LECTURE VI COMPOUND REFLEXES: SUCCESSIVE COMBINATION

We considered last the co-ordination of reflexes in simultaneous combination. We now turn to sequence of reflexes. Reflexes are seen to follow one another in consecutive combination. And in this chaining together of successive reflexes in different instances different kinds of processes seem traceable. Of these, one consists in the reaction to one external stimulus bringing about an application of an external stimulus for a second reflex. The dart-reflex of the frog’s tongue provoked by the seen fly provides, if successful, the stimulus (contact with the mucosa of the mouth) which provokes closure of the mouth, and this probably insures the stimulus for the ensuing deglutition, and so on. Exner has dealt with this kind of chaining together of reflexes by one stimulus leading to another in his “Entwurf einer physiologischen Erklärung psychischer Erscheinungen.” Loeb has illustrated it luminously in his “Physiology of the Brain.” He calls sequence of reflexes proceeding by this process from one segmental reflex to another “chain-reflexes” (Ketten-reflexe). Mosso,⁶⁶ Kronecker and Meltzer,^{89, 90, 101} Chauveau,²¹⁸ and Zwaardemaker,^{235, 272} have traced such reflexes analytically in deglutition.

Mosso⁶⁶ showed that in the oesophageal stage of deglutition each reflex in a part of the tube above is immediately succeeded by a reflex ensuing in the adjoining part below, and yet in this sequence the distant bulbar centre is itself concerned, since the sequence ceases if the branches of the vagus containing the nerve paths to and from that centre be severed. Biedermann has recently demonstrated a very similar procession of reflexes in the crawling of the earthworm, and there again the sequence involves reflexes conducted through the central nervous system. It appears that the action of each preceding segment provides a stimulus for the reflex act of the next succeeding segment.

Orderly sequence of movement characterizes the outward behaviour of animals. Not least so where, as in the earthworm crawling,²⁷³ or the insect in flight, or the fish swimming, every observer admits the coadjustment is essentially reflex. One act succeeds another without confusion. Yet, tracing this sequence to its external causes, we recognize that the usual thing in nature is not for one exciting stimulus to begin immediately after another ceases, but for an array of environmental agents acting concurrently on the animal at any moment to exhibit correlative change in regard to it, so that one or other group of them becomes—generally by increase in intensity—temporarily prepotent. Thus there dominates now this group, now that group, in turn. It may happen that one stimulus ceases coincidently as another begins, but as a rule the stimuli overlap one another in regard to time. Thus each reflex in the unmutilated animal breaks in upon a condition of relative equilibrium; and this latter is itself reflex.

It was shown that reflex movements can be grouped as regards their mutual relation into those which have allied relation and mutually facilitate and reinforce, and those which are mutually antagonistic. Antagonistic reflexes do not enter into simultaneous combination. Simultaneous combination unites “allied” reflexes only. But into reflex combinations of successive kind, reflexes both allied and antagonistic enter as components.

If the scratch-reflex be excited from a spot in its receptive skin-field and then while the reflex is in progress another scratch-reflex is excited from a receptive point not far removed from the first one, the scratch-reflex under the double excitation may differ very little in appearance from that first excited, and on the first stimulation being discontinued the reflex persists, maintained by the second stimulation, and hardly or not perceptibly altered in character from its outset. In this case the second reflex which succeeded the first resembles the first; that is, it is to outward appearance and for practical purposes merely a prolongation of it.

But if the point of application of the second stimulus be, although still in the receptive field of the scratch-reflex, widely distant from that of the first stimulus, the reflex becomes obviously modified when the second stimulus is thrown in. Thus, if the stimulus be so located in the receptive field that the first excites the low form of the reflex and the second the high form, the reflex, though initiated in the low form, assumes the high form when the second stimulus—if that is of appropriate strength—is thrown in. Or, conversely, it assumes the low form if the second stimulus be appropriately located for producing that form.

And between the component scratch-reflexes there are grades of likeness corresponding with degrees of proximity of the points of application of the stimuli in the receptive field. When, therefore, a reflex occurs in immediate sequence to a reflex to which it is allied, it smoothly maintains the reaction that is already in progress, and if its own character differs in some respect from the foregoing reflex, impresses that character on the reflex without, however, any hitch or hindrance of the reflex.

For a reflex to be immediately succeeded by a reflex of allied relation to it is of common occurrence. Any stimulus that moves over a receptive field is likely to excite such a sequence. A morsel of food moving over the surface of the tongue, a stimulus moving in the field of vision, an object moving along the skin, and, as an instance of the last, a parasite travelling across the receptive field of the scratch-reflex.²⁵²

In such successive combinations the reflexes are, in the scratch-reflex at least, linked together by more than the mere external circumstance of the incidence of the stimulus. In such a sequence the threshold of each succeeding reflex is lowered by the excitation just preceeding its own.

A subliminal stimulus applied at a point A will render a subliminal stimulus applied at a point B near A supraliminal if the second stimulus follow within a short time, e. g. 500 σ. The space of receptive surface across which this can be demonstrated in the scratch-reflex amounts to 5–6 centimeters. It is best worked out by unipolar application of the induced current through a stigmatic electrode—fine gilt entomological pin. In that way numerical values can be assigned to the results. But the phenomenon is characteristically and simply illustrated²⁸⁷ by the difference between the potency as a stimulus of the edge of a card, say six inches long, pressed simultaneously its whole length against the receptive skin-field, say for 5 seconds, and on the other hand lightly drawing one corner of the card along the same line in the skin-field also for 5 seconds. The former application simply evokes a reflex of a few beats, which then dies out. The latter evokes a vigorous reflex that continues and outlasts the application of the stimulus. A successive line is more effective as stimulus than a simultaneous line of equal length and duration. Again, if a light disc three centimeters in diameter and a fraction of a millimeter thick be freely pivoted in bearings at the end of a handle, so that it turns when pushed by its handle over the skin surface, such a wheel may not, when pushed against a spot of the receptive surface, excite the reflex, but it excites it when it is rolled along it. The same thing is seen with a spur wheel. Even when the points are two centimeters apart, as the spur wheel is rolled over the surface successive summation occurs, and the reflex is evoked as the progress of the wheel proceeds. If a parasite in its travel produces excitation which is but close below the threshold, its progress is likely to so develop the excitability of the surface whither it passes that the scalptor-reflex will be evoked. In the skin and the parasite respectively we have, no doubt, two competing adaptations at work. It is perhaps to avoid the consequences of the spatial spread of the “bahnung” that the hop of the flea has been developed.

This bahnung, which spreads around a stimulated point, must be the same phenomenon which finds still more marked expression in the summation of stimuli individually subliminal applied successively at one and the same point. That it influences other points in the neighbourhood as well as its own seat of application, is another item of evidence of the central conjunction of the neighbouring reflex-arcs of any one type-reflex. It is an influence which, with barely supraliminal stimuli, is short lived; but it is one of the factors in the card-experiment just mentioned. This spread of influence to adjacent points is important because, though short lived, it can contribute effectively to maintain one and the same reflex. It favours the occurrence of sequences of closely allied reflexes. It is convenient to have a term for such a species of “bahnung,” and “immediate induction” seems the most suitable here.

Phenomena akin to these are met with in the physiology of vision. A moving point in the peripheral field is more visible than a line of similar length, direction, and duration. Again, a row of dots individually below the minimum visibile and too far apart for their retinal images to overlap by diffusion, becomes visible. Probably this same process is contributory toward the seeing of lines the diameter of which is narrower than the diameter of the circular minimum visibile. I have found osmic-stained nerve-fibres of 4 μ diameter visible to the naked eye both for myself and other workers in the laboratory. Re-inforcement by positive induction appears to be at work in these visual effects just as in the scratch-reflex.

In the sequence of reflexes the supervening reflex may differ imperceptibly or slightly, though distinctly, from the precedent, or may be in part quite different from that.

If the reflexes are closely similar, it is difficult to say at what moment the transition under overlapping stimuli occurs, since the initial reflex on discontinuance of the first stimulation is maintained unaltered by the second. But if the reflexes are recognizably dissimilar, e. g. a low scratch-reflex and a high scratch-reflex, the moment of transition is obvious, for the reflex then takes the form of the response which the second stimulation would excite, and, on discontinuing the first stimulation, is continued in that form (Fig. 50). In my experience the transition does not, in the case of this reflex at least, include a period of summation of the two reflexes in the sense that the reflex under the two stimulations A and B consists of the response to stimulation A summed with that to stimulation B. So also in the transition from one reflex to another of even greater dissimilarity. There is the same absence of any period of fusion of the two reflexes. Such fusion might be appropriately termed confusion. The rule seems that such confusion is avoided in the transition. In the hind limb a scratch-reflex of the high form presents a certain degree of resemblance to a simple flexion-reflex, inasmuch as there can be distinguished in the former a marked tonic flexion on which the clonic is, so to say, superposed. When a scratch-reflex of this form supervenes on the flexion-reflex the result is commonly that which is seen in Fig. 51. The transition occurs without confusion; even in regard to the tonic contraction, an element possibly common to the two reflexes, there is in the transition no period at which the tonic contraction of the flexion-reflex has added to it that of the scratch-reflex. In the instance figured the amplitude of the tonic contraction of the scratch-reflex was about equal to that of the flexion-reflex; if these summed, therefore, their joint amplitude would be much greater than that of either reflex singly. But the record shows a smooth continuity of flexion-reflex with scratch-reflex in which there is no stage of summated amplitude of the two contractions. This is what I mean by saying that the transition from one reflex to another takes place without confusion.

