LECTURE IV INTERACTION BETWEEN REFLEXES

We have hitherto dealt with reflex reactions under the guise of a convenient but artificial abstraction, — the simple reflex. That is to say, we have fixed our attention on the reaction of a reflex-arc as if it were that of an isolable and isolated mechanism, for whose function the presence of other parts of the nervous system and of other arcs might be negligible and wholly indifferent. This is improbable. The nervous system functions as a whole. Physiological and histological analysis finds it connected throughout its whole extent. Donaldson opens his description of it with the remark: “A group of nerve-cells disconnected from the other nerve-tissues of the body, as muscles and glands are disconnected from each other, would be without physiological significance.” A reflex reaction, even in a “spinal animal” where the solidarity of the nervous system has been so trenchantly mutilated, is always in fact a reaction conditioned not by one reflex-arc but by many. A reflex detached from the general nervous condition is hardly realizable.

The compounding together of reflexes is therefore a main problem in nervous co-ordination. For this problem it is important to recognize a feature in the architecture of the graycentred (synaptic) nervous system which may be termed “the principle of the common path.”³⁰⁰ If we regard the nervous system of any higher organism from the broad point of view a salient feature in its scheme of construction is the following.

At the commencement of every reflex-arc is a receptive neurone extending from the receptive surface to the central nervous organ. This neurone forms the sole avenue which impulses generated at its receptive point can use whithersoever be their destination. This neurone is therefore a path exclusive to the impulses generated at its own receptive point, and other receptive points than its own cannot employ it. A single receptive point may play reflexly upon quite a number of different effector organs. It may be connected through its reflex path with many muscles and glands in many different regions. Yet all its reflex-arcs spring from the one single shank or stem, i. c., from the one afferent neurone which conducts from the receptive point at the periphery into the central nervous organ.

But at the termination of every reflex-arc we find a final neurone, the ultimate conductive link to an effector organ, (muscle or gland). This last link in the chain, e. g. the motor neurone, differs obviously in one important respect from the first link of the chain. It does not subserve exclusively impulses generated at one single receptive source, but receives impulses from many receptive sources situate in many and various regions of the body. It is the sole path which all impulses, no matter whence they come, must travel if they are to act on the muscle-fibres to which it leads.

Therefore, while the receptive neurone forms a private path exclusively serving impulses of one source only, the final or efferent neurone is, so to say, a public path, common to impulses arising at any of many sources of reception. A receptive field, e. g., an area of skin, is analyzable into receptive points. One and the same effector organ stands in reflex connection not only with many individual receptive points but even with many various receptive fields. Reflexes generated in manifold senseorgans can pour their influence into one and the same muscle. Thus a limb-muscle is the terminus ad quem of many reflexarcs arising in many various parts of the body. Its motornerve is a path common to all the reflex-arcs which reach that muscle (cf. infra, Fig. 44, p. 148).

Reflex-arcs show, therefore, the general features that the initial neurone of each is a private path exclusively belonging to a single receptive point (or small group of points); and that finally the arcs embouch into a path leading to an effector organ; and that their final path is common to all receptive points wheresoever they may lie in the body, so long as they have connection with the effector organ in question. Before finally converging upon the motor neurone the arcs converge to some degree. Their private paths embouch upon internuncial paths common in various degree to groups of private paths. The terminal path may, to distinguish it from internuncial common paths, be called the final common path. The motor nerve to a muscle is a collection of final common paths.

Certain consequences result from this arrangement. One of these seems the preclusion of essential qualitative difference between nerve-impulses arising in different afferent nerves. If two conductors have a tract in common, there can hardly be essential qualitative difference between their modes of conduction; and the final common paths must be capable of responding with different rhythms which different conductors impress upon it. It must be to a certain degree aperiodic. If its discharge be a rhythmic process, as from many considerations it appears to be, the frequency of its own rhythm must be capable of being at least as high as that of the highest frequency of any of the afferent arcs that play upon it; and it must be able also to reproduce the characters of the slowest.¹

A second consequence is that each receptor being dependent for final communication with its effector organ upon a path not exclusively its own but common to it with certain other receptors, such nexus necessitates successive and not simultaneous use of the common path by various receptors using it to different or opposed effect. When two receptors are stimulated simultaneously, each of the receptors tending to evoke reflex action that for its end-effect employs the same final common path but employs it in a different way from the other, one reflex appears without the other. The result is this reflex or that reflex, but not the two together.¹⁸³ Excitation of the central end of the afferent root of the eighth or seventh cervical nerve of the monkey evokes reflexly in the same individual animal sometimes flexion at elbow, sometimes extension. If the excitation be preceded by excitation of the first thoracic root the result is usually extension: if preceded by excitation of the sixth cervical root it is usually flexion. Yet though the same root may thus be made to evoke reflex contraction of the flexors or of the extensors, it does not, in my experience, evoke contraction in both flexors and extensors in the same reflex-response. Of the two reflexes on extensors and flexors respectively, either the one or the other results, but not the two together. Thus, in my experience, excitation of the seventh or eighth root never causes simultaneously with reflex contraction of the flexors of elbow a contraction of that part of the triceps which extends the elbow. The flexor-reflex when it occurs seems therefore to exclude the extensor-reflex, and vice versa. If there resulted a compromise between the two reflexes, so that each reflex had a share in the resultant, the compound would be an action which was neither the appropriate flexion nor the appropriate extension. Were there to occur at the final common path algebraical summation of the influence exerted on it by two opposed receptive arcs, there would result in the effector organ an action adapted to neither and useless for the purposes of either.

In the Coelenterate, Carmarina, a mechanical stimulus applied to the subumbrella causes, as in another Geryonid, Tiaropsis indicans,⁷² a reflex movement that brings the free end of the manubrium to the spot touched. Bethe reports²⁶³ that if two stimuli are applied simultaneously to opposite points of the discoid subumbrella, the points chosen being such that the manubrium is midway between them, the manubrium is moved toward the point at which the stimulus applied was the stronger. He adds that if both stimuli are of exactly equal strength the manubrium remains unmoved and uncontracted. To obtain such a result as this last with antagonistic spinal reflexes in the vertebrate would obviously be more difficult, because the more complex the preparation and the nervous system involved, the more difficult it will be at any moment to exactly balance the two reflexes. But, apart from that, the observation on Carmarina is an analogue of that in the monkey’s arm.

