The Integrative Action of the Nervous System

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

The tortoise exhibits a similiar sexual reflex of great spinal potency.ii^a ii2b It would seem a general rule that reflexes arising in species of receptors which considered as sense-organs provoke strongly affective sensation caeteris paribus prevail over reflexes of other species when in competition with them for the use of the ''filial common path'' Such reflexes override and set aside with peculiar facility reflexes belonging to touch organs, muscular sense-organs, etc. As the sensations evoked by these arcs, e.g, "pains," exclude and dominate concurrent sensations, so do the reflexes of these arcs prevail in the competition for pos- session of the common paths. They seem capable oi pre-eminent intensity of action.^^

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

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

The high variability of reflex reactions from experiment to experiment, and from observation to observation, is admittedly one of the difficulties that has retarded knowledge of them. Their variability, though often attributed to general conditions of nutrition, or to local blood-supply, etc., seems far more often due to changes produced in the central nervous organ by its own functional conductive activity apart from fatigue. This functional activity itself causes from moment to moment the temporary opening of some connections and the closure of others. The chains of neurones, the conductive lines, have been, especially in recent years, by the methods of Golgi, Ehrlich, Apathy, Cajal, and others, richly revealed to the microscope. Anatomical tracing of these may be likened, though more difficult to accomplish, to tracing the distri- bution of blood-vessels after Harvey's discovery had given them meaning, but before the vasomotor mechanism was dis- covered. The blood-vessels of an organ may be turgid at one time, constricted almost to obliteration at another. With the conductive network of the nervous system the temporal varia- tions are even greater, for they extend to absolute withdrawal of nervous influence. Under reflex inhibition a skeletal muscle may relax to its post-mortem length,^^ i. e.y there may then be no longer evidence of even a tonic influence on it by its motor neu- rone. The direction of the stream of liberation of energy along

the pattern of the nervous web varies from minute to minute. The final common path is handed from some group of a plus class of afferent arcs to some group of a minus class, or of a rhythmic class, and then back to one of the previous groups again, and so on. The conductive web changes its func- tional pattern within certain limits to and fro. It changes its pattern at the entrances to common paths.^^ The changes in its pattern occur there in virtue of interaction between rival reflexes, " interference." As a tap to a kaleidoscope, so a new stimulus that strikes the receptive surface causes in the central organ a shift of functional pattern at various synapses. The central organ is a vast network whose lines of conduction follow a certain scheme of pattern, but within that pattern the details Eof connection are, at the entrance to each common path, muta- ble. The gray matter may be compared with a telephone ex- change, where, from moment to moment, though the end-points of the system are fixed, the connections between starting points and terminal points are changed to suit passing requirements, as the functional points are shifted at a great railway junction. In order to realize the exchange at work, one must add to its purely spatial plan the temporal datum that within certain limits the connections of the lines shift to and fro from minute to min- ute. An example is the " reciprocal innervation " of antago- nistic muscles — when one muscle of the antagonistic couple is thrown into action the other is thrown out of action. This is only a widely spread case of the general rule that antagonistic reflexes interfere where they embouch upon the same final com- mon paths. And that general rule is part of the general princi- ple of the mutual interaction of reflexes that impinge upon the same common path.

Unlike reflexes have successive but not simultaneous use of the common path ; like reflexes mutually re- inforce each other on their common path. Expressed teleologi- cally, the common pathy although economically subservient for many and various purposes^ is adapted to serve but one purpose at a time. Hence it is a co-ordinating mechanism and prevents con- fusion by restricting the use of the organ^ its minister^ to but one action at a time. In the case of simple antagonistic muscles, and in the in- stances of simple spinal reflexes, the shifts of conductive pattern due to interaction at the mouths of common paths are of but small extent. The co-ordination covers, for instance, one limb or a pair of limbs. But the same principle extended to the reaction of the great arcs arising in the projicient receptor organs of the head, e. g. the eye, which deal with wide tracts of musculature as a whole^ operates with more multiplex shift of the conductive pattern. Releasing forces acting on the brain from moment to moment shut out from activity whole regions of the nervous system, as they conversely call vast other regions into play. The resultant singleness of action from moment to moment is a key-stone in the constructiojt of the individual whose unity it is the specific office of the nervous system to perfect. The interference of unlike reflexes and the alliance of like re- flexes in their action upon their common paths seem to lie at the very root of the great psychical process of " attention."