Not that the onset of one reflex is uninfluenced by the existence of the other—its interval of latency is, for instance, liable to be greatly influenced. In the example furnishing Fig. 51, the scratch-reflex, though its intensity is not increased, shows a hesitancy about the opening of its clonus not present in the reflex when elicited singly. Again, in Fig. 52, which shows transition from the crossed stepping-reflex to the scratch-reflex, though no period of confusion occurs in the transition from the former reflex to the latter, there is yet, on cessation of the latter, evidence of modification of the former. The crossed stepping-reflex returns, but considerably modified and after a longer latency than before. The amount of modification is greater than would in my experience be ascribable with probability to the effect of mere fatigue for the period during which the stimulus was at work, although the movement itself was in abeyance; the rhythm is present but weakened.

If it is advantageous for the transition from one reflex to another of like type to occur without a period of confusion, it is still more advantageous that it should be so in the case of transitions from one reflex to another of converse type. Confusion in the literal sense above would in that case involve not merely inaccuracy at the outset of each new reflex, but would mean mutual destruction of the two reflex effects; an interval of impotent mutual self-hindrance would disadvantageously intervene between successive motor acts of opposite direction. In the transition from one reflex to another of antagonistic kind the avoidance of confusion of the two reflexes emphasizes at the same time the impossibility of co-ordinating them in a simultaneous combination.

Though the stimulus exciting the reflex that is displaced continues while the new reflex is introduced, the displacement of the former reflex occurs without confusion.

Taking the flexion-reflex and the scratch-reflex, the one may temporarily interrupt the other in mid career (Figs. 43, 53) or may cut it short or may defer its onset; in all these cases it does so without a phase of confusion in the transition, although the stimuli belonging to both reflexes continue in contemporary operation (Figs. 43, 51, 52, etc.) all the time. And the same holds between other antagonistic reflexes, e. g. flexion-reflex and crossed extension-reflex (Figs. 30, 32, 33), extension-reflex and scratch-reflex (Figs. 42, 54), etc.

And the direction of the interference is reversible. The flexion-reflex may be made to interrupt the scratch-reflex (Fig. 43) or the scratch-reflex to interrupt the flexion-reflex (Fig. 51). The scratch-reflex may be made to interrupt the crossed stepping-reflex (Fig. 52) or the crossed stepping-reflex to interrupt the scratch-reflex (Fig. 55); and similarly with other pairs of antagonistic reflexes.

Inhibition. The process in virtue of which this transition from one antagonistic reflex to another occurs is obviously one of active intervention. In many cases the form which the intervention takes is inhibition. For instance, where a crossed extension-reflex is interrupted by a brief flexion-reflex, two transitions occur, the first at a moment a from the E-reflex to the F-reflex, the second at a moment β from the F-reflex back to the E-reflex (Fig. 33, p. 101). Although stimuli for the two reflexes are both in operation continuously from a to β, there is a clean transition from one reflex to the other. The tracing is taken from the extensor muscle of the knee. The flexion-reflex expresses itself by inhibition of the reflex contraction in progress in that muscle at that time under the combined influence of the crossed extension-reflex and of the tonic extensor rigidity due to the animal being decerebrate. It causes the contraction due to these conjoined reflexes to cease. In such a case the transition from the extension-reflex to the flexion-reflex evidently occurs by inhibition. So also in the transition from the flexion-reflex to the extension-reflex when the hamstring muscles are examined (Fig. 32, p. 98).

We do not yet understand the intimate nature of inhibition. In the cases before us now, its seat is certainly central, and in all probability is, as argued above, situated at points of synapsis. I have urged that a prominent physiological feature of the synapse is a synaptic membrane. It seems therefore to me that inhibition in such cases as those before us is probably referable to a change in the condition of the synaptic membrane causing a block in conduction. But what the intimate nature of the inhibitory change may be we do not know.

The views of some of those who have authoritatively treated of the nature of inhibition may be cited here, both for their intrinsic interest and the suggestion of lines of investigation. One view has been that, as the process of conduction along nerve-fibres is an undulatory one in the sense that the nerve-impulse travels as a disturbance with wave-like configuration of intensity, inhibition is due to a mutual suppression of two wave-like disturbances impinging on the same point of the conductor but in opposite phases of disturbance.

In those cases where stimulation through one nerve inhibits the action of a tissue acting under another nerve we can imagine a process which leaves the tissue unaffected but simply interferes with another stimulus, as in the physical interference of vibrations. Rosenthal’s³⁰ resistance theory of the action of the afferent vagus-fibres upon the respiratory centre is a supposition of this kind. It recognizes during the inhibition no change of total output of energy in the particular function inhibited. The alteration of a hypothetical resistance only distributes the discharge of the nerve-centre over an altered time-rhythm,—smaller and more frequent discharges representing the same liberation of energy as larger and less frequent. Such a view of the nature of inhibition is that which Gaskell¹⁰³ termed the “neutral” one; according to it the inhibition leaves the tissue in the same ultimate condition as that in which it found it, neither exhausted nor surcharged. “As far as the central nervous system is concerned, there exists a strong general tendency to look upon the inhibitory processes occurring there as ‘neutral’ in their character.”¹⁰³

This view, originally put forward by v. Cyon,³⁹ has been abandoned by him, although it has since been supported by Lauder Brunton^107a and others. It was expressly dissented from by Wundt.⁸³ And the grounds of Wundt’s objection are valid, the kernel of them being that though in a certain sense of the word the nerve-impulse can be described as wave-like it is not an undulatory disturbance at all in the sense in which those reactions are which show physical interference. It is therefore to physical interference that in this view of inhibition the analogy is drawn, but the similarity between the process of nerve-conduction and conduction of light and electrical waves or sound, etc., is not real enough to strictly justify such analogy. Moreover, as we shall see presently, central inhibition is not a neutral process, for, at least in many cases, it leaves the reflex centre surcharged for subsequent response (v. infra, pp. 205–213, “successive induction”).

The most striking thing that we know of inhibition is that it is a phenomenon in which an agent such as in other cases excites or increases an action going on in this case stops or diminishes an action going on. Now, the activity of a tissue can be lowered or abolished by production in it of deleterious changes such as exhaustion or, in the highest degree, death. But there is no evidence that inhibition of a tissue is ever accompanied by the slightest damage to the tissue; on the contrary, it seems to predispose the tissue to a greater functional activity thereafter.

We can imagine that a material continuously produced by a tissue, and yielding on decomposition the particular activity which is inhibited, may by an inhibition be checked in its decomposition, and accumulate, so that at the end of the period of inhibition the tissue contains more of the particular decomposable material than before. This molecular rearrangement would diminish activity for the time being, but lead to increase of activity afterwards. There would ensue a rebound effect. This is, as is well known since Gaskell’s^99a researches, what actually happens in the pure vagus action on the heart. A similar rebound-effect is perfectly obvious in many instances of inhibition in the central nervous system.³⁰⁴ It constitutes a point of resemblance between central and peripheral inhibition.

Such an explanation as this second one may take the chemical structure of the living material, the bioplasm, of the cell as the field of operation for the decomposition and the synthetic process that it pictures. The living cell is constantly liberating energy in its function, and rebuilding its complex structure from nutrient material. Its life is therefore an equilibrium of balanced katabolism and anabolism; at any given moment the one process or the other may predominate in the cell. At a moment when the cell is vigorously discharging some function which involves conversion of internal energy into external energy it is in a katabolic phase. In subsequent relative repose from the discharge of that function the replenishment of its store of potential energy by assimilation may predominate and the cell be in an anabolic phase. Katabolism and activity of external function, anabolism and rest from external function, come on this view to be almost synonymous terms. But Hering, Gaskell, Verworn, and others have taught us to attach important external functions to the assimilatory (anabolic) phase. The first mentioned has dealt with the visual sensations in assimilation-dissimilation pairs. Black-white sensation is thus traced to a pair of reactions affecting the trophism of the cell in exactly opposite directions. Gaskell relates vagus inhibition of the heart to the throwing of the cardiac muscle-cell into a phase predominantly anabolic. Verworn in his “Biogen-hypothese” has developed this trophic theory with further elaboration still. These views all take the actual nutritive activity of the cell as the direct and immediate field in which inhibition has its seat. The tendency to rebound after-effect in the opposite direction is in this view a natural trophic result. Hence Gaskell expressively speaks of the vagus as the “trophic nerve of the heart.” And Hering^48a (1872)¹¹⁸ formulated his experience somewhat as follows: The action of a stimulus affects the cell’s autonomic trophic equilibrium; it may increase or lessen either the cell’s asssimilation or the cell’s dissimilation; in whichever of these ways the stimulus acts the excitation of the cell, owing to a self-regulation proper to it, dwindles for that stimulus and for all stimuli producing a similar change, while the excitability increases for stimuli that produce an opposite effect. On all these views inhibition is in its intimate and essential nature a trophic process.