This dilemma between reflexes would seem to be a problem of frequent recurrence in reflex co-ordination. We note an orderly sequence of actions in the movement of animals, even in cases where every observer admits that the co-ordination is merely reflex. We see one act succeed 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 one stimulus overlaps another in regard to time. Thus each reflex breaks in upon a condition of relative equilibrium, which latter is itself reflex. In the simultaneous correlation of reflexes some reflexes combine harmoniously, being reactions that mutually reinforce. These may be termed allied reflexes, and the neural arcs which they employ allied arcs. On the other hand, some reflexes, as mentioned above, are antagonistic one to another and incompatible. These do not mutually reinforce, but stand to each other in inhibitory relation. One of them inhibits the other, or a whole group of others. These reflexes may in regard to one another be termed antagonistic; and the reflex or group of reflexes which succeeds in inhibiting its opponents may be termed “prepotent” for the time being.

Allied reflexes. The action of the principle of the final common path may be instanced in regard to “allied arcs” in the scratch-reflex as follows. If, while the scratch-reflex is being elicited from a skin point at the shoulder, a second point distant, e. g. 10 cent. from the other point but also in the receptive field of skin, be stimulated, the stimulation at this second point favours the reaction from the first point. This is well seen when the stimulus at each point is of subminimal intensity. The two stimuli, though each unable separately to invoke the reflex, yet do so when applied both at the same time (Fig. 38). This is not due to overlapping spread of the feeble currents about the stigmatic poles of the two circuits used. Weak cocainization of either of the two skin points annuls it. Moreover, it occurs when localized mechanical stimuli are used. It therefore seems that the arcs from the two points, e. g. rα and rβ (Fig. 39 B) have such a mutual relation that reaction of one of them reinforces reaction of the other, as judged by the effect on the final common path.

It is obvious that such reinforcement — immediate spinal induction may occur in either of two ways. The diagram (Fig. 39 B) treats the final common path as if it consisted of a single individual neurone. The single neurone of the diagram stands for several thousands. It may be (1) that when the reflex is excited from rα only a particular group of the motor neurones composing the final common path is thrown into action, and similarly another particular group when the reflex is excited from Rβ. If the two groups in the final common path are separate groups, the explanation of the reinforcement shown in the muscular response may be by mechanical summation of contraction occurring in two separate fields of muscular tissue, the contraction of each too slight to cause perceptible movement by itself without the other. In other words, the reinforcement would be due not to the response in the set of neurones comprising the final common path (fc Fig. 39 B), being neurone for neurone more intense under the combined stimulation of rα and rβ than under stimulation of either singly, but the result would arise from the number of neurones in action in fc being simply greater under the stimulation of the two skin points than under stimulation of one of them only.

On the other hand, it may be (2) that all the neurones composing the final common path constitute together one almost unitary apparatus, so that stimulation at Rα excites or can excite them all, and similarly stimulation at Rβ excites or can excite them all. The question, therefore, regarding the mode of the reinforcement is a question between intensity and extensity. The scratch-reflex affords some opportunity for examining this question. The rhythm of the reflex has practically the same frequency whether the reflex be excited strongly or feebly: thus, whether the amplitude of the contractions be great or small, they recur with practically the same frequence. Suppose the reflex be excited by stimulation of the skin point R α (Fig. 39 B), and suppose the stimulus is weak, producing only a feeble reflex. Then let another skin point R β (Fig. 39 B) be stimulated while R α is being stimulated, and let the stimuli at R β be timed so as to fall alternately with those applied at R α. Then if the two paths impinge on two different sets of units in the compound group of motor neurones composing the final common path, evidence of two rhythms should appear, for the muscle-fibres (of the flexors of the hip) can respond to a much quicker rhythm than four per second. But, in fact, the result is that the rhythm appears unquickened and unaltered (Figs. 18, 19, 20, 21). There is not even a break or interference in it. It might be thought, therefore, that for some reason the stimulation of the second point, Rβ, is remaining ineffective altogether. But that is not so, because the stimulation at Rβ has often the effect of increasing the amplitude (Fig. 18) of the individual beats of the rhythmic reflex, though it does not alter the rhythm. This change in amplitude proves that the reflex is also in action from the second skin point as well as from the first. But there is no interference of the rhythms of the two reflexes. Evidently the central mechanism on which Rβ acts is subjected by Rα to a refractory state which the stimulation at Rβ does not break through. That is, the refractory state obtaining in the central mechanism under action from Ra obtains at the same moment for excitation reaching it from Rβ. The central mechanism acted on by Rβ must therefore belong in common to the reflexes from Rα and Rβ respectively. And since the experiment can be repeated with a great number of different pairs of points in the receptive field, practically the whole of the neurones of fc are common to all the receptive points in the receptive field. Similarly it is shown by Zwaardemaker²⁷² that the refractory phase demonstrated by him in reflex deglutition spreads to the whole of the reflex centre, both right and left.

Again, it was shown above, under the heading of summation, that although a single-induction shock, even though strong, does not in my experience ever evoke a scratch-reflex, a series of even feeble shocks does so by summation. But in order to act by summation the individual shocks must follow each other at not too long an interval of time, the interval being caeteris paribus shorter the less intense the shocks. Suppose an induction shock be applied to Rα at such a frequence, e. g. once a second, that at the intensity chosen they fail to evoke the reflex. Suppose that a series of induction shocks be applied to Rβ similarly unable to evoke the reflex. Then suppose that while the stimuli are being applied to Rα and fail to evoke the reflex, the other series of stimuli are applied to Rβ, and are so applied that each stimulus at Rβ falls at a moment of time midway between the moments of application of the stimuli at Rα. The stimuli thus conjoined suffice to evoke the reflex. Evidently the internal excitatory change is not confined to the arcs to whose receptive ends the external stimulus is actually applied. It spreads to other arcs belonging to the same “type-reflex,” especially to those arising near to those actually stimulated in the receptive field. A subminimal stimulus at one point in the field favours response to a subsequent stimulus at a second point in the field even 8 centimeters distant — so long as the second stimulus follows within summation time; but the summation time is shorter than when stimuli follow each other at one and the same spot, and is especially so when the points stimulated lie distant from one another. Hence we may draw some sort of picture of the extent of the excitatory internal change induced in this reflex mechanism by a single momentary stimulus: as to distribution in time the change fades off gradually from an early maximum to a trace just detectable after 1400 σ, if the stimulus be strong: as to distribution in space it spreads from the peripherally stimulated arcs A A themselves as centre to the intraspinal parts of other arcs of the same type-reflex, but among these it affects those starting in the skin as neighbours to A A more than ones more distant in origin, and it endures less long in these than in its own arcs; hence the shorter summation time. Exner early insisted on the close connection between “facilitation” (bahnung) and summation. The above immediate spinal induction illustrates it well.