Argument : Reflexes as adapted reactions. The purposes of various type-reflexes. Shock a difficulty in deciphering the purpose of re- flexes. Characters of spinal shock. Its incidence confined to the Iaboral side of the transection. Its difference in severity in different reflexes and in different animals. Shock referable not to the irrita- tion of the trauma but to the cutting off by the trauma of some supra- spinal influence. Pseudaffective reflexes afford opportunity for determining the pain- path in the spinal cord. This ascends both lateral columns, chiefly the one crossed from side of stimulation. The " chloroform cry " in decerebrate animals. Mimesis of pleasure as compared with mimesis of pain. The bodily resonance of the emotions. The theory of James, Lange, and Sergi. Emotional expressions in dogs deprived of visceral and largely of bodily sensation.

It is of course as impossible to disprove as to prove that psychical events accompany, or that they do not accompany, the nervous reactions of the ** spinal " animal. It is significant, however, that the best-known controversy (Pfliiger, Lotze) as to the psychical powers of the spinal cord, occurred prior to the advent of the Darwinian theory of evolution. This latter sug- gests how purposive neural mechanisms may arise. It furnishes a key to the genesis and development of adapted reactions and, among these latter, reflexes.

That a reflex action should exhibit purpose is no longer considered evidence that a psychical process attaches to it ; let alone that it represents any dictate of " choice " or " will." In light of the Darwinian theory every reflex must be purposive. We here trench upon a kind of teleology. It is widely and wisely held that natural knowledge pursues the question '* how " rather than the question " why." The " why " involves a judg- ment whose data lie so beyond present human experience and comprehension that self-abnegation in regard to the desire to

attempt it is not only prudent, but to the unbiassed judgment a necessity. Yet the question has its humbler forms as well as its more general and ambitious. Older writings on reflex action concerned themselves boldly with the purpose of the reflexes they described. The language in which they are couched shows that for them the interest of the phenomena centred in their being regarded as manifesta- tions of an informing spirit resident in the organism, lowly or mutilated though that might be. Progress of knowledge has tended more and more to unseat this anthropomorphic image of the observer himself which he projected into the object of his observations. The teleological speculations accompanying such observations have become proportionately discredited.

Self- wounded in this way physiology became for a time ex- tremely reticent about purpose, remaining simply objectively descriptive. The impetus given to biology by the doctrine of adaptation under natural selection, felt so strongly by morphological studies, seems hardly as yet to have begun its course as a motive force in physiology. But signs begin to be numerous that such an era is at hand.* The infinite fertility of the organism as a field for adapted reactions has become more apparent. The purpose of a reflex seems as legitimate and urgent an object for natural inquiry as the purpose of the colouring of an insect or a blossom. And the importance to physiology is, that the reflex reaction cannot be really intelligible to the physiologist until he knows its aim.

In general terms we may say that the effect of any reflex is to enable the organism in some particular respect to better dominate the environment. One often hears objection taken to the epithets — common in writings on biology — "lower" and * Such a "motif" seems constantly present as an undercurrent in much recent writing in experimental pathology, notably that of Ehrlich in respect to his suggestive " Antikorper " hypothesis. It is detectable as a principle in the fine researches by Bayliss and Starling. Most definitely and broadly it is expressed in the writings of A. Tschermak,298» especially in the remarkable Essay, Das Anpassungs-problem in der Physiologic der Gegenwart, and in the contributions of V. Uexkiill from the field of invertebrate physiology.

