be hindered by local anaemia, resulting from aneurism in an artery, or from what is called an embolism— a plugging up of an artery with coagulated blood. Thus paralysis is caused by embolism of the cerebral blood-vessel which supplies tho highest part of the motor tract. The converse facts similarly imply this same general relation. When, other conditions being normal, a nerve-centre is highly charged with arterial blood, it responds with unusual rapidity to the disturbances it receives; and evolves more than ordinary amounts of force, shown in secondary nervous changes, or in muscular motions, or both. Supposing, again, that there is no hypersemia of a nervous centre, it will still happen that if the heart propels blood to it with unusual rapidity, its libero-motor function will be exalted.
At the periphery of the nervous system, like variations of efficiency follow like variations of circulation. A reduction in the quantity of blood present, caused by constriction of the vessels, is probably one cause of the decreased nervous impressibility in a part that is exposed to cold; and to the same cause is perhaps to be ascribed some of the comparative 74 THE DAT^ OF PSYCHOLOGY.
sluggishness with which the muscles of the part respond to motor impulses. If instead of local lack of blood there is retardation or stoppage of the local current of blood,, the nerves of the part similarly become incapacitated in a pro- portionate degree: instance the blindness that results from blocking up the central retinal artery; or instance the gradual disappearance of impressibility in a region of the skin that has had its supplying blood-vessel tied. Conversely,, excess of blood around the peripheral nerve-fibres, causes unusual excitability of them. A gentle touch on the skin in its normal state,, sends through the afferent nerves a disturbance so small as to call forth from the central organs scarcely any response; but "where the skin is highly inflamed, a like touch affects them so much that the disturbance,, when reflected from the central organs, produces a start of the whole body. If in addition to local excess in the quantity of blood there is an accelerated flow of blood, a still greater exaltation of local nervous action follows. It is a familiar truth that, other things remaining the same, an inflamed part is made more irritable by anything which increases the action of the heart.
* § 29. Nervous action depends not alone on the quantity I of blood supplied but also on its quality — on the proportion I of the needful elements contained by it.
* General rather than special warrant must suffice for this proposition. Little is known about variations in the consti- tution of the blood; and still less about the relations between these and variations of nervous activity. That a blood greatly impoverished, as in dropsical persons (whose tissues become infiltrated because the thin serum passes too easily through the walls of the capillaries), is accompanied by ener- vation, is pretty clear; and we can scarcely be wrong in concluding that a blood rich in the constituents of nerve- substance, renders possible a great evolution of nerve-force. Bat there is indirect evidence serving to enforce the scanty TKE CONDITIONS ESSENTIAL TO NERVOUS ACTION.?•"> direct evidence. For we have abundant proof that by adding certain matters to the blood, unusual amounts of nervous action may be evoked. Alcohol, nitrous oxide, the vegeto- alkalies, and other stimulants, are not, indeed, components of nerve-substance; nor is there any reason to suppose that they can serve in place of components. Probably their im- mediate influence is that of setting up or facilitating the change of nerve -substance, and so causing unusual disen- gagement of molecular motion. But by showing that the supply of particular substances to the nervous system exalts nervous activity, they make it more manifest that nervous activity must partly depend on the supply of substances which re-build nerve-tissue as fast as action disintegrates it. We must not omit a further qualitative character of a positive kind. The blood must contain oxygen. What is the special action of oxygen — whether it is a direct disinte- grant of the tissues, including nerve-tissue; or whether it simply facilitates by its presence molecular disintegrations otherwise caused; or whether it serves merely to combine with, and carry away, the products of such disintegrations otherwise caused; are questions about which there are differences of opinion. But there can. be no difference of opinion as to the necessity for an oxygenated blood. And opinions can scarcely differ respecting the general relation that exists between the degree of oxygenation and the de- gree of nervous activity.
§ 30. While, for the maintenance of.nervous,, action^, it is requisite that certain matters shall be. present in the blood, it \ is also requisite that certain other matters shall be^ absent j | or, to speak strictly, that they shall be present in but small f proportions. These are the compounds resulting from de-f composition of the tissues — the nervous tissue included. The I two most important are carbonic acid and urea^ If the exhalation of carbonic acid by the lungs is greatly; retarded, lethargy ensues: disturbances at the periphery ( 76 THE DATA OF PSYCHOLOGY.
of the nervous system fail to call forth the usual responses. If the exhalation is completely arrested, complete insensi- bility is soon produced; followed quickly by arrest of the inferior nervous functions, and consequently of all other functions. And these effects arise still more rapidly if there is an absorption of carbonic acid through the lungs, instead of an arrested excretion of the carbonic acid internally generated.