The mutual reinforcement of action exercised by the two scratch-reflexes one upon another appears therefore to be an affair of intensity. This does not, however, exclude the existence of extensity as a factor also in some degree. There is evidence, adverted to above (p. 76, Lect. III), that makes it likely that in very weak reflexes not all the individual neurones composing the final common path are in action, although in stronger reflex reactions all may be in action.

In the scratch-reflex the mutual reinforcing power between the reflexes falls as the distance between the receptors of the arcs increases. The nearer the skin points of Rα and Rβ lie together the greater the mutual reinforcement between the action of their arcs on fc. This suggests an explanation by physical diffusion of the stimulating currents applied to Rα and Rβ; but for the reasons above mentioned this overlap of stimulus can, I consider, be excluded. Light is however thrown on this proportion between the degree of reinforcement and the degree of nearness of the receptive points by another feature of the reflex. The scratch-reflex in the spinal dog carries the foot approximately toward the place of stimulation. In the spinal dog the reflex does not succeed in bringing the foot actually to the irritated skin point, yet when the irritation lies far forward the foot is carried further forward, and when the irritation is far back the foot is carried further back. A scratch-reflex evoked by a stimulus applied far back and high up in the dorsal skin is therefore not wholly like a scratch-reflex evoked from far forward and low down. These differences are easily registered in graphic tracings of the movement at hip (Fig. 40). It is found that the greater the likeness between the two scratch-reflexes which two separate skin points initiate, the stronger the mutual reinforcement between the action of those two receptive points upon the final common path fc (Figs. 38 and 41). In other words, the coalition between reflexes is greater the greater the likeness between them, and that likeness increases with the nearness of their receptive points to one another in the skin surface. I have seen the mutual reinforcement demonstrable with skin points 20 centimeters apart in the receptive field of the scratch-reflex, but I have failed to find this mutual reinforcement between the most distant arcs of the receptive field. Whether coalition fades into mere indifference or passes over into antagonism I have not at present the evidence to judge.

The whole collection of points of skin surface from which the scratch-reflex can be elicited may conveniently be termed the receptive field of that reflex. And the receptive field of a reflex is analyzable into points from each of which the reflex can be evoked. But the reflex as elicited from various points in its receptive field is not in the case of all the points exactly the same reflex; e. g., the foot is directed to somewhat different places according as the scratch-reflex is elicited from this or that point. A similar feature is seen in the “wisch-reflex” of the spinal frog’s hind leg. That is to say, when we speak of the “scratch-reflex” in general, what we mean strictly speaking is a group of reflexes all more or less alike, all using approximately the same motor apparatus in approximately the same way, and all more or less conforming to the same type. And this group of individual reflexes forms a physiological group not only on account of their similarity, but also because they act harmoniously upon the same final common path, and in many cases reinforcement occurs between them in their action on that common path. Their intraspinal mechanisms are more or less knit together into an harmonious whole. A reflex, e. g. the scratch-reflex, when referred to in general, may be conveniently termed a type-refl x. The kind of harmonious relationship which holds between the individual reflexes comprised under one and the same type-reflex may be indicated by recognizing them as “allied reflexes” and their arcs as “allied arcs.”

Similarly with the various other reflexes. The flexion-reflex of the hind limb, the pinna-reflex, the extensor-thrust, the crossed extension-reflex of the hind limb, the torticollis reflex, etc.; these are each of them type-reflexes. Each is a group of reflexes. The individual reflexes comprised in each of these type-reflexes have such mutual relationship between themselves that they act harmoniously together on the same final common path, and are therefore “allied reflexes” and employ “allied arcs.”

The extent of the receptive field of each type-reflex is usually wide. It is much wider in some type-reflexes than in others; thus, that of the direct flexion-reflex of the hind limb of the dog is more extensive than that of the extensor-thrust of the limb. Within the receptive field of any given type-reflex not all the receptive points equally potently excite the reflex. From certain areas of points the reflex can be most easily evoked, from certain others least easily, and from the rest of the field with intermediate degrees of facility. The area whence the reflex can be evoked with most difficulty is usually the circumferential zone of the field, the width of the zone varying along different radii. The area where the threshold stimulus is lowest lies usually fairly remote, though not equally remote, from all the borders of the field. The reflex effect of a weak stimulus in this central focal area seems to resemble the effect of a stronger stimulus applied in the border zone of the field. Reflexes of an intensity unobtainable from the border zone of the field can be easily provoked by stimulation of the focal area of the field. In the flexion-reflex of the dog’s hind limb the toe-pads and plantar cushion are in the focal area of the receptive field. In the scratch-reflex of the dog the focal area is along that part of the field that lies next to the mid-dorsal line of the trunk, and especially (as seen after low cervical transection) near the posterior end of the scapular region; e.g. in Fig. 39 A, from 5 to 15 in the horizontal figures and dorsal to 9 in the vertical row. The difference between the threshold value of the stimulus for the reflex at different points in the field is very considerable indeed. Although the absolute value of the threshold may vary considerably in one and the same animal at different times, even from day to day, the relative values as between separate areas in the same field is usually about the same. But this relative value may be upset by “local fatigue,” etc. The coalescence of allied reflexes excited from one receptive field tends to make weak stimuli applied to an extensive area equivalent to intenser stimuli applied to a smaller area. G. H. Parker²⁵³ shows that in the positive phototropism of the frog to light falling on its skin the strength of the reaction varies in proportion with the extent of skin exposed to the light.