" higher " as applied to organisms, plant and animal. Such objection seems valid if the phrase assumes that the " lower " organism any less perfectly fulfils its ** purpose " or " design " than does the " higher," or in those respects in which it has commerce with the environment is any less admirably adjusted than is the higher. But "lower" and "higher" maybe used without any connotation of that kind. In the course of evolu- tion a number of organisms have become so adapted to the en- vironment as to dominate it more variously and extensively than do other organisms. In that sense some organisms are higher and some are lower. In that sense man is the highest organism. And if evolution be a process of gradual and more or less un- interrupted course it is obvious that the highest form achieved will also be among the latest of the forms achieved. This grad- ing of rank in the animal scale will be nowhere more apparent than in the nervous system in its office as integrator of the in- dividual. The more numerous and extensive the responses made by a creature to the actions of the world around upon its receptors, the more completely will the bundle of reflexes, which from this standpoint the creature is, figure the complexity of the world around, mirroring it more completely than do the bundles of reflexes composing " lower " creatures.

The study of reflexes as adapted reactions evidently, there- fore, includes reactions of two ranks. With the nervous system intact the reactions of the various parts of that system, the " simple reflexes," are ever combined into great unitary harmo- nies, actions which in their sequence one upon another consti- tute in their continuity what may be termed the " behaviour " (Lloyd Morgan) of the individual as a whole. Into the intri- cate " purposes " (adaptations) traceable in these total reactions which constitute the creature's behaviour as a social unit in the natural economy it is not our part to enter. Our part of the problem is a humbler one. In the analysis of the animal's life as a machine in action there can be split off from its total behaviour fractional pieces which may be treated conveniently, though artificially, apart, and among these are the reflexes we have been attempting to decipher. We cannot but feel that

we do not obtain due profit from the study of any particular type-reflex unless we can discuss its immediate purpose as an adapted act. When we try to assign what we may in this restricted sense call its " purpose " to any particular reflex, the data for an answer are gathered in the main from one or other of the con- ditions attaching to the reaction. The mode of the adequate stimulus is one of these. The time-relations and spatial form of the response are others. The broad pressure applied under the foot-pad which seems the adequate stimulus ^^ for the " ex- tensor-thrust " and the brief forcible straightening of the limb which constitutes the response suggest that that reflex has its purpose in the execution of an act in the series of movements of stepping in the animal's locomotion. And the recent analysis of Philippson ^^^ demonstrates that such an act occurs in the reflex trotting and galloping of the dog. Again, the con- nection between the tickling irritative stimuli which seem " adequate " ^^ for the " scratch-reflex," and the scratching movement itself which results, suggests that the purpose of that reflex is a grooming of the skin to protect that organ against parasites which infest it and would confuse its function as a receptive surface reacting to more significant environmental stimuli.

Grainger's ^^ conclusion was that spinal cutaneous reflexes " are either of a preservative character or resemble the move- ments which the functions of the organ require." From the skin of the spinal creature reflex movements resembling those executed by the normal individual in preening or cleansing itself are of widespread occurrence. Together with the preen- ing actions of the spinal fly,^* grasshopper, Astacus,^'® etc., there fall into this category the movement by which the spinal frog wipes irritants from its back or head ; the " nettoyage " by the tortoise ;^^^ the posturing of the hind limbs and tail of the spinal dog concurrently with reflex defaecation,^^ tending to keep the body from being soiled ; and the ** scratch-reflex " and the " shake-reflex " ^^^a of the spinal dog. The conjunctival reflex protecting the cornea, essentially a cutaneous and from

the broad point of view a spinal reflex, is similarly preserva- tive of the part whence it is initiated. There are of course two modes of preservation, namely, escape and defence. Parts that can move themselves seem reflexly to employ the former. The spinal frog's foot is drawn out of harm's way when irritated ; so also in the cat and dog. But parts that cannot of their own motion withdraw themselves effectively seem to invoke defensive movements from adjacent motile parts. The spinal frog's flank, when irritated, is defended by the hind limb, which comes up and removes the irritant from the flank, the flank itself also shrinking away somewhat. Simi- larly in the scratch-reflex, the distant limb is brought up to the defence of the irritated shoulder or flank. There is indeed one, rarely exemplified, group of reflexes in which the organ is sacri- ficed for the preservation of the rest of the individual. In certain forms, e. g. Asterias, Cometula, Ophiurus, Arachne, Carcinus, a limb pulled upon violently or long suddenly ruptures itself and is shed. These actions have been shown by Fredericq to be re- flexes, employing muscular contraction. Such reactions exhibit well how absolutely the nervous system is adapted to minister to the requirements of the organism as an integrated whole, and the position of that system as a keystone in the upbuilding of the solidarity of the individual.