An hypothesis not built immediately on views of the nutritive processes of the cell has recently been put forward by Macdonald.³⁰⁶ Dealing with nerve and muscle fibres, he does not take the purely chemical structure of the living framework of the cell as the field of operation for either excitation or inhibition. The explanation he offers of these two latter processes is as follows.

From study of the part played by inorganic salts in the function of nerve, he sees in the attachment of these salts to the proteids present, and in their partial detachment from the proteids, normal occurrences underlying the conditions of rest and excitation respectively. He assumes the connection between salt and proteid involved in this matter to be of purely physical nature. The axis-cylinder, for example, is composed of a colloid solution in which there are present minute particles of colloid proteid. These may increase in size—indeed so much as finally to become visible—under the influence of factors determining a tendency towards coagulation. Upon the surface of these particles the major portion of the inorganic salts present is held restrained in a condition of condensed solution. An increase in the size of the particles is accompanied by a diminution in the total surface separating the particles from the solvent in which they lie. Such a diminution in surface is equivalent to a diminution in the forces restraining the motion of the inorganic salts. It occasions the liberation of salt molecules in a state of free motion into the surrounding aqueous solution. This release of hitherto restrained molecules is the cause of alterations in osmotic pressure, of new processes of diffusion, of resultant electrical phenomena, and thus of the phenomena of excitation.

Macdonald thus considers a stimulus as an agency determining an approach to the condition of coagulation. He regards as the important characteristic of the excited state the release of inorganic salts resulting from this change. According to him inhibition is a condition in which the inorganic salts are more securely packed away upon the surface of the “colloid particles” than usual by reason of diminution in the individual size of the particles and an increase in the surface they present to the surrounding solution. He supposes the communication of a negative electrical charge to be a stimulus provoking a tendency to coagulation with all the just-mentioned dynamic consequences of the enlargement of the colloid particles. So he considers that conversely the communication of a positive charge produces a change of an exactly opposite kind in which inorganic salts hitherto in motion are brought to a static condition of rest.

It does not seem at once clear that the condition of greater subdivision of particles should also be a condition of greater stability, although it is clear that such a conception might explain the greater store of potential energy possessed by an inhibited tissue. Macdonald, in fact, does not, I take it, assume this to be the case. To explain this separate fact he appeals to the evidence upon which the conception is based and points out a distinction between the amount of inorganic salt involved in changes above (inhibition) and changes below (excitation) the equilibrium line of the normal resting state. Thus there is quantitative evidence to show that the amount of salt remaining for withdrawal from the resting colloid solution is only a small fraction of the total amount of salt present; the major portion is already withdrawn, and is, so to speak, in reserve in this large quantity for such changes of excitation as occur below the base line of rest. Changes from the normal to the hypernormal, and from the hypernormal to the normal, cannot therefore involve a redistribution of salt comparable in quantity to those taking place down from, and up to, the normal. It is in this way conceivable that the application of a stimulus to an inhibited tissue—although productive of an effect akin to the phenomenon underlying the process of excitation—might yet lead only to such a subminimal change in the amount of inorganic salt in motion as to determine no externally appreciable manifestation of its occurrence. Imagine a tissue which has been placed in a condition of inhibition by the communication of a positive electrical charge. The application of a negative charge to such a tissue would produce no visible effect, although productive of an internal change. The application of a second negative charge would give rise to a characteristic excitation, its efficiency determined by “summation.” Let us on the other hand suppose that the tissue has not only been inhibited, but is maintained in a continuous state of inhibition by the steady arrival of positive charges. In this case the application of a succession of stimuli would result in nothing more than a series of subminimal, and therefore unnoticed, changes.

In an excited tissue, summarizing this conception, an unusual quantity of inorganic salts is in motion. Excitation is ended by the reduction of this excessive motion. Inhibition is the condition in which the possibilities of free motion are most reduced.

This view is fertile in suggestion for further experiment. Based on examination of the physical structure of nerve by electrical methods irreproachably employed, and on the revelation, under the microchemical test which we owe to Macallum,³⁰⁷ of potassium appearing in quantity at injured points of nerve-fibres, and explaining naturally as it does the injury-current of nerve as similar in production to the current of a “concentration” battery²³⁴ the concentrations of which can be known from the current, it merits very careful consideration. The features and conditions of occurrence of inhibition harmonize strikingly with what on Macdonald’s view we should expect them to be.

Interference. Whatever the intimate nature of the inhibition, it is, however, only one part of the processes involved in transition from one antagonistic reflex to another. In the transition from the crossed extension-reflex to the flexion-reflex the inhibition of previous excitation in the extensor neurone is accompanied by excitation of the previously inhibited flexor neurone. And conversely in the transition from flexion-reflex to extension-reflex. Transition from one form of reciprocal innervation to another will obviously involve such changes wherever the transition is from one reflex of simultaneous double-sign to an antagonistic of simultaneous double-sign. There will be inhibition at one set of points and excitation at another. The process of transition, therefore, in many cases is one half of it inhibitory, one half excitatory. It seems advisable, therefore, to avoid employing the term “inhibition” for the displacement in general of one reflex by another. To avoid confusion, some expression of broader scope, including excitation as well as inhibition proper, seems required. The term “interference” already used by Wundt⁸³ and by A. Tschermak¹⁸⁶ in an almost similar way seems to me well suited for this purpose. In employing it Wundt expressly stipulated that it had in this use no reference to its technical employment by physicists for the mutual interaction of vibrations, as in light and sound. The term “interference” as applied to reflexes would mean simply the interaction between antagonistic reflexes, that is, reflexes which are incapable of simultaneous combination. These reflexes are capable of successive combination, and in that process the influence of one reflex replaces that of another upon a common path potentially belonging to each. The replacement may take place by inhibition succeeding excitation, or by excitation succeeding inhibition, or by excitation of one kind succeeding excitation of another kind, as when the steady tonic flexion of the flexion-reflex is succeeded by the rhythmic clonic flexion of the scratch-reflex. In all these cases the process of displacement of one reflex action by the other may be termed interference. We thus get a comprehensive convenient term for embracing the whole series of cases.

The frequency with which, in co-ordination of reflexes by successive combination, the reflex which succeeds to another is antagonistic to this latter is very great. Two important classes of such sequence are especially common. One is that which forms what are known as “alternating reflexes;” the other is the class of “compensatory reflexes.”

Alternating reflexes are seen very clearly in cyclic reversals of direction of movement; thus, when extension succeeds flexion in the stepping-reflex. Here antagonistic reflexes succeed one another alternately at two final common paths. In the motor neurones for the knee, in the stepping-reflex, excitation and inhibition alternately ensue in the flexor neurones, while synchronously with that, inhibition and excitation alternately ensue in the extensor neurones.

The essence of an alternating reflex is that excitation and inhibition ensue in succession at two (or more) final common paths, a sequence of antagonistic reflexes possessing them in turn. In an ordinary rhythmic reflex a periodic excitation (and a periodic refractory or inhibitory state) is recurrently produced in the reflex-arc at rhythmic intervals. Every alternating reflex, therefore, is a rhythmic reflex, but not every rhythmic reflex is an alternating reflex. The movements of the vertebrate limb in locomotion give instances of alternating reflexes; probably the movement of the tail of fishes in swimming is a similar instance, but has not yet been analyzed as to its reflex composition. Alternating reflexes form an excellent field for examination of reciprocal innervation of antagonistic muscles.

It is particularly by the case of “alternating reflexes” that Macdougall illustrates his view of the central process in reciprocal innervation. His scheme also offers an explanation for the transition from one antagonistic reflex to another. Based more immediately on the behaviour of visual images, it is applied by Macdougall specifically to the case of the reciprocal innervation of antagonistic muscles. “Let us,” he writes,²⁶² “imagine each arc in a simple schematic form as a chain of three neurones afferent (a₁), central (a₂), efferent (a₃), and let us call them a₁, a₂ and a₃, and b₁, b₂, and b₃ , in the two arcs respectively” (Fig. 55). “When a strong stimulus is applied to the afferent neurone of arc a it generates neurin rapidly, so that it becomes very rapidly charged, and the resistance of synapse a₁—a₂ is lowered until a series of discharges takes place from a₁ to a₂, and again from a₂ to a₃. The problem is, then, to imagine such a mode of connection between arc a and arc b as will cause arc a during stimulation to drain off from the afferent and central neurones of b the smaller quantities of neurin generated in them. Several forms of such a connection may be imagined, but I think that probably it takes the form of a collateral fibre coming from neurone b₂, and taking part with the axone of a₂ in forming a synapse with a₃.” “Whatever the exact constitution of this synapse may be, we may assume that, when its resistance is lowered by the stimulation of a, and consequent charging of a₂, the collateral of b₂, making connection with a₃, through this synapse, becomes the path of least resistance for the escape of neurin from b₁ and b₂. These neurones are therefore drained by a₃, while b₃ ceases to receive any neurin from b₂, and the tone of the muscle-group supplied by it is abolished.” “In a similar way, if both a₁ and b₁ be stimulated, but one more strongly than the other, the more strongly stimulated arc will drain the afferent and central neurones of the less strongly stimulated arc, because the resistance of synapses of the former will be reduced to a lower level than that of the synapses of the latter.”