Reflex complication. One and the same field of receptive surface may, and usually does, contain receptive points of more than a single species. Thus, a skin-field may contain receptors some of which are adapted for mechanical stimuli, some for chemical, some for thermal, and so on. In this case receptors of two different species may not both of them initiate reflexes which belong to the same type-reflex, i. e. which have the relation to one another of “allied reflexes.” For instance, in the planta of the dog’s foot receptors coexist²⁵² of which one set are excited by mechanical stimuli of harmless (tactual) kind, the other set by stimuli of nocuous kind. The reflexes elicited from the limb through these two kinds of receptors respectively do not reinforce each other but oppose each other. On the other hand, in the tentacles of the Actinian, Aiptasis saxicola, there coexist at the surface receptors of two species,¹⁴⁶ one receptive for tactual stimuli the other for certain chemical stimuli (Nagel). The reflexes elicited through these by combination of mechanical with certain chemical stimuli seem to combine harmoniously and mutually reinforce each other (Nagel). And a similar occurrence seems evidenced by observations on the barblets of Siluroid fishes, e. g. Ameiurus²⁵⁰ (C. J. Herrick). The combining of such reflexes is comparable with the associative combination of disparate sensations for which Herbart¹²⁶ introduced the term “complication.”

Analogy exists here, as it should, between the compatibility of reflex movements from two receptors of different species and the compatibility of sensations which, judging by inference from our own introspection, might be initiated from such receptors. Skin-pain is sensually incompatible with pure touch, the dolorous suppressing the tactual, just as the noci-ceptive reflex in the “spinal” dog’s hind leg suppresses the merely tango-ceptive. But gustatory and tactual sensations excited from the same receptive surface, e. g. the tongue, habitually blend harmoniously.

Proprio-ceptive reflexes. There exists a further important class of cases in which reflexes have “allied” relation. Throughout a vast range of animal types the bulk formed by the organism presents to the environment a surface sheet of cells, and, beneath that, a mass of cells more or less screened from the environment by the surface sheet. Many of the agencies by which the environment acts on the organism do not penetrate it far enough to reach the cells of the deep mass inside. Bedded in the surface layer of the organism are numbers of receptor cells constituted in adaptation to the stimuli delivered by environmental agencies. But the organism itself, like the world surrounding it, is a field of ceaseless change, where internal energy is continually being liberated, whence chemical, thermal, mechanical, and electrical effects appear. It is a microcosm in which forces which can act as stimuli are at work as in the macrocosm around. The deep tissues underlying the surface sheet are not provided with receptors of the same kinds as those of the surface, yet they are not devoid of receptors. They have receptors specific to themselves. The receptors which lie in the depth of the organism are adapted for excitation consonantly with changes going on in the organism itself, particularly in its muscles and their accessory organs (tendons, joints, blood-vessels, etc). Since in this field the stimuli to the receptors are given by the organism itself, their field may be called the proprio-ceptive field.

There exist, therefore, two primary distributions of the receptor organs, each a field in certain respects fundamentally different from the other. The surface field lies freely open to the numberless vicissitudes of the environment. It has felt for countless ages the full stream of the varied agencies forever pouring upon it from the outside world. This field, extero-ceptive as it may be called, is rich in the number and variety of receptors which adaptation has evolved in it.

The excitation of the receptors of the proprio-ceptive field in contradistinction from those of the extero-ceptive is related only secondarily to the agencies of the environment. The proprio-ceptive receive their stimulation by some action, e. g. a muscular contraction, which was itself a primary reaction to excitation of a surface receptor by the environment. The primary reaction is excited in the majority of cases by a receptor of the extero-ceptive field, that field so rich in the number and the variety of its receptors. Reflexes arising from proprioceptive organs come therefore to be habitually attached and appended to certain reflexes excited by extero-ceptive organs. The reaction of the animal to stimulation of one of its extero-ceptors excites certain tissues, and the activity thus produced in these latter tissues excites in them their receptors, which are proprio-ceptors. Thus, in a muscular movement induced by a stimulus to the skin of the spinal dog, the change in form and tension of the muscles, the movements of the joints, etc., excite the receptors in these structures, and these in turn initiate a reflex in their own arcs and their reaction often has an “allied” relation to the reflex reaction excited from the skin.

Alliance of proprio-ceptive with extero-ceptive reflexes. In one of the type-reflexes previously described, namely the scratch-reflex, the reflex-arcs which provoke the reflex arise in a large continuous area of skin, and all excite the same motor neurones, that is, are mutually related as allied arcs. The area of skin whence these arcs arise we termed the receptive field of the reflex. The afferent nerves of the muscles which execute the scratching movement do not, when themselves excited, evoke the scratch-reflex; nor does the severance of the afferent nerves of the muscles obviously impair or alter the scratch-reflex. With the flexion-reflex of the limb it is different. The reflex, like the scratch-reflex, has a cutaneous field of origin. It is provocable from arcs arising in a large area of the skin covering the hind limb. But the flexion-reflex can in addition be excited from various of the afferent nerves of the muscles of the limb. Thus stimulation of the central end of the nerve of the flexor muscles themselves excites the reflex. It is similarly elicitable from the afferent nerve of the extensor muscle (vasto crureus) of the knee. And the reflex excited from the muscles of the limb allies itself with the reflex excited from the skin of the limb. A subliminal stimulation of the afferent nerve of the hamstring muscles applied simultaneously with a subliminal stimulation of the skin of the foot results in a marked flexion-reflex.

In the case of the flexion-reflex, therefore, the receptive field includes not only reflex-arcs arising in the surface field, but reflex-arcs arising in the depth of the limb. Combined therefore with an extero-ceptive area, this reflex has, included in its receptive field, a proprio-ceptive field. The reflex-arcs belonging to its extero-ceptive and proprio-ceptive components co-operate harmoniously together, and mutually reinforce each other’s action. In this class of cases the reflex from the muscle-joint apparatus seems to reinforce the reflex initiated from the skin.

Reflex flexion of the leg is induced by stimulation of the central end of the nerve of a hamstring muscle. Since mechanical stimulation of these flexor muscles, e. g. kneading or squeezing them, excites a reflex inhibition of the contraction of their antagonists, which as we have seen is part of the flexion-reflex itself, it would seem likely that their own contraction will excite a flexion-reflex. A flexion-reflex excited from the skin would thus in its progress tend to induce a secondary flexion-reflex which would reinforce the primary one, for when excited apart the reflexes excited from an afferent nerve of the foot and from the hamstring nerve are closely similar (Fig. 37). The case therefore resembles that of the reflexes from two adjacent spots in the receptive field of the scratch-reflex. The reflex elicited from the skin of the foot and that elicited from the hamstring muscle are “allied” reflexes. There is here alliance and “bahnung” between a reflex of the proprio-ceptive field and a reflex of the extero-ceptive field.

