The Integrative Action of the Nervous System

Figure 40. — Tracing of the hip flexion in the '* scratch-reflex." The reflex was elicited by- unipolar faradization of a point of skin rather far back and near the dorsum in the recep- tive field. Considerable tonic flexion is seen to accompany the clonic scratching move- ments. Time in fifths of seconds. The lower signal marks the period of stimulation. Compare figures 14, 18, 19, in which the skin points excited lay farther forward and more ventral in the field and the subtratum of steady flexion is much less.

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 demon- strable 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 Figure 41. — Similar to Fig. 38, but with greater separation of the two skin points stimu- lated : in A the separation of the two points was 15 centimeters, in B it was 20 centi- meters.

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 indi- vidual 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-reflex. 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 ** al- lied 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 in-

termediate 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 unob- tainable 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 rela- tive 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. Parker ^^ 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 rela- tion to one another of " allied reflexes." For instance, in the

planta of the dog's foot receptors coexist ^^^ 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,^^^ one re- ceptive 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. Ameiurus^^ (C J. Herrick). The combining of such reflexes is comparable with the associa- tive combination of disparate sensations for which Herbart^^ 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 dolor- ous 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 re- ceptive surface, e. g. the tongue, habitually blend harmoniously.

Proprio-tcptive reflexea. There exists a further important class of cases mvWhich 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 envi- ronment acts 6n 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 acces- sory 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 re- ceptor 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 re- ceptors which adaptation has evolved in it.

The excitation of the receptors of the proprio-ceptive field in contradistinction from those of the exteroceptive 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 pri- mary reaction is excited in the majority of cases byra receptor of the extero-ceptive field, that field so rich in the number and the variety of its receptors. Reflexes arising from proprio- ceptive 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 recep- tive field of the reflex. The afferent nerves of the muscles which execute the scratching movement do not, when them- selves excited, evoke the scratch-reflex; nor does the sever- ance 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 cutane- ous 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 there- fore with an extero-ceptive area, this reflex has, included in its receptive field, 2. 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 reiiiforce 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 me- chanical 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 aff"erent 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 afl'erent 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 monograph ^^ (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 character- istic of strychnine is due to excitation of muscular (proprio- ceptive) reflexes (Baglioni) ^^''' ^^^ 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 men- tioned 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 stimula- tion 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 01 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 coa- lesce in the same way in flexion of the hind leg. Exner^^ has shown that in exciting different points of the central ner- vous 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 diff'erent 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 differ- ent parts conjoin and pour their influence harmoniously into the same muscle. The motor neurones of a muscle of the knee are the terminus ad qiiem of reflex-arcs arising in re- ceptors not only of its own foot, but from the crossed fore foot and pinna, and tail, also undoubtedly from the otic laby- rinth, olfactory organs, and eyes.

Thus, if we take as a stand- point 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 groups because they all in their course converge together upon this motor mechanism, this final common paihy 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 dis- charger of nervous impulses. But the same final common path can be shown to be connected also with certain reflexes initi- able 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 be- fore 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 rein- force 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 mutu- ally 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 men- tioned 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.y 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 con- tinuance 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 refrac- tory phase as a kind of inhibition, then these reflexes are, as we have seen, reflexes of successive double-sign. And these also can be " aUied" 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 ex- ecuting 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 com- mon 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.^^ 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 em- bouching 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 eff"ect precludes the other's taking effect as far as the final common path is con- cerned; 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 arrested ^^ (F'g- 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-cep- tive, 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 ex- istent 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 RyS, Fig. 39 B. The motor neurone FC is a path common to it and to the scratch-reflex

Figure 42 (opposite). — Interference of the reflex from the skin of the opposite foot with the scratch-reflex. FC, the flexor muscle of the left hip (Fig. 39 B, fc). r, the signal line the notch in which marks the beginning, continuance, and conclusion of a skin stimulation of the right foot (Fig. 39 B, r.) s, signal line similarly marking the period of stimulation of the skin of the left shoulder (Fig. 39 B, Ra). The ability of stimulus s to produce the scratch-reflex takes effect only on concluding stimulus r ; that is, s obtains connection with ihtjinal common path (the motor neurone of the flexor muscle) only on r's relinquishing it. Stimulus r, while excluding s from fc, causes slight contraction of fc's antagonist, and coincident slight relaxation of fc itself. Time in fifths of seconds. Read from left to right.

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 eff"ect 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-cep- tive 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 Figure 43 (opposite). — Interference between the reflex action of the left hip flexor, FC, caused by the nervous arc from the left foot (l, Fig. 39 B) and the scratch-reflex. The stimula- tion of the dorsal skin (Fig. 39 A) inducing the scratch-reflex began at the beginning of the notch in the signal line s, and continued throughout the period of that notch. Later, for the period marked by the notch in signal line L, the stimulation of the foot was made. This latter stimulation interrupts the clonic scratch-reflex in the manner shown. The time is registered above in fifths of seconds. The tracing reads from left to right. It is note- worthy that the interruption of the scratch-reflex by the foot-reflex is not established directly the foot-stimulus begins, and that it outlasts for a short time the application of the foot-stimulus.