But the assignment of a particular purpose to a particular reflex is often difficult and hazardous. The difficulty is inversely as the amplitude of the field covered by the reflex-effect. A slight movement confined to a single limb, or a transient rise of blood-pressure observed alone, is open to many interpretations and admits of no security of inference. It is a fractional reaction that may belong to any of many general reactions of varied aim.

When a reflex is elicited faintly in a spinal animal it occurs simply at the focus, so to say, of its area of distribution, and owing to the restricted character of its features its meaning may be difficult or impossible to read. It is in my experience only by repeated observations of a reflex under various circumstances of its development that as a rule its significance becomes clear. The accessory parts of it are often instructive concerning the

whole. In the scratch-reflex of the dog, besides the rhythmic scratching movement of the hind hmb, say of the right, there is steady extension of the left hind limb, and steady extension with some abduction of the two fore limbs. The accessory parts of the reflex, namely those in the three limbs which are not scratching, are also contributory to the same effect as in the scratching movement in the right hind limb itself They steady the dog and secure the stabiHty of its body during the perform- ance of the scalptor act.

In the " flexion-reflex " of the hind limb excited by noxious stimuli, e. g. a prick or a faradic current, the limb itself is drawn up, — if weakly, chiefly by flexion at the knee ; if strongly, by flex- ion at hip as strongly as at knee. At the same time the crossed hind limb is thrown into action, primarily in extension, but this is soon followed by flexion, and alternating extension and flexion is the characteristic result. The rate of this alternation is about twice a second. That is to say, the foot which has stamped on the thorn is drawn up out of way of further wounding, and the fellow hind limb runs away; and so do the fore legs when — which is more difficult to arrange, owing to the height of the necessary spinal transection — they also are included, fairly free from shock, within the " spinal " animal.

Spinal shock. One of the experimental difficulties in de- ciphering the purport of a spinal reflex is the phenomenon known as " shock." " If in a frog the spinal marrow be divided just behind the occiput, there are for a very short time no dias- taltic actions in the extremities. The diastaltic actions speedily return. This phenomenon is * shock.' " (Marshall Hall.)^ Whytt had, a century previous to Hall, drawn attention to the same phenomenon, although assigning to it no descriptive term. The whole of that depression or suppression of nervous functions which ensues forthwith upon a mechanical injury of some part of the nervous system and is of temporary nature may be conveniently included as " shock." Goltz considered it entirely a collection of inhibition phenomena. Among labora- tory animals it is in the monkey that, on the whole, " spinal shock " appears at maximum.

Spinal shock appears to take effect in the aboral direction Qj^ly 183, 205 Section behind the brachial enlargement disturbs little if at all the reactions of the fore limb, although the number of headward running channels of conduction ruptured by such a section is enormous. Striking instances of the absence of head- ward spread of the depression due to " shock " are afforded by transections abutting on the lower edge of the fifth cervical seg- ment; these depress the respiratory activity of the phrenic motor cells hardly at all, even momentarily. On the aboral side of the transection depression is profound. Analogously, the sudden cutting off of all that stream of centripetal impulses continually pouring for conscious and subconscious elaboration into the encephalon from the cutaneous, articular, and muscular senscHDrgans of the tail, limbs, trunk, and neck, and from the viscera, seems to disturb the reactions of the head and brain little or not at all.