Similarly, if the knee-jerk is accepted as a sign of a tonic reflex originated by the afferent nerve-endings in the knee-jerk muscles themselves, many reflexes elicitable from the extero-ceptive surface are well known to reinforce it. A comprehensive account of these was furnished in Sternberg’s monograph¹⁴² (1893). Here again the reflexes which are “allied,” exhibiting reinforcement and “bahnung,” belong not in the ordinary sense to the same category, but have reflex-arcs commencing in receptive organs of different species. Yet the arcs are “allied arcs,” for they act harmoniously on the same final common path.

That the prolongation of the reflex contractions characteristic of strychnine is due to excitation of muscular (proprioceptive) reflexes (Baglioni)^{207, 245} secondary to a reflex elicited from other receptors is again a further illustration of the secondary relation of proprio-ceptive reflexes to extero-ceptive pointed out above.

Wider combinations of reflexes. And reflexes whose arcs commence in receptive fields even wider apart than those mentioned above may also have “allied” relation. In the bulbo-spinal dog stimulation of the outer digit of the hind foot will evoke reflex flexion of the leg, and stimulation of each of the other digits evokes practically the same reflex; and if stimulation of several of these points be simultaneously combined the same reflex as a result is obtained more readily than if one only of these points is stimulated. And to these stimulations may be added simultaneously stimulation of points in the crossed fore foot; stimulation there yields by itself flexion of the hind leg; and under the simultaneous stimulation of fore and hind foot the flexion of the leg goes on as before, though perhaps more readily; that is, the several individual reflexes harmonize in their effect on the hind limb. Further, to these may be added simultaneous stimulation of the tail, and of the crossed pinna; and the reflexes of these stimulations all coalesce in the same way in flexion of the hind leg. Exner⁹⁹ has shown that in exciting different points of the central nervous system itself, points widely apart exert bahnung for one another’s reactions, and for various reflex reactions induced from the skin. Thus reflexes originated at different distant points, and passing through paths widely separate in the brain, converge to the same motor mechanism (final common path) and act harmoniously upon it. Reflex-arcs from widely different parts conjoin and pour their influence harmoniously into the same muscle. The motor neurones of a muscle of the knee are the terminus ad quem of reflex-arcs arising in receptors not only of its own foot, but from the crossed fore foot and pinna, and tail, also undoubtedly from the otic labyrinth, olfactory organs, and eyes. Thus, if we take as a standpoint any motor-nerve to a muscle it consists of a number of motor neurones which are more or less bound into a unit mechanism; among the reflex-actions of the organism a number can all be brought together as a group, because they all in their course converge together upon this motor mechanism, this final common path, activate it, and are in harmonious mutual relation with regard to it. They are in regard to it what were termed above “allied” reflexes.

Allied inhibitory reflexes. The examples of allied reflexes cited so far have had for their result on the final common path an increase of its activity; that is to say, of its activity as a discharger of nervous impulses. But the same final common path can be shown to be connected also with certain reflexes initiable from other receptive points which depress its activity as a discharger of nervous impulses. The reflexes exerting this influence are “inhibitory,” whereas the reflexes mentioned before may be termed “excitatory.” Inhibitory reflexes are accessible to study chiefly through the kind of refractory state which they impress upon the commencement of the efferent part of their arc, as tested by concurrent excitations of reflexes which should excite it.

Just as in regard to one and the same final common path certain excitatory reflexes act harmoniously together and reinforce one another, so also do certain inhibitory reflexes. Thus, reflex inhibition of the flexors of the knee (spinal dog) is regularly excitable by stimulation of the skin of a digit of the crossed hind foot; and the concurrent stimulation of two or more digits and of the dorsum pedis of the crossed foot mutually combine and reinforce in their reflex inhibition of the knee-flexor: and to these may be added stimulation of the homonymous fore foot: all these reflexes combine harmoniously together in exerting a conjoint inhibitory influence on the knee-flexors. The alliance between reflexes in regard to any one final common path may be as wide and strong when the end-result of those reflexes is in the form of inhibition as when it is in the form of excitation. In addition, therefore, to the category of “allied excitatory” reflexes above mentioned there is a category of “allied inhibitory” reflexes. Under this latter category come subgroups analogous to the four already mentioned under allied excitatory reflexes. Thus: the reflex from the proprio-ceptive nerves of the hamstring muscles combines with and reinforces the flexion-reflex from the skin of the foot of the same leg in a resultant reflex inhibition of the extensors of the homonymous knee.

But there are, as we have seen, reflexes which are neither purely excitatory nor purely inhibitory. For instance, the flexion-reflex of the hind leg (cat and dog) is, as we have seen, at one and the same time excitatory of the flexor neurones (knee) and inhibitory of the extensor neurones (knee).

These reflexes of simultaneous double-sign may have “allied” relation with one another, e. g., the individual reflexes of the flexion type-reflex.

Also there are other reflexes neither purely excitatory nor purely inhibitory, namely, the reflexes which during the continuance or repetition of the exciting stimulus exhibit refractory period. Several rhythmic reflexes seem of this character, e. g. the swallowing reflex, the scratch-reflex. If we regard refractory phase as a kind of inhibition, then these reflexes are, as we have seen, reflexes of successive double-sign. And these also can be “allied” in their relation one to another.

