The Integrative Action of the Nervous System

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 in- hibits not only the scratch-reflex but also the homonymous flexion-reflex.

Further, in all these interferences between reflexes the direc- tion 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 cor- relation of reflexes about the flexor neurone in the leg so that some reflexes are mutually allied and some are mutually antag- onistic 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.^^

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, afl"erent 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. Lenhoss^k, 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 den- drites with one another and with the axone is commonly the perikaryon. This last is therefore a nodal point in the conduc- tive 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 Verworn^o^ that the interference pro- duced 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,*^ Belmondo and Oddi,^^^ ^^^^^ according to which the threshold of direct excitability of the motor root is lowered by stimulation of the afferent root. This points to an exten- sion 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, z. 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 synap- ses 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 phe- nomena 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 contrib- utes 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.^^ 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 ex- tensors 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 rela- tion 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, espe- cially 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, 17 yd), 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^ There can be little doubt that in the scratch- reflex the long descending proprio-spinal neurone (Fig. 39 B, Pa or P/3) is connected not with one but with a whole group of afferent neurones (private paths) from the scalptor receptors in

hat part of the skin-field of the scratch-reflex which corresponds i)vith its own spinal segment. Its internuncial path is therefore ;ommon to impulses transmitted to the central organ by many receptive paths. Again, the structure of the retina (Cajal), olfac- tory 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-pedun- cular paths converge. Each internuncial path is therefore

Iisually, to some extent, a common path,^^ just as usually the eceptive neurone, /. e. private path, itself is common to a small lumber of receptors. The ultimate path, therefore, differs from he intermediate paths only in that it exhibits communism in he 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 inter- ference of their actions, each structure at which it occurs is a mechanism for co-ordination?^

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 ex- traordinary 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) enter- ing 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 re- ceptor 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 repre- sented 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.^^ This gives some idea of the enormous forma- tion 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 con- siderations the question arises, Are there in the body no re- flexes 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 ob- tained 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.

wo reflexes may be neutral to each other when both are weak, ut 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 verte- brate 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 indif- ferent 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 I 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 knee- jerk, 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 diff'erent 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. inter- ruption, displacement, or extinction at the final common path of one reflex by another.

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 Figure 44. — Explanation mainly in text, s stands for scratch-receptor, e and/ are extensor and flexor muscles of knee respectively.

together next the final common path. Reflexes with excitatory- effect (+ sign) are brought together on the left, those with in- hibitory (— 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 dia- gram 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.

Argument : Combination of reflexes simultaneously proceeding. Spread of reflex-response about a focus. Gray matter and lines of reflex resistance. "Short" reflexes and "long" reflexes. Rules decipher- able in the spread of reflex reaction. Pfltiger's " laws " of spinal irradiation. The " reflex figure." Variability of reflex result. Irra- diation of a reflex attaches itself to the problem of the simultaneous combination of reflexes. Co-ordination of reflex result obtains even when large mixed afferent nerve-trunks are stimulated. The move- ment excited by stimulation of the motor spinal nerve-root does not really resemble a movement evoked reflexly or by the will. Extent of simultaneous combinations of reflexes. Simultaneous stimuli arrange themselves naturally in constellations in which some com- ponent is usually of pre-eminent intensity. The resulting compound reaction has both positive and negative sides.

A LARGE part of co-ordination consists in the orderly com- bining of reflexes. In studying this co-ordination vi^e have to deal with and discriminate between simultaneous combinations and successive combinations of reflexes. We may proceed to attempt the former problem. Irradiation. If by appropriate stimulation of the skin of the foot, say by unipolar faradization of a spot of the plantar skin of a digit, the ordinary flexion-reflex of the hind limb of the dog be evoked, the extent of the reflex increases with increase in the intensity of the stimulus. The reflex-effect spreads over a larger and larger field, irradiating as it were in various direc- tions from a focus of reflex-discharge which takes effect on the limb itself.