In an analogous but less rapid manneir, a decrease and final stoppage of nervous action is caused by an accumula- ' tion in the blood of urea, or of those nitrogenous products I represented by it. If the kidneys fail to perform their func- tion, or if the waste nitrogenous products which they have separated from the blood are prevented from escaping out of the body, and are re-absorbed; there results a nervous in - action, ending presently in coma and finally in death.
§ 31. Such, stated as fully as is needful here, are the con- ditions essential to nervous action. Qualifications have been passed over; and much evidence has been omitted. In summing up these leading facts which alone concern the psychologist, we may with advantage observe how they harmonize with the general views of nerve-structure and nerve-function set forth in foregoing chapters. All these pre-requisites to nervous action obviously admit of being grouped as pre-requisites to the genesis of molecular motion, and pre-requisites to the conveyance of molecular motion.
\ That molecular motion may be disengaged there must be decomposition; and, therefore, for the discharge of molecular motion to be maintained, decomposition must be facilitated. The quantity of waste being a measure of the quantity of force evolved, it follows that the nervous system, requires ja good supply, and quick exchange, of blood; since in the tolood are brought the matters that favour disintegration. Similarly with respiration, considered as a process of ft.
THE CONDITIONS ESSENTIAL TO NERVOUS ACTION. 1 i absorbing that oxygen which directly or indirectly aids ~ the metamorphosis. And so likewise with the excre- tion of those waste products which hinder the metamor- phosis. But perpetual waste must be met by perpetual repair. If its action is to continue, nervous tissue must be re-composed as fast as it is decomposed. Hence the reason why there is needed a blood that is rich in nerve- constituents. Hence the fact that abundant blood must be present wherever there is much nervous action. And hence, also., the necessity for an efficient circulation to replace by fresh blood,, the blood that has been used.
Equally well do the several conditions essential to the transmission of nervous disturbance, conform to the hypo- thesis that the disturbance transmitted is a wave of isomeric change. For if it is, we at once see why there must be not merely contact-continuity of nerve-fibre, but molecular continuity. We are helped to understand how pressure, by deranging that delicate molecular balance which makes possible the alternation of isomeric states, may prevent the passage of nervous discharges. And we are supplied with an explanation of the fact that the presence of free molecular motion or heat, is needful to enable a nerve con- tinually to resume its fitness for conveying a wave of change.
Before closing the chapter it should be pointed out that these many conditions essential to nervous action, are never all fulfilled at one time in the same degree, but are usually fulfilled in various degrees and combinations • and that by now conspiring and now conflicting, they produce results that are complicated and often perplexing. Thus, for in- stance, substances which directly stimulate the nervous system, are usually substances which retard the exchange of gases in the lungs, and by so doing depress the state of the nervous system; and these conflicting actions, diffe- rent in their proportions in different individuals, and in the same individual at different times, often work opposite effects, or work first one effect and then the other. Again, 78 THE DATA OF PSYCHOLOGY.
richness of blood, by facilitating high nutrition of nerve- centres, conduces to nervous activity. Yet there is a plethoric state which is not nervously active; and starva- tion, with its greatly impoverished blood, has a phase at which delirium sets in, in consequence of the unduly rapid disintegration of the nerve-centres. Analogous incongruities,, too numerous to specify here, continually occur. This en- tanglement of the conditions must be borne in mind and allowed for in each case* CHAPTER T- CHAPTER T- NEttVOUS STIMULATION AND NERVOUS DISCHARGE.
§ 32. Every agent capable of altering the molecular state of a nerve, causes the nerve to produce the particular change which it habitually produces. Experiments prove that eachf nerve is made to work the same kind of effect by stimuli ol aU orders; or, to speak strictly, it is found that the effect is of the same kind wherever its kind renders it accessible to observation.