This scheme fits a number of facts of reciprocal inhibition. Thus, in reciprocal innervation, as the term itself implies, the inhibition at one part always appears as the negative aspect of positive excitation at another. To “alternating” reflexes, which are common as spinal reactions, Macdougall applies the scheme thus: “We must suppose the collateral connection of the arc a (to which we may suppose the stimulus to be applied) with the neurone b₃ to be but little inferior in conducting capacity to its direct connection with a₃. When, then, the neurone a is stimulated continuously, the arc will first discharge into a₃ and drain b₁ and b₂ (i. e. inhibit b₃) until after some little time fatigue causes the resistance of synapse a₂—a₃ to become slightly greater than that of synapse b₂—b₃, when a₂ will discharge with b₂ wholly into b₃, and a₃ will be in turn “inhibited.” So the charges of the afferent neurones of both arcs will be discharged into b₃ until fatigue causes rise of resistance of the synapse on b₃.” The high influence of intensity of reaction in determining whether the reaction shall or shall not replace another reaction is expressed by this scheme very lucidly. Also the occurrence under strychnine³⁰⁴ and tetanus toxin of excitation at a synapse where inhibition is otherwise the rule seems a contingency which the view answers well. If these agents (strychnine presumably from the afferent side, tetanus toxin from the efferent) reduce the resistance at synapse a₂—b₃, there will, on stimulation of a₁, be conversion of the inhibition of b₃ in excitation, just as in the second phase of an alternating reflex, but the resistance at synapse a₂—a₃ not being raised, as it is in the second phase of the alternating reflex, a₃ will also be excited as usual, and both antagonistic muscles will contract, as I have shown they do in fact in strychnine reflexes and as they do in the convulsions of strychnine poisoning. The scheme makes it clear, too, that this double discharge, or leakage, will prove rapidly exhausting on the central arcs, and indeed the phasic character of the attacks in strychnine poisoning does seem due to rapid exhaustion following each convulsive discharge.

Again, in working with tetanus toxin, I have,³⁰⁴ in the gradual progress of the disease, several times found the afferent nerves produce a slight reflex inhibition of the extensor of the knee, if the initial posture at the knee were at the time extension (i. e., the extensor arc in high activity), and yet produce distinct excitation of the extensor if the initial posture at the time were flexion (i. e., the extensor arc in less activity). These effects of strychnine and tetanus toxin the view of Macdougall seems well fitted to meet, though they were not known at the time the view was formulated. The view seems also applicable to some of the results obtained in the interesting experiments by v. Uexkull on Invertebrata.

One difficulty however seems to me presented by Macdougall’s view, and it is that the diversion of the influence of a₁ through a₂ away from a₃ which constitutes inhibition does not suggest a reason for the superactivity (successive induction) in a consequent on the inhibition. And a more serious difficulty, attaches, in my thinking, to the view,—at least, as it at present stands—in that it seems to sever this central nervous inhibition—of which I regard reciprocal innervation of antagonistic muscles as but one widely spread case—from other forms of inhibition met peripherally in the heart, blood-vessels, and viscera, rather than to connect it with them. It appears to me unlikely that in their essential nature all forms of inhibition can be anything but one and the same process.

Another important class of sequence of antagonistic reflexes is that which gives the “compensatory reflex.” A compensatory reflex occurs where the reflex is a return to a state of reflex equilibrium which had been disturbed by an intercurrent reflex to which the compensatory reflex is the diametrical antagonist. Some compensatory reflexes are excited by passive movements, others by active movements. It is the latter which come under consideration here. Many reflex movements are intercurrent reactions, breaking in on a condition of neural equilibrium itself reflex. Take the case of the flexion-reflex of the leg (spinal dog) induced by a brief stimulus during “decerebrate rigidity,” where the animal may be regarded as a bulbo-spinal machine. Suppose the animal suspended with spine horizontal and limbs pendent. The limbs are then in slight active extension, much as if the animal were standing. This slight extension is active, for it is reflex, and the peripheral source of the maintained reflex pose is traceable to arcs arising in the extensor muscles, in alliance probably with some from the otic labyrinth. The creature breathes quietly and regularly. Its skeletal musculature otherwise exhibits no movements, although its reflex activity is considerable and in progress all the time, as shown by the steady reflex extension of the limbs. If, then, the foot be excited by a brief stimulus and thus a flexion-reflex induced in the limb, the limb is drawn up at hip, knee, and ankle. The movement is brief; after being drawn up, the limb returns to its previous pendent posture. It is easy in many instances to perceive that the pose of extension, as resumed, is more marked than it was prior to the intercurrent reflex or flexion. It is also equally easy to perceive that in the replacement of the limb the extension is not a mere passive drop under gravity, but is a reversion to the previous posture by an active movement. In such a case the reflex effect of the intercurrent stimulus seems to cease with the cessation of the intercurrent reflex (flexion) which the stimulus immediately provoked. But closer examination shows that this is not really the case. There is an active reflex return to the pre-existing pose. Thus, the disturbing stimulus brought about not only the flexion-reflex, but, secondarily to that, a reflex antagonistic to that. This latter antagonistic (extension) reflex is “allied” to that which originally held the field. When the flexion-reflex disturbed the neural equilibrium it dispossessed the opposed reflex of extension from certain final paths common to that and itself. In other words, its own reaction induces an after-coming reflex antagonistic to itself, and this brings resumption of the original reflex attitude that, under the condition (gravity, etc.) obtaining at the time, satisfies neural equilibrium. The whole intercurrent reflex disturbance is really ended by a “compensatory reflex.” The compensatory reflex in this case seems traceable primarily to proprio-ceptive (muscular) afferents from the muscles, joints, etc., of the limb. But compensatory reflexes are particularly evident in reflex reactions started by labyrinthine afferents (Ewald, Lee, Loeb, Lyon, Muskens, Nagel, and others), and in the decerebrate animal this compensatory reflex of the limb is presumably due to muscular afferents of the limbs and to labyrinthine afferents acting in reflex alliance.

Among the afferent nerves of importance to this compensating reflex there seems particularly that of the vasto-crureus muscle itself, the extensor of the knee. Electrical stimulation of the central end of that nerve excites contraction of the flexors of hip and knee and inhibits the vasto-crureus itself. It therefore reinforces the flexion-reflex, but its stimulation is, on being discontinued, immediately succeeded by contraction of the extensor muscles. This rebound (successive induction) is particularly marked in the vasto-crureus itself. These instances exemplify further what was said as to the close connection of secondary, not immediate, kind between reflexes initiated by receptors of the extero-ceptive (e. g. skin) surface and reflexes initiated by receptors of the deep, i. e. proprio-ceptive field. But in the instances given earlier the proprio-ceptive reflex which was initiated secondarily in consequence of the foregoing exteroceptive (skin) reflex is antagonistic to the latter; the two reflexes are related as antagonistic reflexes. The secondary association which, as pointed out earlier, holds so generally between certain extero-ceptive proprio-ceptive pairs of reflexes and connects them, forms some of its pairs from “allied reflexes” and others from “antagonistic reflexes.” In the former case the coupling is “simultaneous,” in the latter “successive.” Proprio-ceptive reflexes may themselves be coupled in antagonistic pairs; of this, one example is the reflex contraction of the hamstring muscles, which sometimes undoubtedly ensues on stimulation of the central end of the nerve of the extensor of the knee (I was at first inclined to attribute this to escape of current, but am convinced it occurs truly reflexly sometimes); and another example is Mislawski’s and Baglioni’s interesting expiratory reflex elicitable from the afferent fibres of the phrenic nerve.

Factors determining the sequence. The formation of a common path from tributary converging afferent arcs is important because it gives a co-ordinating mechanism. There the dominant action of one afferent arc, or set of allied arcs in condominium, is subject to supercession by another afferent arc or set of allied arcs, and the supercession normally occurs without inter-current confusion. Whatever be the nature of the physiological process occurring between the competing reflexes for dominance over the common path, the issue of their competition, namely, the determination of which one of the competing arcs shall for the time being reign over the common path, is largely conditioned by four factors. These are “spinal induction,”⁸⁰⁴ relative intensity of stimulus, relative fatigue, and the functional species of the reflex.

I. Spinal induction. The first of these occurs in two forms, one of which has been already considered, namely, “immediate induction.” It is a form of “bahnung.” The stimulus which excites a reflex tends by central spread to facilitate and lower the threshold for reflexes allied to that which it particularly excites. A constellation of reflexes thus tends to be formed which reinforce each other, so that the reflex is supported by allied accessory reflexes, or if the prepotent stimulus shifts, allied arcs are by the induction particularly prepared to be responsive to it or to a similar stimulus.