The centrifugal discharge elicited by any reflex seems as regards its spatial distribution to be focussed about a centre round which its irradiation varies according to circumstance. In the scratch-reflex the pretibial muscles that dorso-flex the ankle seem to he at the focus of the motor discharge. In he " flexion-reflex," if the reaction evoked is very weak a band of the deep inner hamstring muscle has often in my experience emed the only part of the musculature thrown into action, n the other hand, when the reflex is evoked with medium strength it can often be seen that after the reflex (the exc-iting stimulation being continued unaltered) has been in progress for a few seconds, flexion at hip adds itself to the flexion at the ^^nee (see Fig. 45). And by strong stimulation, strong flexion I^Bit hip occurs together with that at knee and practically from ^^Hhe very outset. In my experience the condition of ** spinal ^^Bhock" is very favourable for noting. the seat of the focus of |^|the motor discharge in a reflex, because in that condition it happens often that the piece of musculature which is at focus of the discharge is the only one which can be got to give the reflex-response. It seems possible in this way to determine what reflex in, for instance, a " spinal " monkey corresponds with this or that reflex in a " spinal " dog. In the monkey the severity and long duration of spinal shock allows merely the focal reply in the musculature. Thus a feeble tightening of a part of a amstring muscle in the *' spinal" monkey affords fair evidence, response to a stimulus of the foot, that the flexor-reflex — for e full extent of which one must turn to the " spinal dog " — is oked. In man spinal shock seems still more severe and last- ng than in the monkey. The situation of the weak brief con- actions evoked can still reveal which they correspond with among reflexes better open to study in the lower mammals.

The more intense the spinal reflex — apart from strychnine and similar convulsant poisoning — the wider, as a general rule, the extent to which the motor discharge spreads around its focal area. Thus, as stimulation of the planta causing the flexion- reflex is increased there is added ^^2» ^'^ to the flexion of the homonymous hind limb extension of the crossed hind limb, then in the homonymous fore limb extension at elbow and retraction at shoulder, then at the crossed fore limb flexion at elbow, extension at wrist, and some protraction at shoulder; also turning of the head toward the homonymous side, and often opening of the mouth, also lateral deviation of the tail.

According to circumstance, especially according to intensity of stimulation, the field of end-effect of the flexion-reflex may vary from a minute field occupying part of a flexor muscle of the knee to a field including musculature in all four limbs and neck and head and tail. That the reaction should spread in its spatial extent is not surprising. The afferent neurone on entering the central organ, the spinal cord, enters a vast network of conduction of paths interlacing in all directions. A glance at any Weigert prepara- tion of the spinal cord shows a tangle of branching nerve-fibres, the richness and intricacy of which seems practically infinite. Into this forest the receptive neurone conducts the impulses, and can itself be traced, breaking up into many divisions that pass in many directions and to various distances. And this web of conductive channels into which the centripetal impulses of the reflex are thus launched is known to be practically a continuum in the sense that no part of the nervous system is isolated from the rest. " A group of nerve-cells disconnected from the other nerve-tissues of the body, as the muscles or glands are discon- nected from each other, would be without physiological sig- nificance. To understand the physiology of the nervous system it is important to keep in mind the fact that by histology it is found to be continuous throughout its entire extent." ^^ And there is the generally accredited statement that on exhibition of strychnine centripetal impulses poured in via any afferent nerve, excite reflex-discharge over the efferent channels of the whole nerve-system. This, even if not strictly true, is sufficiently ap- proximate to the truth to show the enormous interconnections between any afferent channel and the congeries of arcs of the