Thus, if an exposed end of a nerve which goes to a muscle is roughly touched, the muscle contracts. If it is eroded by an alkali or an acid, the muscle contracts. If it is galvanized, the muscle contracts. If it is suddenly heated, still the muscle contracts. Similarly with a vaso-inotor nerve. No matter whether the disturbing agent be me- chanical, chemical, thermal, or electric, there results at the peripheral extremity a like change in the state of the ad- jacent arteries.
An allied truth is that whether a nerve be irritated at the end which normally receives the disturbance, or whether it be irritated at some place between this and the organ acted upon by it, the effects wrought are alike — in nature, at least, if not in degree. As already said, the quantity of change set up increases with the length of the nerve through which the impulse is transmitted. But the quality of this 80 THE DATA OF PSYCHOLOGY.
change remains identical be tlie stimulus applied at a near point or a remote point.
i This last truth, equally with the first, harmonizes with the i supposition on which we have thus far proceeded. If the ^disturbance that travels along a nerve is a wave of isomeric ^transformation, the kind of effect produced by the wave at She place it eventually reaches, will be the same whatever stimulus set it up, or wherever it commenced.
§ 33. Nerve is not capable of continuous stimulation or continuous discharge. Persistent action of whatever kind on a nerve-termination or the cut end of a nerve, does not produce a persistent effect on the connected nerve-centre, or on the connected peripheral organ.
Supposing the nerve supplying some muscle has been dis- sected out and cut in two; then, if the exposed part be sud- denly pressed the muscle will suddenly contract; but main- tenance of the pressure will not cause maintenance of the contraction. Or if this nerve is made part of an electric circuit, then, at the moment of completing the circuit, the muscle will contract j but its contraction is only momentary, and the subsequent continuance of the current works no visible effect. To keep up niuscular contraction, it is requi- site to send through the nerve a quick succession of.separate disturbances. If the nerve forms part of an electric circuit in which there is an apparatus for breaking and completing the circuit; then, at each completion of the circuit, the muscle contracts; and when the alternate breaks and com- pletions follow one another very rapidly, the contraction of the muscle becomes practically persistent. This truth is demonstrable by experiment on a dead frog, and also by experiment on the living human subject. A man who grasps the two metallic cylinders forming the poles of a magneto-electric ma.chine, cannot leave hold of the cylinders when the intermittent current is passed through his arms. The like result occurs when the disturbances are mechanical NERVOUS STIMULATION AND NERVOUS DISCHARGE. 81 instead of electric. If tlie cut end of a motor nerve IP, sub- ject to a rapid series of taps,, the muscle it supplies is thrown into a state of tetanus.
The fact tliat tlie so-called nerve-current consists of successive pulses, is one of great significance. We sliall find Iiereafter that it has many important corollaries. For the present it will suffice "to observe how entirely congruous it is with the hypothesis on which we have thus far pro- ceeded. If a nervous disturbance travels as a wave of mole- cular change — if this wave is such that the molecules of nerve-substance fall from one of their isomeric states to the other y then, having fallen in passing on and increasing the pulse or shock, they remain incapable of doing anything more until they have resumed their previous isorneric state. Hence the very nature of the process necessitates the inter- mittent character of nerve-action.
§ 34. The transmission of a disturbance through a nerve/ takes an appreciable time. The rate of transmission, as measured by Professor Helmholtzflias^ varA from about 28 yards per second to 32 yards per second.j Difference of constitution is douBttess tne caiise^oSTTETsI variation — a variation to which is due that individual pecu- liarity recognized by astronomers in what they call {C the personal equation/'' This peculiarity affords yet another confirmation of the belief that a nervous discharge is a wave of isomeric trans- formation. If the disturbance "propagated through any series of molecules is one that does not permanently change their relative positions; then the transfer of the disturbance may be excessively rapid, because the amount of molecular momentum to be generated is excessively minute. But if the molecules have to be transposed — if, as in isomeric transfor- mation, the components of each compound molecule have their relative positions altered; then the quantity of molecular momentum generated must be comparatively very large; a 82 THE DATA OF PSYCHOLOGY.