Immediate induction only occurs between allied reflexes. Its tendency in the competition between afferent arcs is to fortify the reflex just established, or, if transition occur, to favour transition to an allied reflex. Immediate induction seems to obtain with highest intensity at the outset of a reflex, or at least near its commencement. It does not appear to persist long.

The other form of spinal induction is what may be termed successive induction. It is in several ways the reverse of the preceding.³⁰⁴

In peripheral inhibition exemplified by the vagus action on the heart the inhibitory effect is followed by a rebound aftereffect opposite to the inhibitory (Gaskell). The same thing is obvious in various instances of the reciprocal inhibition of the spinal centres. Thus, if the crossed-extension reflex of the limb of the spinal dog be elicited at regular intervals, say once a minute, by a carefully adjusted electrical stimulus of defined duration and intensity, the resulting reflex movements are repeated each time with much constancy of character, amplitude, and duration. If in one of the intervals a strong prolonged (e. g. 30″) flexion-reflex is induced from the limb yielding the extensor-reflex movement, the latter reflex is found intensified after the intercurrent flexion-reflex.³⁰⁴ The intercalated flexion-reflex lowers the threshold for the aftercoming extension-reflexes, and especially increases their after-discharge (Figs. 57, 58). This effect may endure, progressively diminishing, through four or five minutes, as tested by the extensor-reflexes at successive intervals. Now, as we have seen, during the flexion-reflex the extensor arcs were inhibited: after the flexion-reflex these arcs are in this case evidently in a phase of exalted excitability. The phenomenon presents obvious analogy to visual contrast. If visual brightness be regarded as analogous to the activity of spinal discharge, and visual darkness analogous to absence of spinal discharge, this reciprocal spinal action in the example mentioned has a close counterpart in the well-known experiment where a white disc used as a prolonged stimulus leaves as visual after-effect a gray image surrounded by a bright ring (Hering’s “Lichthof”). The bright ring has for its spinal equivalent the discharge from the adjacent reciprocally correlated spinal centre. The exaltation after-effect may ensue with such intensity that simple discontinuance of the stimulus maintaining one reflex is immediately followed by “spontaneous” appearance of the antagonistic reflex. Thus the “flexion-reflex” if intense and prolonged may, directly its own exciting stimulus is discontinued, be succeeded by a “spontaneous” reflex of extension, and this even when the animal is lying on its side and the limb horizontal,—a pose that does not favour the tonus of the extensor muscles. Such a “spontaneous” reflex is the spinal counterpart of the visual “Lichthof.” To this “spinal induction,” as it may be termed, seems attributable a phenomenon commonly met in a flexion-reflex of high intensity when maintained by very prolonged excitation. The reflex-flexion is then frequently broken at irregular intervals by sudden extension movements (Figs. 45, 61). It would seem, therefore, that some process in the flexion-reflex leads to exaltation of the activity of the arcs of the opposed extension-reflex. And electrical stimulation of the proximal end of the severed nerve of the extensor muscles of the knee (cat), though it does not in my experience directly excite contraction of the extensors of the knee, is on cessation often immediately followed by contraction of them.

As examples of the rebound exaltation following on inhibition the following may also serve.³⁰⁴ The so-called “mark-time” reflex of the “spinal” dog is an alternating stepping movement of the hind limbs which occurs on holding the animal up so that its limbs hang pendent. It can be inhibited by stimulating the skin of the tail. On cessation of that stimulus the stepping movement sets in more vigorously and at quicker rate than before (Fig. 59). The increase is chiefly in the amplitude of the movement, but I have also seen the rhythm quickened even by 30 per cent of the frequence.

This after-increase might be explicable in either of two ways. It might be due to the mere repose of the reflex centre, the repose so recruiting the centre as to strengthen its subsequent action. But a similar period of repose obtained by simply supporting one limb—which causes cessation of the reflex in both limbs, the stimulus being stretch of the hip-flexors under gravity—is not followed by after-increase of the reflex (Fig. 60).

Or the after-increase might result from the inhibition being followed by a rebound to superactivity. This latter seems to be the case. The after-increase occurs even when both hind limbs are passively lifted from below during the whole duration of the inhibitory stimulus applied to the tail. It is the depression of inhibition and not the mere freedom from an exciting stimulus that induces a later superactivity. And the reflex inhibition of the knee-extensor by stimulation of the central end of its own nerve is especially followed by marked rebound to superactivity of the extensor itself.

Again, the knee-jerk, after being inhibited by stimulation of the hamstring nerve, returns, and is then more brisk than before the inhibition (Fig. 29, p. 89).

By virtue of this spinal contrast, therefore, the extension-reflex predisposes to and may actually induce a flexion-reflex, and conversely the flexion-reflex predisposes to and may actually induce an extension-reflex. This process is qualified to play a part in linking reflexes together in a coordinate sequence of successive combination.³⁰⁴ If a reflex arc A during its own activity temporarily checks that of an opposed reflex-arc B, but as a subsequent result induces in arc B a phase of greater excitability and capacity for discharge, it predisposes the spinal organ for a second reflex opposite in character to its own in immediate succession to itself. I have elsewhere¹⁸³ pointed out the peculiar prominence of “alternating reflexes” in prolonged spinal reactions. It is significant that they are usually cut short with ease by mere passive mechanical interruption of the alternating movement in progress. It seems that each step of the reflex movement tends to excite by spinal induction the step next succeeding itself.

Much of the reflex action of the limb that can be studied in the “spinal” dog bears the character of adaptation to locomotion. This has been shown recently with particular clearness by the observations of Phillipson. In describing the extensor thrust of the limb I drew attention at the time to its significance for locomotion. “Spinal induction” obviously tends to connect to this extensor-thrust flexion of the limb as an after-effect. In the stepping of the limb the flexion that raises the foot and carries it clear of the ground prepares the antagonistic arcs of extension, and, so to say, sensitizes them to respond later in their turn by the supporting and propulsive extension of the limb necessary for progression. In such reflex sequences an antecedent reflex would thus not only be the means of bringing about an ensuing stimulus for the next reflex, but would predispose the arc of the next reflex to react to the stimulus when it arrives, or even induce the reflex without external stimulus. The reflex “stepping” of the “spinal” dog does go on even without an external skin stimulus: it will continue when the dog is held in the air. The cat walks well when anaesthetic in the soles of all four feet.

Each reflex movement must of itself generate stimuli to afferent apparatus in many parts and organs—muscles, joints, tendons, etc. This probably reinforces the reflex in progress. The reflex obtainable by stimulation of the afferent nerve of the flexor muscles of the knee excites those muscles to contraction and inhibits their antagonistics: the reflex obtainable from the afferent nerve of the extensor muscles of the knee excites the flexors and inhibits their antagonists.

Where a reflex by spinal induction tends to eventually bring about the opposed reflex, the process of spinal induction is therefore probably reinforced by the operation of any reflex generated in the movement. This would help to explain how it is that a reflex reaction, when once excited in a spinal animal, ceases on cessation of the stimulus as quickly as it generally does. Such a reaction must generate in its progress a number of further stimuli and throw up a shower of centripetal impulses from the moving muscles and joints into the spinal cord. Squeezing of muscles and stimulation of their afferent nerves and those of joints, etc., elicit reflexes. The primary reflex movement might be expected, therefore, of itself to initiate further reflex movement, and that secondarily to initiate further still, and so on. Yet on cessation of the external stimulus to the foot in the “flexion-reflex” the whole reflex comes usually at once to an end. The “scratch-reflex,” even when violently provoked, ceases usually within two seconds of the discontinuance of the external stimulus that provoked it.

We have as yet no satisfactory explanation of this. But we remember that such reflexes are intercurrent reactions breaking in on a condition of neural equilibrium itself reflex. The successive induction will tend to induce a compensatory reflex, which brings the moving parts back again to the original position of equilibrium.

II. Fatigue. Another condition influencing the issue of competition between reflexes of different source for possession of one and the same final common path is “fatigue.”³⁰⁰ A spinal reflex under continuous excitation or frequent repetition becomes weaker, and may cease altogether. This decline is progressive, and takes place earlier in some kinds of reflexes than it does in others. In the spinal dog the scratch-reflex under ordinary circumstances tires much more rapidly (Fig. 45) than does the “flexion-reflex.”

A reflex as it tires shows other changes besides decline in amplitude of contraction. Thus, in the “flexion-reflex” the original steadiness of the contraction decreases (Figs. 45, 61); it becomes tremulous, and the tremor becomes progressively more marked aud more irregular. The rhythm of the tremor in my observations has often been about 10 per second. Then phases of greater tremor tend to alternate with phases of improved contraction as indicated by some regain of original extent of flexion of limb and diminished tremor. Apart from these partial evanescent recoveries the decline is progressive. Later, the stimulation being maintained all the time, brief periods of something like complete intermission of the reflex appear, and even of a replacement of flexion by extension. These lapses are recovered from, but tend to recur more and more. Finally, an irregular phasic tremor of the muscles is all that remains. It is not the flexor muscles themselves which tire out, for these, when under fatigue of the “flexion-reflex” they contract nolonger for that reflex, contract in response to the scratch-reflex which also employs them.