Figure 45. — Maintenance of the scratch-reflex A and the flexion-reflex B respectively under unipolar faradic stimulation of comparable intensity. The diffuse electrode (anode) was on the fore limb in each case; the primary circuit and its rate of interruption was the same, and the secondary coil of the inductorium remained at the same distance from the pri- mary. For the scratch-reflex the needle electrode was set in the skin of the loin, for the flexion-reflex in the plantar skin of the outermost digit. After giving 28 beats the scratch-reflex died out, having lasted about seven seconds. Further continuation of the stimulation was cut short as useless after four more seconds. The flexion-reflex, on the other hand, is in full intensity at the end of the 13th second of continued application of the stimulus, and its amplitude is perfectly maintained at the end of the 20th, although the reflex is rather tremulant. At the 44th second, when it has become more tremulant

whole central nervous system. It is therefore not surprising that the reflex reaction should spread. On the other hand, the data leave unexplained certain features of the spread. How is it that the spread, as the reflex is intensified, does not extend everywhere, as it is said to do in strychnine poisoning? How is it that in the flexion-reflex, of the cat for instance, the spread does not extend to the muscles of the pinna of the ear? It is easy from certain parts to obtain the brisk reflex retraction of the pinna. Yet in my experience the stimulation of the foot that causes the flexion-reflex and all its various irradiations may be pushed without evoking retraction or other movement of the pinna. In other words, the irradiations of the reflex occur along certain lines only and not along others, and the line to the pinna is of these latter.

Evidently the irradiation from each entrant path tends to run in certain directions and not in all. This fact is sometimes stated in the form that gray matter offers to the entrant path lines of conduction possessing different degrees of resistance. To say this merely of course restates the fact in terms suggesting analogy between nerve-paths and electric circuits. Before the Golgi and methylene-blue methods had thrown doubt on the intricate forest of nerve-fibres in the gray matter being a net- work structurally continuous in all directions, as supposed from the Gerlach preparations and the universal irradiation under strychnine, the diff'erences in conductive resistance were attrib- uted mainly to diff'erences in the length of the network to be traversed by some reflexes as compared with others. The longer that path in the gray matter the higher was thought to be the resistance. Evidence indicating slow travel of impulses in gray matter was taken as evidence of resistance in gray matter. In certain reactions the impulses were supposed to have very long paths of travel in gray matter. Thus, impulses of pain were supposed to ascend along the spinal gray matter to the brain. The path of impulses connected with pain does plunge into the gray matter very soon after entering the spinal cord ; it then, probably after a short course, emerges into the lateral white columns, preponderantly of the side crossed from that on

which it entered. This short in-and-out traverse of the spinal gray matter seems typical of all paths in the. gray matter ; they are probably all quite short.^^ If all synapses lie in the gray matter, each path where it involves a passage from one link to another of the neural chain must enter the gray matter to es- tablish its linkage; it probably soon emerges thence again. Neurone -threshold. But one finds still very generally ex- pressed the view that the differences of resistance to irradiation in different directions are referable to different conductive re- sistance offered by different fibres in the gray matter. The different resistance seems more probably referable to differences in the facility of conduction at different synapses. At each synapse there is a neurone-threshold.^®^ At each synapse a small quantity of energy, freed in transmission, acts as a releas- ing force to a fresh store of energy not along a homogeneous train of conducting material as in a nerve-fibre pure and simple, but across a barrier which whether lower or higher is always to some extent a barrier. There is abundant evidence that different synapses differ from one another. That neurones should differ in the threshold value of the stimulus necessary to excite them seems only natural. The arguments adduced by Goldscheider point in this same direction. Many of the phenomena consid- ered in the first three lectures are easiest explicable by such differences. The distinctions between different synapses in re- gard to ease of alteration by strychnine and by tetanus toxin emphasize this probability further. On this view the fact that irradiation of a reflex reaction spreads along certain conductive arcs more readily than along others, can be schematically figured as in the diagram (Fig. 46). A receptive neurone A enters the cord and forms synaptic connections with three neurones, the neurone-threshold at the synapse with one of the neurones is higher than that at the synapses with the others. The threshold heights (resistances) are represented by whole numbers, two and one respectively.