and as the genesis of tHs momentum takes place in eacn molecule before the next is affected, the transfer of the disturbance must be greatly retarded.* * Perhaps too much, has already been said respecting the nature of nerve-action. But before finally leaving the subject, I must add some important illustrative facts that have come to my knowledge while writing this chapter. They are contained in a paper by Mr. Gore, published in the* Transactions of the Royal Society for 1858, describing the allotropic changes undergone by electro-deposited antimony. Antimony so depo- sited assumes, according to the conditions, two forms — a dark, amorphous, or unstable form; and a grey, crystalline, or stable form. When a mass of the amorphous antimony is disturbed at one end, there begins a change into the crystalline antimony, which spreads almost instantly throughout the whole mass, with great evolution of heat. A slight tap with a hard substance suffices to initiate this transformation. Touching one of the angles with a hot body equally produces the effect. And it is also produced by an electric spark. A temperature below that of boiling water sufficed to cause the change; and Mr. Gore found that in proportion as the whole mass was raised towards this temperature, the metamorphosis, more easily set up, travelled with greater rapidity. AVhen a copper wire was coated with a film of this amorphous antimony, the allotropic change progressed along it at a rate varying from 12 to 30 feet in a minute. Absorption of the evolved heat by the copper wire, was found to be the cause of retardation when the change advanced slowly; whence it follows that were none of the disengaged heat allowed to escape, -the wave of change would travel much faster. A further significant fact is that when this transformation was propagated through some amorphous anti- mony that had been previously reduced to powder, part of it was oxidized — the disturbance caused by the allotropic change initiated chemical change, in parts of the substance that were favourably circumstanced for chemical change. See then the parallelism. We have the transformation set up indifferently, as in nerve, by mechanical force, heat, electricity. We have it facilitated, as in nerve, by raised temperature. We have it travelling from end to end of a mass with a velocity which, though far less than that of the nerve-wave, is still considerable. And we have allotropic change initiating chemical change, just as we concluded that isomeric change in a nerve-fibre sets up chemical change in a nerve-vesicle. Let me not omit an interpretation of nerve-structure, which is suggested by Mr. Gore's experience that the transmission of the allotropic change is rapid in proportion as the evolved heat is retained. In developed animals, nerve- fibres are surrounded by sheaths of medullary matter; and we saw reasons for concluding that this medullary matter is an insulator. Now it has recently been discovered that white or fibrous nerve-tissue, is chemically distinguished from grey or vesicular nerve-tissue, by the XERYOES STIMULATION AKD NERVOUS DISCHARGE. 83 § 35. Allied with tie fact tiat a nerve-disturbance takes/ an appreciable time to travel from periphery to centre or/ from centre to periphery, is the fact that the effect produced presence in large quantity of a substance called protagon; and this sub- stance proves to be of excessively complex composition — has a molecule more highly compounded than any other known molecule. But in pro- portion as molecules become complex and large, the masses formed of them become bad conductors of molecular motion. It is inferable, then, that the essential nerve-fibre is imbedded in a substance especially distin- guished by inability to absorb the molecular motion disengaged during the isomeric change of the nerve-fibre.
I have hitherto passed over without remark, the hypothesis at one time current, and still surviving in some minds, that the nervous force is either electricity or some form of force allied to it. In addition to the many foregoing reasons for adopting another hypothesis, it may be well to set down here the reasons for rejecting this. The highest rate of the nervous discharge is some 32 yards per second. The electric discharge travels at the rate of 280,000 miles per second. The one velocity is thus nearly 16,000,000 times the other. That a force allied to the electric should have a velocity so enormously different, seems very unlikely. Again, an electric current, so long as its source is unexhausted and the circuit unbroken, is a continuous current; but the nerve-current is not continuous. Hence if the nerve-force is of a kind allied to the electric, its mode of alliance is quite exceptional; for the other allied forces, heafc, light, and magnetism, are not intermittent. Once more, nervous transmission is facilitated by heat; whereas heat is an obstacle to electric transmission, and diminishes or destroys magnetic action. The fact is that but for the accidental observation of G-alvani, the suspicion that the nerve-force is electric or qaasi-electric, would probably never have been entertained; and it should have been abandoned as soon as it was found that other disturbing agents, physical and chemical, work just the same effects. The conception has, indeed, been kept alive by the discovery that electricity is generated by certain fishes. But the supposed support is wholly imaginary. If because the Torpedo evolves electricity by the help of nerves ramifying through its electric organ, it is inferred that the nerve-force is electricity; it may in like manner be inferred that the nerve-force is sensible motion, because it generates sensible motion in muscles. But, it may be asked, do not the experiments of Du Bois-Reymond yield support to the hypothesis? A very doubtful support I think. The phenomena he describes may well be merely incidental accompaniments of actions that are in themselves neither electric nor quasi-electric. The truth that both molar and molecular changes in the distribution of matter habitually destroy the electric equilibrium, would be a sufficient general justification for this belief. But there is a special justification. Direct proof exists that the 8 j. TEE DATA OF PSYCHOLOGY.