Antagonistic reflexes. But not all reflexes connected to one and the same final common path stand to one another in the relation of “allied reflexes.” Suppose during the scratch-reflex a stimulus be applied to the foot not of the scratching side but of the opposite side (Fig. 39 B, r). The left leg, which is executing the scratch-reflex in response to stimulation of the left shoulder skin is cut short in its movement by the stimulation of the right foot, although the stimulus at the shoulder to provoke the scratch movement is maintained unaltered all the time. The stimulus to the right foot will temporarily interrupt a scratch-reflex, or will cut it short or will delay its onset; which it does of these depends on the time-relations of the stimuli (Fig. 42). The inhibition of the scratch-reflex occurs sometimes when the contraction of the muscles innervated by the reflex conflicting with it is very slight. There is interference between the two reflexes and the one is inhibited by the other. The final common path used by the left scratch-reflex is also common to the reflex elicitable from the right foot. This latter reflex evokes at the opposite (left) knee extension; in doing this it causes steady excitation of extensor neurones of that knee and steadily inhibits the flexor neurones.³⁰⁰ But the scratch-reflex causes rhythmic excitation of the flexor neurones. Therefore these flexor neurones in this conflict lie as a final common path under the influence of two antagonistic reflexes, one of which would excite them to rhythmical discharge four times a second, while the other would continuously repress all discharge in them. There is here an antagonistic relation between reflexes embouching on one and the same final common path.

In all these forms of interference there is a competition, as it were, between the excitatory stimulus used for the one reflex and the excitatory stimulus for the other. Both stimuli are in progress together, and the one in taking effect precludes the other’s taking effect as far as the final common path is concerned; and the precise form in which that occurs depends greatly on the time-relations of application of the two stimuli competing against each other.

Again, if, while stimulation of the skin of the shoulder is evoking the scratch-reflex, the skin of the hind foot of the same side is stimulated, the scratching may be arrested³⁰⁰ (Fig. 43). Stimulation of the skin of the hind foot by any of various stimuli that have the character of threatening the part with damage causes the leg to be flexed, drawing the foot up by steady maintained contraction of the flexors of the ankle, knee, and hip. In this reaction the reflex-arc is (under schematic provisions similar to those mentioned in regard to the scratch-reflex schema) (i) the receptive neurone (Fig. 39 B, l ), noci-ceptive, from the foot to the spinal segment, (ii) the motor neurone (Fig. 39 B, fc) to the flexor muscle, e.g. of hip (a short intra-spinal neurone; a Schalt-zelle (v. Monakow) is probably existent between (i) and (ii) but omitted for simplicity). Here, therefore, there is an arc which embouches into the same final common path fc as do Ra and Rβ, Fig. 39 B. The motor neurone fc is a path common to it and to the scratch-reflex arc; both these arcs employ the same effector organ, namely, the knee-flexor, and employ it by the common medium of the final path fc. But though the channels for both reflexes embouch upon the same final common path, the excitatory flexor effect specific to each differs strikingly in the two cases. In the scratch-reflex the flexor effect is an intermittent effect; in the noci-ceptive flexion-reflex the flexor effect is steady and maintained. The accompanying tracing (Fig. 43) shows the result of conflict between the two reflexes. The one reflex displaces the other at the common path. Compromise is not evident. The scratch-reflex is set aside by that of the noci-ceptive arc from the homonymous foot. The stimulation which previously sufficed to provoke the scratch-reflex is no longer effective, though it is continued all the time. But when the stimulation of the foot is discontinued the scratch-reflex returns. In that respect, although there is no enforced inactivity there is an interference which is tantamount to, if not the same thing as, inhibition. Though there is no cessation of activity in the motor neurone, one form of activity that was being impressed upon it is cut short and another takes its place. A stimulation of the foot too weak to cause more than a minimal reflex will often suffice to completely interrupt, or cut short, or prevent onset of, the scratch-reflex.

The kernel of the interference between the homonymous flexion-reflex and the scratch-reflex is that both employ the same final common path fc to different effect—just as in the interference between the crossed extension-reflex and the scratch-reflex. Evidently, the homonymous flexion-reflex and the crossed extension-reflex both use the same final common path FC. And they use it to different effect. The motor neurone to the flexor of the knee being taken as representative of the final common path, the homonymous flexion-reflex excites it to discharging activity, but the crossed extension-reflex inhibits it from discharging. Hence if, while the direct flexion-reflex is in progress the crossed foot is stimulated, the reflex of the knee-flexor is inhibited. The crossed extension-reflex therefore inhibits not only the scratch-reflex but also the homonymous flexion-reflex.

Further, in all these interferences between reflexes the direction taken by the inhibition is reversible. Thus, the scratch-reflex is not only liable to be inhibited by, but is itself able to inhibit, either the homonymous flexion-reflex or the crossed extension-reflex; the homonymous flexion-reflex is not only capable of being inhibited by the crossed extension-reflex (Fig. 32, p. 98), but conversely in its turn can inhibit the crossed extension-reflex (Figs. 33, 35, p. 100). These interferences are therefore reversible in direction. Certain conditions determine which reflex among two or more competing ones shall obtain mastery over the final common path and thus obtain expression.

Therefore, in regard to the final common path fc the reflexes that express themselves in it can be grouped into sets, namely those which excite it in one way, those which excite it in another way, and those which inhibit it. The reflexes composing each of these sets stand in such relation to reflexes of the same set that they are with them “allied reflexes.” But a reflex belonging to any one of these sets stands in such relation to a reflex belonging to one of the other sets that it is in regard to the latter an “antagonistic” reflex. This correlation of reflexes about the flexor neurone in the leg so that some reflexes are mutually allied and some are mutually antagonistic in regard to that neurone, may serve as a paradigm of the correlation of reflexes about every final common path, e. g. about every motor nerve to skeletal muscle.³⁰⁰

As to the intimate nature of the mechanism which thus, by summation or by interference, gives co-ordination where neurones converge upon a common path it is difficult to surmise. In the central nervous system of vertebrates, afferent neurones A and B in their convergence toward and impingement upon another neurone Z, towards which they conduct, do not make any lateral connection directly one with the other—at least there seems no clear evidence that they do. It seems then that the only structural link between A and B is neurone Z itself. Z itself should therefore be the field of coalition of A and B if they transmit “allied” reflexes.