Similar results are furnished by the scratch-reflex with certain differences in accord with the peculiar character of its discharge.²⁸⁷ One of these latter is the feature that the individual beats of the scratch-reflex usually become slower and follow each other at slower frequency (Fig. 62). Also the beats, instead of remaining fairly regular in amplitude and frequency tend to succeed in somewhat regular groups. The beats may disappear altogether for a short time, and then for a short time reappear, the stimulus continuing all the while (Fig. 63). Here, again, the phenomena are not referable to the muscle, for when excited through other reflex channels, or through its motor-nerve directly, the muscle shows its contraction well. Part of the decline of these reflexes under electrical stimulation in the spinal dog may be due to reduction of the intensity of the stimulus itself by physical polarization. That does not account in the main for the above described effects. The graphic record of fatigue of the flexion of the scratch-reflex obtained by continued mechanical stimulation does not appreciably differ from that yielded under electrical stimulation. The different speed of the decline due to fatigue proceeds characteristically in different kinds of reflex, and in the same kind of reflex under different physiological conditions e. g. “spinal shock”: this indicates its determination by other factors than electrical polarization. Polarization has in a number of cases been deferred as far as possible by using equalized alternate shocks applied in opposite directions through the same gilt needle; this precaution has not yielded results differing appreciably from those given by ordinary double shocks or by series of make or break shocks of the same direction. The slowing of the beat in “fatigue” is also against the explanation by polarization, since merely weakening the stimulus does not lead to a slower beat.

When the scratch-reflex elicited from a spot of skin is fatigued, the fatigue holds for that spot but does not implicate the reflex as obtained from the surrounding skin.^{287, 300} The reflex is when tired out to stimuli at that spot easily obtainable by stimulation two or more centimeters away (Fig. 63). This is seen with either mechanical or electrical stimuli. When the spot stimulated second is close to the one tired out, the reflex shows some degree of fatigue, but not that degree obtaining for the original spot. This fatigue may be a local fatigue of the nerve-endings in the spot of skin stimulated, to which in experiments making use of electric stimuli some polarization may be added. Yet its local character does not at all necessarily imply its reference to the skin. It may be the expression of a spatial arrangement in the central organ by which reflex-arcs arising in adjacent receptors are partially confluent in their approach toward the final common path, and are the more confluent the closer together lie their points of origin in the receptive field. The resemblance between the distribution of the incidence of this fatigue and that of the spatial summation previously described argues that the seat of the fatigue is intraspinal and central more than peripheral and cutaneous; and that it affects the afferent part of the arc inside the spinal cord, probably at the first synapse. Thus, its incidence at the synapse Ra—Pa and at Rβ—Pβ (Fig. 13, B, or 39, B) would explain its restrictions, as far as we know them, in the scratch-reflex.

The local fatigue of a spinal reflex seems to be recovered from with remarkable speed, to judge by observations on the reflexes of the limbs of the spinal dog. A few seconds’ remission of the stimulus suffices for marked though incomplete restoration of the reaction (Fig. 63). In a few instances I have seen return of a reflex even during the stimulation under which the waning and disappearance of the reflex occurred. The exciting stimulus has usually in such cases been of rather weak intensity. In my experience, these spinal reflexes fade out sooner under a weak stimulus than under a strong one. This seeming paradox indicates that under even feeble intensities of stimulation the threshold of the reaction gradually rises, and that it rises above the threshold value of the weaker stimulus before it reaches that of a stronger stimulus. This is exemplified by Fig. 64, where the scratch-reflex which has ceased to be elicited by the stimulus A is immediately evoked—often without any sign of fatigue in its motor response—by increasing the intensity of the stimulus (applied at the same electrode) to A + a, 5 ohms having been short-circuited from the current in the primary circuit. But the occurrence of “fatigue” earlier under the weaker stimulus than under the stronger also shows that the fatigue consequent under the weaker stimulus may often be relatively to the production of the natural discharge greater than when a stronger stimulus is employed. This which has been of frequent occurrence in my observations on the leg of the spinal dog if obtaining widely in reflex actions has evident practical importance.

It is easy to avoid in some degree the local fatigue associated with excitation of the scratch-reflex from one single spot in the skin by taking advantage of the spatial summation of stimuli applied at different points in the receptive field. Brief-lasting stimuli can be shifted from point to point in the field. When this is done, a curious result has met me. The provocation of the reflex has been made through ten separate points in the receptive field, the distance between each member of the series of points and the point next to it being about four centimeters. Each point is stimulated by a double-induction shock delivered twice a second. When this is done a series of scratch movements is elicited, and continues longer than when the stimuli are applied at the same interval, not to succeeding series of skin points but to one point. Thus three or four hundred beats can be elicited in unbroken series. But the series tends somewhat abruptly to cease. If then, in spite of the cessation of the response, the stimulation be continued without alteration during three or four minutes or more, the scratching movement breaks out again from time to time and gives another series of beats (Fig. 65), perhaps longer than the first. These experiments indicate that physical polarization at the stigmatic electrode is not answerable for the fading out of the scratch-reflex. It shows also the complexity of the central mechanisms involved in the reflex. The phenomenon recalls Lombard’s¹¹⁶ phases of briskness and fatigue in series of records obtained with the ergograph.

It is interesting to note certain differences between the cessation of a reflex under “fatigue” and under inhibition. Figs. 60 and 43 can be compared. The reflex ceasing under inhibition is seen to fade off without obvious change in the frequency of repetition of the “beats,” or in the duration of the individual beats. The reflex ceasing under fatigue is seen to show a slower rhythm and a sluggish course for the latter beats, especially for the terminal ones.

Among the signs of fatigue of a reflex action are several suggesting that in it the command over the final common path exercised for the time being by the receptors and afferent path in action becomes less strong, less steady, and less accurately adjusted. Under prolonged excitation their hold upon the final common path becomes loosened. This view is supported by the fact that its connection with the final common path is then more easily cut short and ruptured by other rival arcs competing with it for the final common path in question. The scratch-reflex interrupts the flexion-reflex more readily when the latter is tired out than when it is fresh.

In the hind limb of the spinal dog the extensor-thrust is inelicitable during the flexion-reflex. That is to say, when the flexion-reflex is evoked with fair or high intensity I have never succeeded in evoking the extensor-thrust, though the flexed posture of the limb is itself a favouring circumstance for the production of the thrust if the flexion be a passive one. But when the flexion-reflex is kept up by appropriate stimulation of a single point over a prolonged time, so that it shows fatigue, the “extensor-thrust” becomes again elicitable. Its elicitability is, then, not regular nor facile, but it does become obtainable, usually in quite feeble degree at first, later more powerfully. In other words, it can dispossess the rival reflex from a common path when that rival is fatigued, though it cannot do so when the rival action is fresh and powerful.

Again, the crossed “extension-reflex” cannot inhibit the flexion of the flexor-reflex under ordinary circumstances if the intensity of the stimulation of the competing arcs be approximately equal; but it can do so when the flexion-reflex is tired.

The waning of a reflex under long maintained excitation is one of the many phenomena that pass in physiology under the name of “fatigue.” It may be that in this case the so-called fatigue is really nothing but a negative induction. Its place of incidence may lie at the synapse. It seems a process elaborated and preserved in the selective evolution of the neural machinery. One obvious use attaching to it is the prevention of the too prolonged continuous use of a “common path” by any one receptor.³⁰⁰ It precludes one receptor from occupying for long periods an effector organ to the exclusion of all other receptors. It prevents long continuous possession of a common path by any one reflex of considerable intensity. It favours the receptors taking turn about. It helps to insure serial variety of reaction. The organism, to be successful in a million-sided environment, must in its reactions be many-sided. Were it not for such so-called “fatigue,” an organism might, in regard to its receptivity, develop an eye, or an ear, or a mouth, or a hand or leg, but it would hardly develop the marvellous congeries of all those various sense-organs which it is actually found to possess.

The loosening of the hold upon the common path by so-called “fatigue” occurs also in paths other than those leading to muscle and effector organs. If instead of motor effects sensual are examined, analogous phenomena are observed. A visual image is more readily inhibited by a competing image in the same visual field when it has acted for some time than when it is first perceived (W. Macdougall).²²³

One point, on a priori grounds a natural corollary from the “principle of the common path,” is indicated by the experimental findings relative to the incidence of fatigue. The reflex-arcs, each a chain of neurones, converge in their course so as to impinge upon and conjoin in links (neurones) common to whole varied groups—in other words, they conjoin to common paths. This arrangement culminates in the convergence of many separately arising arcs in the final efferent-root neurone. This neurone thus forms the instrument for many different reflex arcs and acts. It is responsive to them in various rhythm and in various grades of intensity. In accordance with this, it seems from experimental evidence to be relatively indefatigable.³⁰⁰ It thus satisfies a demand that the principle of the common path must make regarding it.