Jat the centre or at the periphery lasts an appreciable ftime. That muscular contraction is continuous though the •i stimulus is intermittent, goes to show this. The genesis of molecular modification in muscle by the molecular modi- fication in the nerve-fibres permeating it, hao a duration that bridges the interval between each pulse of stimulus and the next. We have no direct proof that a like con- tinuity of state results from, the successive waves propa- gated to a nerve-centre; for the actions of which nerve- centres are the seats are not objectively perceptible. But we shall presently find abundant indirect proof that these changes also last for measurable periods.
This general truth., like its predecessor, may be regarded as a corollary from what has gone before. The transforma- tions classed as chemical take time, equally with those classed as isorneric. It is true that explosions due to chemical action are what we call instantaneous (a descrip- tion of them which is not, however, scientifically accurate; as may be perceived when the matter exploded is of con- particular kind of molecular change we have supposed to take place in iierve, and in muscle, is adequate to produce tlie phenomena observed by Du Bois-Reymond. Mr. Gore found that if a copper-wire, coated with amorphous antimony, formed part of an electric circuit, it happened that whenever the allotropic change propagated along the antimony stopped, tho galvanometer-needle was deflected. Now, since during the maintenance of a muscular contraction, nerve-pulses are continually arriving and ceasing, and the muscular fibres (never all in action together) are at every instant some of them contracting and some relaxing, it follows that there will be a succession of stoppages of isomeric changes. Consequently there will be a maintenance of deflection in the galvanometer if a contracted muscle forms part of the electric circuit.
[Since this note has been in type, I have referred to the lecture delivered by M. Du Bois-Reymond at the Royal Institution on April 13, 1SG;'.>, " On the Time required for the Transmission of Volition a/id Senaatiuti, through the Nerves," for the purpose of verifying one of the statements above made; and I find that at the close of this lecture he goes a long way towards abandoning his hypothesis. Though he says " it would be rash, as the matter stands, entirely to dismiss the notion of electricity being con- cerned;" and though he sketches out a theory of nerve-composition such as makes it conceivable that an electric disturbance mi "lit travel alontf a nerve NEHv5lJS STIMULATION AND NUEVOUS DISCHARGE. 85 siderable bulk). But explosions occur only in those excep- tional cases where the elements concerned are either, as in detonating compounds, distributed among one another moleculaiiy, or, as in gunpowder, with minute intimacy. In ordinary cases, where sensible masses of the elements concerned are external to one another, the chemical actions, limited to the surfaces of contact, proceed with compara- tive slowness. ISTow the granular protoplasm, contained in and around nerve-vesicles, forms, with its permeating liquids and the blood in adjacent capillaries, a mass of which the components are but imperfectly interfused; and therefore a chemical change cannot pass through it in- stantly. Hence between the reception of a pulse of mole- cular motion by a nerve-centre, and the emission of a gush of molecular motion, or discharge, some little time must elapse.
§ 36. If a nerve-centre that receives a stimulus through an incoming nerve, undergoes a chemical change and sends at the observed rate (basing this, however, on the gratuitous assump- tion that the molecules of nerve-matter have north and south poles); yet he admits that much evidence points another way. He says that **to identify it (the nervous agent) with the electric current as it circulates in a telegraph-wire must appear hopeless, even if a circuit, such as would be necessary for the supposed nerve current to circulate in, were anatomically demonstrated. Thus to the other arguments against this view of the nervous agent — tint the resistance of the nerve-tubes would be far too great for any battery to send an available current through them — that the physiological insulation of the nerve-tubes from each other would be im- possible to explain — that the effect of ligature or of cutting the nerve and causing its ends to meet again, would be equally obscure — to these argu- ments, unanswerable as they are in themselves, the researches sketched in this lecture have added corroborative evidence of the highest order. What we have termed the nervous agent, if we look upon its very small velocity, in all probability is some internal motion, perhaps even some chemical change, of the substance itself contained in the nerve-tubes, spreading along