It was argued above (Lecture III), from the morphology of the perikaryon, that it must form, in numerous cases, a nodal point in the conductive lines provided by the neurone. The work of Ramon-y-Cajal, van Gehuchten, v. Lenhossék, and others with the methods of Golgi and Ehrlich, establishes as a concept of the neurone in general that it is a conductive unit wherein a number of branches (dendrites) converge toward, meet at, and coalesce in a single outgoing stem (axone). Through this tree-shaped structure the nervous impulses flow, like the water in a tree, from roots to stem. The conduction does not normally run in the reverse direction. The place of junction of the dendrites with one another and with the axone is commonly the perikaryon. This last is therefore a nodal point in the conductive system. But it is a nodal point of particular quality. It is not a nodal point where lines meet to cross one another, nor one where one line splits into many. It is a nodal point where conductive lines run together into one which is the continuation of them all. It is a reduction point in the system of lines. The perikaryon with its convergent dendrites is therefore just such a structure as spatial summation and immediate induction would demand. The neurone Z may well, therefore, be the field of coalition, and the organ where the summational and inductive processes occur. And the morphology of the neurone as a whole is seen to be just such as we should expect, arguing from the principle of the common path.

With the phenomenon of “interference” the question is more difficult. There it is not clear that the field of antagonism is within the neurone Z itself. The field may be synaptic. We have the demonstration by Verworn²⁰⁷ that the interference produced by A at Z for impulses from B is not accompanied by any obvious change in excitability of the axone of Z. Z, if itself the seat of inhibition, might have been expected to exhibit that inhibition throughout its extent. This, as tested by its axone, it does not do. There exist, it is true, older experiments by Uspensky,⁴² Belmondo and Oddi,¹²² etc., according to which the threshold of direct excitability of the motor root is lowered by stimulation of the afferent root. This points to an extension of the facilitation effect through the whole motor neurone, conversely to Verworn’s demonstration for central inhibition. Verworn’s experiment and its result is very clear. It leads us to search for some other mechanism common to A and B to which might be attributable their mutual influence on each other’s reactions. But if we admit the conception, argued above (Lecture I), that at the nexus between A and Z, i. e. at synapse A Z, and similarly between B and Z, i. e. at synapse B Z, there exists a surface of separation, a membrane in the physical sense, a further consequence seems inferable. Suppose a number of different neurones A, B, C, etc., each conducting through its own synapse upon a neurone Z. The synapses A Z, B Z, C Z, etc., are all surfaces or membranes into which Z enters as a factor common to them all. A change of state induced in neurone Z might be expected to affect the surface condition or membrane at all of the synapses, since the condition of Z is a factor common to all those membranes. Therefore a change of state (excitatory or inhibitory) induced in Z by any of the neurones A, B, C, etc., playing upon it would enter as a condition into the nervous transmission at the other synapses from the other collateral neurones. In harmony with this is the spread of refractory state in the neurones as mentioned above (p. 122). A change in neurone Z induced by neurone A, playing upon it, in that case seems to affect its point of nexus with the other neurones B, C, etc., also. It is conceivable that the phenomena of interference may be based in part at least on such a condition. The neurone threshold of Z for stimulation through B will be to some extent a function of events at synapses A Z.

Partial interference. It has to be remembered, however, that the total final common path, although a functional unity, is often, especially in compound reflexes, a complex one. It frequently happens that the set of final paths of one complex reflex is partly coextensive with the set of final common paths of another reflex. With two complex reflexes it can happen that the reflexes are “allied reflexes” in regard to one part of their multiple final common path and are antagonistic reflexes in regard to another part of it. We may illustrate this from the scratch-reflex again. The scratch-reflex was mentioned above as being unilateral. That is not strictly the case. It is true that if the right scapular region be stimulated, the right hind leg scratches; and if the left scapular region be stimulated the left hind leg scratches. But if both shoulders be stimulated at the same time, one or the other leg scratches, but not the two together. This shows that the scratch-reflex, though at first sight it appears unilateral, is not strictly so. Suppose the left shoulder stimulated, the left leg then scratches; but if the right leg is examined it is found to present slight steady extension with some abduction.

This extension of the crossed hind leg which accompanies the scratching movement of the homonymous hind leg contributes to support the animal on three legs while it scratches with the fourth. Suppose stimulation at the left shoulder evoking the scratching movement of the left leg, and the skin of the right shoulder then appropriately and strongly stimulated. This latter stimulus often inhibits the scratching movement in the opposite leg and starts it in its own.³⁰⁰ That is, the stimulus at the right shoulder not only sets the flexor muscles of the leg of its own side into scratching action, but it inhibits the flexor muscles of the opposite leg, because with excitation of the extensors of the latter leg goes inhibition of their antagonists, the flexors. The motor neurones of the flexor muscles of the left leg are part of the final common path not only of the scratch-reflex of the left shoulder, but also of the scratch-reflex of the right shoulder; but in the former case the final common path is thrown into rhythmic discharging activity, in the latter case it is steadily inhibited from discharging activity.

Again, the homonymous flexion-reflex of the hind leg (spinal dog) is only the main part of a larger complex reflex which is bilateral (Fig. 37), and consists of flexion of the same side leg and extension of the crossed leg (the crossed extension-reflex). This being so, the mutual relation between the complete scratch-reflex, e. g. of left foot, and the complete noci-ceptive reflex of the same foot, is that the homonymous uncrossed parts of each reflex interfere and are related mutually as antagonistic reflexes; but the crossed parts of each reflex coalesce in excitation of the extensor neurones and inhibition of the flexor neurones of the right leg, and are related mutually as allied reflexes.

It is the transference of the final common path from the group of one set of reflexes to another which constitutes the change which occurs at each step of the orderly sequence of reaction that we see normally succeed each other in animal behaviour—leaving aside all question of consciousness in relation to the sequence. This transference is most obvious when the sets of reflexes between which the final common path is exchanged are antagonistic reflexes. Two classes of this kind of case of specially common occurrence are “alternating reflexes” and “compensatory reflexes” (Lecture VI.).

