The Integrative Action of the Nervous System

For stimulation of the central ends of that vast medley of afferents, from different sources and of various species, collected together in one afferent spinal root {e.g. eighth cervical) to evoke no inharmonious confusion of various reflexes, such as the com- ponent fibres in it must, taken individually, represent, but one allied group of harmonious reflexes, is a result that, though regularly obtainable, is surely not what the observer might have expected would occur. Stimulation of the central end of the tibial nerve behind the ankle causes flexion at knee, hip, and ankle, in which the normal inhibitions of extensor muscles ac- company the contractions of the flexor muscles. Yet in that nerve are included the afferent fibres from the planta which evolve the powerful extension-reflex, the extensor-thrust. How is it that this reflex does not appear conjoined with the flexion- reflex produced by the other afferent fibres in the posterior root or tibial nerve? The principle of interference of antagonistic reflexes central to the mouth of the common path on which both embouch precludes such confusion of reflexes. Such simulta- neous combination of flexion and extension reflexes would be inco-ordination. The formation of a common path from tribu- tary paths is a mechanism ensuring co-ordination against that. The stimulation of the nerve containing admixed receptive paths of different and antagonistic reflexes excites reflexly through the central organ an effect in the skeletal musculature which is co-ordinate and synergic. It is as though a solvent were simul- taneously supersaturated with two crystalloids, and as though when a pair of these crystals were simultaneously dropped into the solution the crystallization out took place of one salt or of

he other, but not of both together. What happens resembles what occurs when there is presented to the eye one of the plane figures suggesting visual perspective, but equivocally in either of the two ways ; the whole of the beaker or of the flight of steps appears set in one way or in the other, never partly in one way, partly in the other. And this is a really germane analogy. The mode of preclusion of the antagonistic reflexes seems so closely akin to the process which occurs when one reflex in its supervention on another dispossesses an antagonistic reflex from the common path that its discussion may be deferred until treating of the co-ordination of successive reflexes. But it is obvious that in the irradiation of a reflex so as to produce a combined movement of remote parts we have really a syn- thesis of simultaneous reflexes. The parts of the reflex finding simultaneous expression in the eff'erent paths of other reflexes are combined by a process which tends to exclude the antago- nistic reflex for each component part. It is obvious that while " allied " reflexes can be compounded together both in simulta- neous and successive combination, antagonistic reflexes can be combined only in successive combination.

The collection of fibres in a motor spinal root does not repre- sent a "reflex figure." The supporters of the view that the motor-fibres gathered together in a motor spinal root form a collection assorted so as to represent the fibres that normally are excited together in willed and other actions adduce the fact that antagonistic muscles are together thrown into contraction on exciting this or that spinal motor nerve root supplying a limb — e. g., the arm. This fact, as I pointed out some years ^^ ago, is in reality one of the clearest evidences that their view is erroneous, because most commonly in normal movements the antagonistic muscles, far from being thrown into contraction together, are reciprocally innervated, one antagonist being made to contract and the other to relax. Further, far from a normal action ever throwing into activity all the motor-fibres of a single motor root, still less using that one root thus solely without other roots, in reality the evidence is that in all normal

actions, reflex or voluntary, especially in the limb regions, the centrifugal discharge to the muscles takes place through scattered motor-fibres contained in several roots, even when the action pro- voked and the movement effected are weak. Outside the limb region, those who argue that the aggregation of motor-fibres in each efferent spinal root represents some definite synergic con- traction of muscles for a co-ordinate movement must disregard the observation of Newell Martin, and HartwelF^ that in the normal breathing of the dog the action that goes forward in the internal and external intercostal muscles is alternating in them. One relaxes as the other contracts ; yet both the external and the internal intercostal muscle of the space receive their motor supply from one and the same motor spinal root.