III. Intensity. In the transition from one reflex to another a final common path changes hands and passes from one master to another. A fresh set of afferent arcs becomes dominant on the supercession of one reflex by the next. Of all the conditions determining which one of competing reflexes shall for the time being reign over a final common path, the intensity of reaction of the afferent arc itself relatively to that of its rivals is probably the most powerful. An afferent arc strongly stimulated is caeteris paribus more likely to capture the common path than is one excited feebly. A stimulus can only establish its reflex and inhibit an opposed one if it have intensity. This explains why, in order to produce examples of spinal inhibition, recourse has so frequently been made in past times to strong stimuli. A strong stimulus will inhibit a reflex in progress, although a weak one will fail. Thus in Goltz’s inhibition of micturition in the “spinal” dog a forcible squeeze of the tail will do it, but not, in my experience, a weak squeeze. So likewise any condition which raises the excitability and responsiveness of a nervous arc will give it power to inhibit other reflexes just as it would if it were excited by a strong stimulus. This is much as in the heart of the Tunicate. There the prepotent spot whence starts the systole lies from time to time at one end and from time to time at the other. The prepotent region at one end which usually dominates the common path is from time to time displaced by local increase of excitability at the other under local distension of the blood-sinuses there.

In judging of intensity of stimulus the situation of the stimulus in the receptive field of the reflex has to be remembered. One and the same physical stimulus will be weak if applied near the edge of the field, though strong if applied to the focus of the field.

Crossed reflexes are usually less easy to provoke, less reliable of obtainment, and less intense than are direct reflexes. Consequently we find crossed reflexes usually more easily inhibited and replaced by direct reflexes than are these latter by those former. Thus the crossed stepping-reflex is easily replaced by the scratch-reflex (Fig. 52), though its stimulus be continued all the time, and though the scratch-reflex itself is not a very potent reflex. But the reverse can occur with suitably adjusted intensity of stimuli.

Again, the flexion-reflex of the dog’s leg is, when fully developed, accompanied by extension in the opposite leg. This crossed extensor movement, though often very vigorous, may be considered as an accessory and weaker part of the whole reflex of which the prominent part is flexion of the homonymous limb. When the flexion-reflex is elicitable poorly, as, for instance, in spinal shock or under fatigue or weak excitation, the crossed extension does not accompany the homonymous flexion and does not appear. But, where the flexion-reflex is well developed, if not merely one but both feet be stimulated simultaneously with stimuli of fairly equal intensity, steady flexion at knee, hip, and ankle results in both limbs (Fig. 66) and extension occurs in neither limb.²⁰⁵ The contralateral part of each reflex is in-hibited by the homolateral flexion of each reflex. In other words, the more intense part of each reflex obtains possession of the final common paths at the expense of the less intense portion of the reflex. But if the intensity of the stimuli applied to the right and left feet be not closely enough balanced, the crossed extension of the reflex excited by the stronger stimulus is found to exclude even the homonymous flexion that the weaker stimulus should and would otherwise evoke from the leg to which it is applied.

It was pointed out above that in a number of cases the transference of control of the final common path FC from one afferent arc to another is reversible. The direction of the transference can caeteris paribus be easily governed by making the stimulation of this receptor or that receptor the more intense. A factor largely determining whether a reflex succeed another or not is therefore intensity of stimulus.³⁰⁰

IV. Species of reflex. A fourth main determinant for the issue of the conflict between rival reflexes seems the functional species of the reflexes.³⁰⁰

Reflexes initiated from a species of receptor apparatus that may be termed “noci-ceptive”²⁵² appear to particularly dominate the majority of the final common paths issuing from the spinal cord. In the simpler sensations we experience from various kinds of stimuli applied to our skin there can be distinguished those of touch, of cold, of warmth, and of pain. The adequate stimuli for the first mentioned three of these are certainly different; mechanical stimuli, applied above a certain speed, which deform beyond a certain degree the resting contour of the skin surface, seem to constitute adequate stimuli for touch. Similarly the cooling or raising of the local temperature, whether by thermal conduction, radiation, etc., are adequate for the cold and warmth sensations. The organs for these three sensations have by stigmatic stimuli been traced to separate and discrete tiny spots in the skin. In regard to skin-pain it is held by competent observers, notably by v. Frey¹⁴⁸ and Kiesow,²⁷⁰ that skin-pain likewise is referable to certain specific nerve-endings. In evidence of this it is urged that mechanical stimuli applied at certain places excite sensations which from their very threshold upward possess unpleasantness, and as the intensity of the stimulus is increased, culminate in “physical pain.” The sensation excited by a mechanical stimulus applied to a touch-spot does not evoke pain, however intensely applied, so long as the stimulation is confined to the touch-spot. The threshold value of mechanical stimuli for touch-spots is in general lower than it is for pain-spots; and conversely the threshold value of electrical stimuli for touch-spots is in general higher than it is for the spots yielding pain. Similarly it is said that stimulation of a cold spot or of a warm spot does not, however intense, evoke, so long as confined to them, sensations of painful quality. But pain can be excited not only by strong mechanical stimuli and by electrical stimuli, but by cold and by warmth, though the threshold value of these latter stimuli is higher for pain than for cold and warm spots. If these observations prove correct there exist, therefore, numerous specific cutaneous nerve-fibres evoking pain.

A difficulty here is that sensory nerve-endings are usually provided with sense-organs which lower their threshold for stimuli of one particular kind while raising it for stimuli of all other kinds; but these pain-endings in the skin seem almost equally excited by stimuli of such different modes as mechanical, thermal conductive, thermal radiant, chemical, and electrical. That is, they appear anelective receptors. But it is to be remarked that these agents, regarded as excitants of skin-pain, have all a certain character in common, namely this, that they become adequate as excitants of pain when they are of such intensity as threatens damage to the skin. And we may note about these excitants that they are all able to excite nerve when applied to naked nerve directly. Now there are certain skin surfaces from which, according to most observers, pain is the only species of sensation that can be evoked. This is alleged, for instance, of the surface of the cornea—a modified piece of skin. The histology of the cornea reveals in its epithelium nerve-endings of but one morphological kind; that is, the ending by naked nerve-fibrils that pass up among the epithelial cells. Similar nerve-endings exist also in the epidermis generally. It may therefore be that the nerve-endings subserving skin-pain are free naked nerve-endings, and the absence of any highly evolved specialized end-organ in connection with them may explain their fairly equal amenability to an unusually wide range of different kinds of stimuli. Instead of but one kind of stimulus being their adequate excitant, they may be regarded as adapted to a whole group of excitants, a group of excitants which has in relation to the organism one feature common to all its components, namely, a nocuous character.

With its liability to various kinds of mechanical and other damage in a world beset with dangers amid which the individual and species have to win their way in the struggle for existence we may regard nocuous stimuli as part of a normal state of affairs. It does not seem improbable, therefore, that there should under selective adaptation attach to the skin a so-to-say specific sense of its own injuries. As psychical adjunct to the reactions of that apparatus we find a strong displeasurable affective quality in the sensations they evoke. This may perhaps be a means for branding upon memory, of however rudimentary kind, a feeling from past events that have been perilously critical for the existence of the individuals of the species. In other words, if we admit that damage to such an exposed sentient organ as the skin must in the evolutionary history of animal life have been sufficiently frequent in relation to its importance, then the existence of a specific set of nerves for skin-pain seems to offer no genetic difficulty, any more than does the clotting of blood or innate immunity to certain diseases. That these nerve-endings constitute a distinct species is argued by their all evoking not only the same species of sensation but the same species of reflex movement as regards “purpose,” intensity, resistence to “shock,” etc. And their evolution may well have been unaccompanied by evolution of any specialized end-organ, since the naked free nerve-endings would better suit the wide and peculiar range of stimuli, reaction to which is in this case required. A low threshold was not required because the stimuli were all intense, intensity constituting their harmfulness; but response to a wide range of stimuli of different kinds was required, because harm might come in various forms. That responsive range is supplied by naked nerve itself and would be cramped by the specialization of an end-organ. Hence these nerve-endings remained free.

It is those areas stimulation of which, as judged by analogy, can excite pain most intensely, and it is those stimuli which, as judged by analogy, are most fitted to excite pain which, as a general rule, excite in the “spinal” animal—where pain is of course non-existent—the prepotent reflexes. If these are reactions to specific pain-nerves, this may be expressed by saying that the nervous arcs of pain-nerves, broadly speaking, dominate the spinal centres in peculiar degree. Physical pain is thus the psychical adjunct of an imperative protective reflex. It is preferable, however, since into the merely spinal and reflex aspect of the reaction of these nerves no sensation of any kind can be shown to enter, to avoid the term “pain-nerves.” Remembering that the feature common to all this group of stimuli is that they threaten or actually commit damage to the tissue to which they are applied, a convenient term for application to them is nocuous. In that case what from the point of view of sense are cutaneous pain-nerves are from the point of view of reflex reaction conveniently termed noci-ceptive nerves.

In the competition between reflexes the noci-ceptive as a rule dominate with peculiar certainty and facility. This explains why such stimuli have been so much used to evoke reflexes in the spinal frog, and why, judging from them, such “fatality” belongs to spinal reflexes.