Number of common paths. The interaction of reflexes has been here so far spoken of chiefly in regard to the final common path, as if the arcs of reflexes met at the final common path only. But, as stated above, reflex-arcs, especially the longer ones and those commencing in receptors far apart, converge and meet to some extent before they reach their final common path. The receptive neurones, i. e. private paths of the receptors, usually—perhaps always—reach internuncial paths (J. Hunter, 1770), which in turn conduct and converge to final paths or to further internuncial paths. The internuncial paths are thus themselves in various degrees common to groups of receptive neurones impinging upon them. They are therefore themselves, to some extent, common paths.³⁰⁰ There can be little doubt that in the scratch-reflex the long descending proprio-spinal neurone (Fig. 39 B, pa or pβ) is connected not with one but with a whole group of afferent neurones (private paths) from the scalptor receptors in that part of the skin-field of the scratch-reflex which corresponds with its own spinal segment. Its internuncial path is therefore common to impulses transmitted to the central organ by many receptive paths. Again, the structure of the retina (Cajal), olfactory bulb (Cajal), etc., gives evidence that the conducting fibres of whole groups of receptors impinge together upon individual neurones of the next relay. Thalamic neurones form a path upon which the dorsal-column-fillet and spino-cerebellar-peduncular paths converge. Each internuncial path is therefore usually, to some extent, a common path,¹⁸⁶ just as usually the receptive neurone, i. e. private path, itself is common to a small number of receptors. The ultimate path, therefore, differs from the intermediate paths only in that it exhibits communism in the highest degree; it is to distinguish it from internuncial common paths that it was termed above the final common path.

Since each instance of convergence of two or more afferent neurones upon a third, which in regard to them is efferent, affords, as shown above, an opportunity for coalition or interference of their actions, each structure at which it occurs is a mechanism for co-ordination.³⁰⁰

Whatever may be the intimate nature of this mechanism which gives co-ordination by the formation of a common path from tributary paths, such common paths exist in extraordinary profusion in the architecture of the gray-centred nervous system of vertebrates. Two features of that system indicate this clearly. Enumerations by Donaldson and his co-workers^{264, 265} show that the afferent fibres (private paths) entering the human spinal cord three times outnumber the efferent (final common paths) which leave it. Add the cranial nerves and the so-called optic nerves (in the latter, of course, formation of common paths having already begun in the retina the afferent paths are reduced in proportion) and the afferent fibres may be taken to be five times more numerous than the efferent. The receptor system bears, therefore, to the efferent paths the relation of the wide ingress of a funnel to the narrow egress. Further, each receptor stands in connection not with one efferent only but with many—perhaps with all, though as to some of these only through synapses of high resistance. The simile to a funnel will therefore be bettered by supposing that within the general systematic funnel, of which the base is five times wider than the egress, the conducting paths from each receptor may be represented as a funnel inverted so that its wider end is more or less coextensive with the whole plane of emergence of the final common paths.³⁰⁰ This gives some idea of the enormous formation of common paths from tributary paths which must take place.

Again, there is the accredited fact that under poisoning by strychnine a muscle can be excited from practically any afferent nerve in the body; in other words, that each final common path is in connection with practically each one of all the receptors of the body. It is not necessary to accept this literally; even if approximately true, it shows the profusion in which common paths exist.

Mutual indifference between reflexes. In view of such considerations the question arises, Are there in the body no reflexes absolutely neutral and indifferent one to another? That is, in regard to any one reflex using a given common path cannot another reflex be found which is wholly separate from it, and neither allied with it nor antagonistic to it? It was pointed out above that the coalition between scratch-reflexes gradually decreases as the interval between the receptive points at the skin surface becomes wider. Whether coalition fades into mere indifference or passes over into antagonism my own observations do not answer. But there are reflexes that do in the spinal dog appear neutral and indifferent to the scratch-reflex. For instance, a weak reflex of the tail may be obtained without any obvious interference between it and the scratch-reflex. The stronger two reflexes are, the less do they remain neutral one to another. Thus, a weak reflex may be excited from the tail of the spinal dog without interference with the stepping-reflex of the hind limb; but a strong reflex (strong stimulus) in the tail inhibits (Goltz) the stepping-reflex. The spatial field of response of a reflex increases with its intensity. Two reflexes may be neutral to each other when both are weak, but may interfere when either or both are strong; when weak they remain “local.”

But to show that reflexes may be neutral to each other in a spinal dog is not evidence that they will be neutral in the animal with its whole nervous system intact and unmutilated. It is a cardinal feature of the construction of the higher vertebrate nervous system that longer indirect reflex-arcs, attached as extra circuits to the shorter direct ones, all pass through the brain. With those former intact the number of reflexes neutral one to another might be fewer. In presence of the arcs of the great projicient receptors (Lect. IX) and the brain there can be few receptive points in the body whose activities are totally indifferent one to another. Correlation of the reflexes from points widely apart is the crowning contribution of the brain towards the nervous integration of the individual.

Our conception comes therefore to this. About any final common path a great number of, or all, the receptive arcs of the nervous system are arranged and are divisible into sets that do not act alike upon it. It might at first be thought that there would be simply two such sets, namely, those that excite it and those that inhibit it. But it must be remembered that we are only at the beginning of knowledge of differences of time-relations between different type-reflexes. Thus (Lectures II, III) at the knee of the spinal dog the time-relations of the extensor-thrust are vastly different from those of the crossed extension-reflex, and these again from the extensor tonus that supports the kneejerk, and these again from the scratch-reflex, and so on. Of the reflexes that excite a final common path some evidently excite it in a manner very different from that in which some others excite it; their excitations if concurrent interfere. We must therefore allow that the sets may be more than two, if the criterion for distinguishing the sets be interference, i. e. interruption, displacement, or extinction at the final common path of one reflex by another.

The final common path is therefore an instrument passive in the hands of certain groups of reflex-paths. I have attempted to depict this very simply in Fig. 44. There certain type-reflexes are indicated by lines representing their paths. The final common path (fc) selected is the motor neurone of the vasto-crureus of the dog or cat. Reflexes that act as “allied reflexes” on fc are represented as having their terminals joined together next the final common path. Reflexes with excitatory effect (+ sign) are brought together on the left, those with inhibitory (− sign) on the right. Of the reflex pairs formed by the two reflexes which two symmetrical receptive points, one right and one left, yield in regard to the final common path, one of the pair only is represented, in order to simplify the diagram. To have a further indication of the reflexes playing upon fc, all that is required is to add to the reflexes indicated in the diagram for fc, a set of reflexes similar to those given in the diagram for fc′, for they must be added if the remaining members of the right and left reflex pairs from various parts of the body be taken into account. It is noteworthy that in many instances the end-effect of a spinal reflex initiated from a surface point on one side is bilateral and takes effect at symmetrical parts, but is opposite in kind at those two parts, e. g. is inhibition at one of them, excitation at the other. Hence reflexes initiated from points corresponding one with the other in the two halves of the body are commonly antagonistic.