Those who hold the view that the assortment of the fibres of the motor root is functional, state that the movements which result from stimulation of these individual roots in the brachial region are not mere contractions more or less strong of various muscles, but are a highly co-ordinated functional synergy in each case. To this one may reply that the mere superficial resemblance of the position assumed by the limb, to one of the manifold positions assumed by it in the normal activity, is a slender analogy. The hind limbs of a frog, when it tries to climb the side of the bell-jar which confines it, assume an attitude of extreme extension, in outward semblance like that of a strychnine cramp, or that due to excitation of the eighth root ; but is it permissible from that resemblance to argue that excitation of the eighth root produces a co-ordinate move- ment of the limb? The same analogy would argue that the strychnic cramp is also a co-ordinate movement of the limb, whereas it is definitely known to be inco-ordinate.

Myself I have not been struck by resemblance between the movement produced by excitation of the motor spinal root of a limb-plexus and the co-ordinate normal movements of the limb. It may be urged that in order to obtain the resemblance the excitation employed must be strong, so as to bring into full action every component of the complex entity of the root. When I have done this the resulting movement has seemed to me, e. g. in the case of the sixth subthoracic root of the monkey, like a strychnine cramp rather than a movement of co-ordinate adjustment. If, on the other hand, it be urged that minimal excitation must be used, I have not been able to obtain in that way any more obvious relation to a co-ordinate movement. For instance, in the lumbo-spinal region of the monkey, excitation when just effective induces through the ninth subthoracic motor root abduction of the tail and flexion of the toes without any movement elsewhere. Similar excitation of the eighth motor root induces reflexion of minimus and hallux without the inter- vening digits, not infrequently accompanied by pursing of the anus. Such combinations strike the observer as bizarre and give little suggestion of the bringing into play of a highly co-ordinate functional synergy.

On the view that the compound muscular contraction obtained by excitation of one whole motor root is highly co- ordinated and due to a group of contractions combined in accordance with some plan for a functional result, it might be expected that the severance of one such motor root in a limb region would result in loss of some particular co-ordinate move- ment, and that the disappearance of that movement might be fairly clearly detectible. I was unable to detect such a result and saw no evidence in support of its existence. The severance of a single motor root seemed to produce not the complete loss of any one particular movement, but a weakened condition of many movements. Even when two of the motor spinal roots were cut the effect on the movements of the limb was rather weakness of movement than inco-ordination of movement. When the number of consecutive nerves cut was more than two there appeared limitation of the range of movement by loss in some particular direction. I inferred from my experiments that the mechanism for specific movement of each part of a limb {e. g. a digit) is so placed in the cord that the efferent fibres debouch- ing from it into the motor roots pass via many root filaments and via at least two, usually more, spinal roots. Thus there is\ not in any one motor root filament, nor even in any one motor root, a perfect representation of any one movement, but only an

imperfect representation of several adjacent local movements, though not for each equally imperfect. Against the view that the aggregation of efferent fibres in a motor spinal root represents a functionally co-operative collection is the fact that between them there may intervene a high resist- ance; that is to say, afferent impulses that easily throw some of them into action have great difficulty in throwing others of them into action. Thus, the ninth subthoracic motor root of the monkey sometimes contains efferent fibres to the urinary bladder and to the muscles of the leg. It is easy to obtain a reflex on the latter through the afferent root of the ninth, but relatively difficult to obtain a reflex upon the bladder. The synergetic view of the character of the collection of fibres in the spinal motor root presupposes or infers that mere spatial juxtaposi- tion possesses curiously high value for sf)inal co-ordination ; but are the separate spinal and bulbar elements of the respiratory centre less perfectly associated in function because in the neural axis they are placed apart? Likeness of quality rather than proximity in space insures the harmony of their reactions. I conclude, therefore, that the collection of fibres in a spinal motor root is not a functional collection in the sense that it is representative of any co-ordination.^^

The receptive field of a reflex does not conform with the field of distribution of an afferent spinal root. Similarly with the afferent root. The distribution in the skin of any afferent root does not correspond with the receptive field of any cutaneous reflex. The skin-field of the scratch-reflex is made up of parts of the skin-fields of many adjacent spinal roots (compare Fig. 49 with Fig. 39, p. 121). The skin-field of the flexion-reflex of the hind limb similarly; that of the fore limb similarly. Even the relatively limited receptive field ^^^ of the extensor- thrust is a patch into which parts of the fields of at least two spinal roots enter. Nor do the limits of the receptive fields of cutaneous reflexes respect the limits of spinal root skin-fields. Again, the afferent nerve of the extensor cruris muscle evokes the same reflex as does that of the flexor muscle, yet the two belong to wholly different spinal nerves.