One and the same skin surface will in the hind limb of the spinal dog evoke one or other of two diametrically different reflexes according as the mechanical stimulus applied be of noxious quality or not, a harmful insult or a harmless touch.²⁵² A needle-prick to the planta causes invariably the drawing up of the limb — the flexion-reflex. A harmless smooth contact, on the other hand, causes extension — the extensor-thrust above described. This flexion is therefore a noci-ceptive reflex. But the scratch-reflex — which is so readily evoked by simple light irritation of the skin of the shoulder — is relatively mildly noci-ceptive. When the scratch-reflex and the flexion-reflex are in competition for the final neurone common to them, the flexion-reflex more easily dispossesses the scratch-reflex from the final neurone than does the scratch-reflex the flexion-reflex. If both reflexes are fresh, and the stimuli used are such as, when employed separately, evoke their reflexes respectively with some intensity, in my experience it is the flexion-reflex that is usually prepotent (Fig. 43). Yet if, while the flexion-reflex is being moderately evoked by an appropriate stimulus of weak intensity, a strong stimulus suitable for producing the scratch-reflex is applied, the steady flexion due to the flexion-reflex is replaced by the rhythmic scratching movement of the scratch-reflex (Fig. 51), and this occurs though the stimulus for the flexion-reflex is maintained unaltered. When the stimulus producing the scratch is discontinued the flexion-reflex reappears as before. The flexion-reflex seems more easily to dispossess the scratch-reflex from the final common paths than can the scratch-reflex dispossess the flexion-reflex. Yet the relation is reversible — by heightening the intensity of the stimulus for the scratch-reflex or lowering that of the stimulus for the flexion-reflex.

In decerebrate rigidity, where a tonic reflex is maintaining contraction in the extensor muscles of the knee, stimulation of the noci-ceptive arcs of the limb easily breaks down that reflex. The noci-ceptive reflex dominates the motor neurone previously held in activity by the postural reflex. And noci-ceptive reflexes are relatively little depressed by “spinal shock.”

Noci-ceptive arcs are, however, not the only spinal arcs which in the intact animal, considered from the point of view of sensation, evoke reactions rich in affective quality. Beside those receptors attuned to react to direct noxa, the skin has others, concerned likewise with functions of vital importance to the species and colligate with sensations similarly of intense affective quality; for instance, those concerned with sexual functions. In the male frog sexual clasp is a spinal reflex.⁶ The cord may be divided both in front and behind the brachial region without interrupting the reflex. Experiment shows that from the spinal male at the breeding-season, and also at other times, this reflex is elicited by any object that stimulates the skin of the sternal and adjacent region. In the intact animal, on the contrary, other objects than the female⁴⁰ are, when applied to that region, at once rejected, even though they be wrapped in the fresh skin of the female frog and in other ways made to resemble the female. The development of the reflex is not prevented by removal of the testes, but removal of the seminal reservoirs is said to depress it, and their distension, even by indifferent fluids, to exalt it. If the skin of the sternal region and arms is removed, the reflex does not occur. Severe mutilation of the limbs and internal organs does not inhibit the reflex, neither does stimulation of the sciatic nerve central to its section. The reflex is however depressed or extinguished by strong chemical and pathic stimuli to the sternal skin, at least in many cases.

The tortoise exhibits a similiar sexual reflex of great spinal potency.^{112a 112b}

It would seem a general rule that reflexes arising in species of receptors which considered as sense-organs provoke strongly affective sensation caeteris paribus prevail over reflexes of other species when in competition with them for the use of the “final common path.” Such reflexes override and set aside with peculiar facility reflexes belonging to touch organs, muscular sense-organs, etc. As the sensations evoked by these arcs, e. g. “pains,” exclude and dominate concurrent sensations, so do the reflexes of these arcs prevail in the competition for possession of the common paths. They seem capable of pre-eminent intensity of action.³⁰⁰

Of all reflexes it is the tonic reflexes, e. g. of ordinary posture, that are in my experience the most easily interrupted by other reflexes. Even a weak stimulation of the noci-ceptive arcs arising in the foot often suffices to lower or abolish the knee-jerk or the reflex extensor tonus of the elbow or knee. If various species of reflex are arranged, therefore, in their order of potency in regard to power to interrupt one another, the reflexes initiated in receptors which considered as sense-organs excite sensations of strong affective quality lie at the upper end of the scale, and the reflexes that are answerable for the postural tonus of skeletal muscles lie at the lower end of the scale. One great function of the tonic reflexes is to maintain habitual attitudes and postures. They form, therefore, a nervous background of active equilibrium. It is of obvious advantage that this equilibrium should be easily upset, so that the animal may respond agilely to the passing events that break upon it as intercurrent stimuli.

Therefore, intensity of stimulation, fatigue and freshness, spinal induction, functional species of reflex, all these are physiological factors influencing the result of the interaction of reflex-arcs at a common path. It is noticeable that they all resolve themselves ultimately into intensity of reaction. Thus, intensity of stimulus means as a rule intensity of reaction. Those species of reflexes which are habitually prepotent in interaction with others are those which are habitually intense; those specially impotent in competition are those habitually feeble in intensity, e. g., skeletal muscular tone. The tonic reflexes of attitude are of habitually low intensity, easily interfered with and temporarily suppressed by intercurrent reflexes, these latter having higher intensity. But these latter suffer fatigue relatively early, whereas the tonic reflexes of posture can persist hour after hour with little or no sign of fatigue. Fatigue, therefore, in the long run advantageously re-dresses the balance of an otherwise unequal conflict. We can recognize in it another agency working toward that plastic alternation of activities which is characteristic of animal life and increases in it with ascent of the animal scale.

The high variability of reflex reactions from experiment to experiment, and from observation to observation, is admittedly one of the difficulties that has retarded knowledge of them. Their variability, though often attributed to general conditions of nutrition, or to local blood-supply, etc., seems far more often due to changes produced in the central nervous organ by its own functional conductive activity apart from fatigue. This functional activity itself causes from moment to moment the temporary opening of some connections and the closure of others. The chains of neurones, the conductive lines, have been, especially in recent years, by the methods of Golgi, Ehrlich, Apathy, Cajal, and others, richly revealed to the microscope. Anatomical tracing of these may be likened, though more difficult to accomplish, to tracing the distribution of blood-vessels after Harvey’s discovery had given them meaning, but before the vasomotor mechanism was discovered. The blood-vessels of an organ may be turgid at one time, constricted almost to obliteration at another. With the conductive network of the nervous system the temporal variations are even greater, for they extend to absolute withdrawal of nervous influence. Under reflex inhibition a skeletal muscle may relax to its post-mortem length,³⁰⁴ i. e., there may then be no longer evidence of even a tonic influence on it by its motor neurone. The direction of the stream of liberation of energy along the pattern of the nervous web varies from minute to minute. The final common path is handed from some group of a plus class of afferent arcs to some group of a minus class, or of a rhythmic class, and then back to one of the previous groups again, and so on. The conductive web changes its functional pattern within certain limits to and fro. It changes its pattern at the entrances to common paths.³⁰⁰ The changes in its pattern occur there in virtue of interaction between rival reflexes, “interference.” As a tap to a kaleidoscope, so a new stimulus that strikes the receptive surface causes in the central organ a shift of functional pattern at various synapses. The central organ is a vast network whose lines of conduction follow a certain scheme of pattern, but within that pattern the details of connection are, at the entrance to each common path, mutable. The gray matter may be compared with a telephone exchange, where, from moment to moment, though the end-points of the system are fixed, the connections between starting points and terminal points are changed to suit passing requirements, as the functional points are shifted at a great railway junction. In order to realize the exchange at work, one must add to its purely spatial plan the temporal datum that within certain limits the connections of the lines shift to and fro from minute to minute. An example is the “reciprocal innervation” of antagonistic muscles — when one muscle of the antagonistic couple is thrown into action the other is thrown out of action. This is only a widely spread case of the general rule that antagonistic reflexes interfere where they embouch upon the same final common paths. And that general rule is part of the general principle of the mutual interaction of reflexes that impinge upon the same common path. Unlike reflexes have successive but not simultaneous use of the common path; like reflexes mutually reinforce each other on their common path. Expressed teleologically, the common path, although economically subservient for many and various purposes, is adapted to serve but one purpose at a time. Hence it is a co-ordinating mechanism and prevents confusion by restricting the use of the organ, its minister, to but one action at a time.

In the case of simple antagonistic muscles, and in the instances of simple spinal reflexes, the shifts of conductive pattern due to interaction at the mouths of common paths are of but small extent. The co-ordination covers, for instance, one limb or a pair of limbs. But the same principle extended to the reaction of the great arcs arising in the projicient receptor organs of the head, e. g. the eye, which deal with wide tracts of musculature as a whole, operates with more multiplex shift of the conductive pattern. Releasing forces acting on the brain from moment to moment shut out from activity whole regions of the nervous system, as they conversely call vast other regions into play. The resultant singleness of action from moment to moment is a key-stone in the construction of the individual whose unity it is the specific office of the nervous system to perfect. The interference of unlike reflexes and the alliance of like reflexes in their action upon their common paths seem to lie at the very root of the great psychical process of “attention.”