Reinforcement. The overflow of reflex action into channels belonging primarily to other reflex-arcs than that under stimula- tion leads to the production by the single stimulus of a wide, compound reflex which is tantamount in effect to a simultaneous combination of several allied reflexes. When in the spinal animal the one fore foot is stimulated, flexion of the hind leg of the crossed side is often obtained. Stimulation of that hind foot itself also causes a like reflex of that limb. When these two are concurrently stimulated, the flexion movement is obtained more easily than from either singly. These widely separate reflex-arcs therefore reinforce one another in their action on the final common paths they possess in common. Similarly with certain reflex-arcs arising from the skin of the pinna of the crossed ear. In them excita- tion reinforces that of the just mentioned arcs from the fore foot and opposite hind foot.

This reinforcement is significant of the solidarity of the whole spinal mechanism ; but significant of more extensive solidarity still are results observed by Exner.^»^*^ A sound conveyed to the ear of a chloralized rabbit, he found, increased the am- plitude of a reflex movement of the foot, induced by the stimulus applied to the foot a moment later. Sternberg has studied similar summation of reflexes. ^^^ The same significance proba- bly attaches to the influence of various precurrent stimuli on the knee-jerk in man, studied by Jendrassik,^^^ Mitchell and Lewis,^^^ Lombard,^^^ and by Bowditch and Warren.^ In these cases of course cerebral as well as subcerebral arcs were in action. And in regard to these, we have the observations by Bubnoff and Heidenhain,^ and by Exner,^ in the narcotized dog and rabbit. In their experiments gentle stimuli to the skin of a limb exerted a reinforcing influence on closely following stimuli applied to the hmb region of the cortex of the brain. Exner's observations proved that minimal electrical stimuli applied near together in point of time to the fore-limb region of the rabbit's cortex and to the skin of the crossed foot, ex- erted a facilitating influence, " bahnung," on each other. He points out that this reinforcement occurs when the cortex itself

has been removed, and the stimulation of the brain is applied direct to the underlying white matter. He argues, therefore, that the seat of production of the facilitation lies in the spinal centres. With that view, the argument followed here is in com- plete accord. The co-ordination, in some of the instances taken, has cov- ered but one limb or a pair of Hmbs. But the same principle extended to the reactions of the great arcs arising in the projicient receptor organs of the head, e. g. the eye, that deal with wide tracts of musculature as a whole, involves further- reaching co-ordination. The singleness of action from moment to moment thus assured is a keystone in the construction of the individual whose unity it is the specific office of the nervous system to perfect. And in the instance taken, namely, concur- rent stimulation of the one fore foot and the crossed hind foot, the co-ordination can be easily traced further ; the crossed fore foot is extended at elbow and retracted at shoulder under the combination of the two stimuli, and the homonymous hind limb is extended at knee and hip. We might also add to these movements others, also caused by the same stimulus, of the eyeballs, the lips, the larynx, and the arterial wall of the splanchnic area. But these would not for the present purpose emphasize the main point further.

As remarked above, it is not usual for the organism to be exposed to the action of only one stimulus at a time. It is more usual for the organism to be acted on by many stimuli Figure 49 (opposite). — The skin-fields of the afferent spinal roots of the monkey (Macacus rhesus), showing their general arrangement in the trunk and hind limb. On the right side only the posterior limit of each field is shown, on the left side only the anterior limit. The fields were observed by the method of " remaining aesthesia." After determination of the limits of the field for a spinal root in a number of individuals the mean of the observations for that root was transferred to a plaster cast of Macacus rhesus, and the lines thus gradually built up on the model. The dotted line extending from the mid-dorsum outward along the dorsal aspect of the thigh is the " dorsal line " of the hind limb, and to it the fields of the sensory roots distributed to the skin of the limb behave as do those of the skin-fields of the trunk to the mid-dorsal line of the body. It will be noticed that the boundaries of the spinal root-fields neither in the limb nor in the trunk conform with the limits of the " re- ceptive-fields " of cutaneous reflexes. The cutaneous fields of the "scratch-reflex," the "flexion-reflex," the " extensor-thrust," are areas which in nowise fit in with the pattern of the cutaneous fields of the afferent spinal roots. Compare this figure with Figure 13 A, Lecture II, p. 46.

concurrently, and to be driven reflexly by some group of stimuli which is at any particular moment prepotent in action on it. Such a group often consists of some one pre-eminent stimulus with others of harmonious relation reinforcing it, forming with it a constellation of stimuli, that, in succession of time, will give way to another constellation which will in its turn become prepotent. The concurrent stimuli keep a number of arcs in active touch with their final paths, and a number of other arcs out of active touch with the final paths belonging to them. In the particular instance taken, they keep arcs of one fore limb and one hind limb in action upon final common paths of flexion of those limbs, upon final common paths of extension in the diago- nal pair of limbs, and upon final common paths of flexion of the neck. And the concurrent stimuli simultaneously check other arcs from getting into active touch with the final common paths of extension of one fore limb and hind limb, namely, those of the seat of stimulations, and of flexion in the opposite fore limb and hind limb, and of retraction of the neck. Further, these reactions certainly receive reinforcement through the arcs of re- ceptors in the muscles and through arcs arising in the receptors of the otic labyrinth. An instance of reinforcement of this very kind from muscular receptors we have already given (Lecture IV, p. 131). n Thus at any single phase of the creature's reaction, a simul- taneous combination of reflexes is in existence. In this combi- nation the positive element, namely, the final common paths (motor neurone groups) in active discharge, exhibits a harmo- nious discharge directed by the dominant reflex-arc, and rein- forced by a number of arcs in alliance with it. The dominant reflex-arc in the instance taken is that from the noci-ceptors of the right hind foot. The reinforcing arcs are at this phase of the re- action certain direct extension arcs, certain proprio-ceptive arcs, and certain labyrinthine arcs. But there is also a negative ele- ment in this simultaneous combination of reflexes. The reflex not only takes possession of certain final common paths and dis- charge nervous impulses down them, but it takes possession of

e final common path whose muscles would oppose those into which it is discharging impulses, and checks their nervous dis- charge responsive to other reflexes. This negative part of the field of influence of the reflex is more difficult to see, but it is as important as the positive, to which it is indeed comple- mental. Therefore it is that the reflex initiated by one group of receptors while in progress excludes in various directions the re- flexes of other receptors, although these latter may be being stimulated. In this way the motor paths at any moment accord in a united pattern for harmonious synergy, co-operating for one eff"ect.

The notion, therefore, that we arrive at of such a motor reflex reaction is that it is referable to a constellation of con- gruous stimuli of which one is prepotent, and that the reac- tion taken in its totality gives the nervous intercommunications of the central organ a certain pattern, which pattern may ramify through a great extent of the central organ. This reac- tion has its positive side traceable as active discharge from a number of end-points of the nervous network, and its negative side symmetrically opposed to its positive and traceable con- versely by check, depression, or absence of nervous discharge. Even in extensive reflexes of the bulbo-spinal animal it is prob- able that though great fields of the nervous centres are involved in the reaction at any one time, large parts are still left outside the reaction. This part of the neural network would therefore be indifferent to that particular reaction. That amounts to say- ing that it is open during the reaction to be thrown into activity by some concurrent and distinct other reaction. But this possible neutrality and discreteness of reflex reaction and its fields is probably far less in the intact higher vertebrate than in the lower or in the mutilated higher vertebrate.^^ In the pres- ence of the brain the knitting together of the whole nervous network is probably much greater than in its absence.

A question arises concerning the simultaneous combination of reflexes which is closely related to that regarding the grad- ing of intensity of a reflex. ing of intensity of stimulus. The flexion-reflex is an instance. | There as the skin stimulus is increased the height to which I the foot is flexed is increased. But it seems obvious that : such an effect is not to be expected in all reflexes. Where, ; as, for instance, in the scratch-reflex, the foot has, in response ; to irritation at a certain spot, to be moved to that spot, it I would defeat the use of the reflex for a strong stimulus to flex * the limb further, so as to carry it beyond the spot required. \ And we see that as the scratch-reflex is increased in intensity \ the increase does not appreciably increase the amount of tonic flexion exhibited by the reflex, but spends itself in increasing • the clonic beat of the reflex, which still oscillates about the ; same median position. When the scratch-reflex is elicited by simultaneous combination of two reflexes initiated from spots ■ near together in the receptive field the tonic flexion under- 1 lying either reflex does not in my experience appear to sum | with that of the other; the summation that appears seems ^ confined to the more vigorous clonic beating of the combined reflex. ^!

It seems therefore likely that in the simultaneous combina- tion of reflexes the reinforcement that goes on, although it is \ sometimes expressed as greater amplitude of contraction, is not ■ necessarily so expressed in all cases. Just as various type- ; reflexes exhibit extreme individuality of time-relations, intensity ; grading, etc., so also they exhibit in their mode of simultaneous \ combination individual differences of high degree. I Argument: Co-ordination of reflex sequences. Chain-reflexes (Loeb).

i Overlapping of successive stimuli in time. The sequence of allied reflexes. Spread of bahnung, " immediate induction." Sequence of antagonistic reflexes. The role of inhibition in this transition. Views of the nature of inhibition : Rosenthal, Wundt, E. Hering, Gaskell, Verworn, J. S. Macdonald. The " interference " of reflexes. " Alter- nating reflexes." VV. Macdougall's view of "drainage of energy." "Compensatory reflexes." Factors determining the issue of the competition between antagonistic reflexes. " Successive induction." Rebound-effects in spinal reactions ; tend to restore reflex equilibrium. Fatigue in reflexes. Relative high resistance to fatigue possessed by the final common path, /. e. motor neurone. Intensity of reac- tion a decisive factor in the competition of afferent arcs for posses- ;- sion of the final common path. Noci-ceptive nerves. Prepotency

I^K of reflexes generated by receptors that considered as sense organs I^H initiate sensations with strong affective tone. Resistance of tonic I^B reflexes to fatigue. All these factors render the conductive pattern ^H of the central nervous system mutable between certain limits. We considered last the co-ordination of reflexes in simultane- ous combination. We now turn to sequence of reflexes. Re- flexes are seen to follow one another in consecutive combination. And in this chaining together of successive reflexes in differ- ent 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 Erklarung psy- chischer 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,^^ Kronecker and Meltzer, ^' ^' ^^^ Chauveau,^^^ and Zwaardemaker, 2^^' '^^'^ j^^yg traced such reflexes analytically in deglutition. Mosso^ 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,2'3 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 cor- relative change in regard to it, so that one or other group of them becomes — generally by increase in intensity — tempo- rarily 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 over- lap 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 rela- tion 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 suc- cessive 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 difl'er 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 per- ceptibly 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 be- comes 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 as- sumes 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, there- fore, 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.^^ 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 <r. 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 illustrated ^^ 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 centi- meters apart, as the spur wheel is rolled over the surface succes- sive summation occurs, and the reflex is evoked as the progress of the wheel proceeds. If a parasite in its travel produces ex-

citation 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 compet- ing adaptations at work. It is perhaps to avoid the conse- quences of the spatial spread of the " bahnung " that the hop of the flea has been developed. This bahnungy which spreads around a stimulated point, must be the same phenomenon which finds still more marked ex- pression in the summation of stimuli individually subliminal applied successively at one and the same point. That it in- fluences 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 im- portant because, though short lived, it can contribute efl"ectively 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 induc- tion " 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 ft 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 Figure 50. — The scratch-reflex initiated from one receptive spot A in the form of a " low " reflex and then concurrently from a receptive spot B, lying more dorsal than A, in the form of a "high" reflex, the tonic flexion at hip supporting the clonic being greater. Time above in .2 seconds. Below, the upper signal marks the period of excitation of spot A, the lower signal that of spot B. The line between the two signals indicates the rhythm of the induction shocks delivered as stimuli.