After high cervical transection, *' shock " appears more severe in the fore Hmbs than in the hind. For an hour or so it may be difficult to elicit any reflex movement from skin inner- vated behind the transection, whether by mechanical, thermal, or electrical stimuli. The view of Goltz and his school that ** spinal shock " is a long lasting inhibition due to irritation by trauma is not, I think, really tenable. The argument implies, if it does not explicitly state, that the trauma, by its damage and by its subsequent processes of inflammatory reaction, formation of scar-tissue, etc., acts as a stimulus, exciting inhibition that de- presses or suppresses reflex activity in adjacent and even remote arcs of the central nervous system. Against this explanation militate several facts. Firstly, the shock takes effect almost exclusively in the aboral direction. Were the mere irritative action of the trauma the cause, it is not easy to see why the nervous centres near the trauma should not be depressed on either side of, for instance, a spinal transection, headward as well as backward. Secondly, experiments of the following kind give results difficult to reconcile with the view. When in the dog complete transection of the spinal cord through the eighth cer-

vical segment is practised, a severe fall in the general arterial blood-pressure ensues, and vasomotor reflexes cannot be elic- ited. But in the course of some days this is largely recovered from, and after some weeks the blood-pressure will, with the animal in the horizontal position, often be found practically normal. When the animal is then anaesthetized and curarized, artificial respiration being maintained, it is usually easy to obtain on stimulation of the central ends of divided afferent or mixed nerves, for instance of the internal saphenous nerve, good and often very large vasomotor reflexes, the blood- pressure rising fifty millimeters and more (Fig. 6^), These reflexes upon the vascular musculature are purely spinal, since the cord has been divided just headward of the thoracic region. Then, while these spinal vasomotor reflexes are regularly elicitable and serve as a guide to the reflex activity of the cord behind the transection, I have transected the cord again a couple of segments behind the original transection. This section excites an immediate transient rise in the arterial pressure, lasting about a minute, and succeeded by a gradual fall. The arterial pressure, then, in my experience sinks to an equilibrium of pressure hardly lower than its mean prior to this second transection. There is none of that deep

Figure 67. — Spinal vasomotor reflex; dog; 300 days after spinal transection at 8th cervical level; chloroform and curare. Electrical stimulation of central end of a digital nerve of hind limb during the time marked by signal on second line from bottom. The arterial pressure (carotid) rises from 90 mm. Hg to 208 mm. Hg. Time marked below in 2 seconds. depression which ensued on the first trauma, though the second trauma has been practically qua trauma a complete repetition of the former one. If the fall of general blood-pressure be regarded as part, and a severe part, of the " spinal " shock which ensues on spinal transection in the cervical region, the absence of that fall on repeating practically the same trauma must signify that the second trauma is not followed by the shock that followed the first trauma. Moreover, reflex heightenings of blood-pressure such as were regularly obtainable just prior to the second tran- section are obtainable immediately, /. e. four minutes, after the second spinal transection. The first trauma causes temporary 1 deep depression of the spinal tonus of the vascular system and ; temporary abolition of vascular reflexes. The second trauma \ causes practically no depression, even transient, of the re-estab- lished tonus of the vascular system nor of the pressor spinal vas- cular reflexes that have become similarly re-established. It may perhaps be objected that the vascular tonus established subse- quent to the first spinal transection is of peripheral mechanism and outside the spinal cord itself. That that is not its main factor is shown by the further deep depression of vascular tonus which occurs when the spinal cord in the thoracic region is itself not merely transected but destroyed.

The trauma qua trauma is as severe in the first instance as in the second. In these experiments, therefore, the " shock " is not due to the trauma qua trauma. It seems to depend simply on solution of continuity of nervous channels, and this solution is practically equally great whether the actual trauma itself be relatively slight (a clean, sharply cut transection) or relatively severe (a contused and jagged transrupture), so long as in the two cases it involves an equal amount of the transverse area of; the cord. The practical absence of spinal ** shock " on repeti- \ tion of the trauma further back is explicable by its then causing 1 little further aggravation of the interruption of the nervous channels concerned with vascular tone and vascular reflexes, j those channels having already been ruptured by the previous transection somewhat further headward.

Similarly the flexion-reflex of the hind limb, though it suffers considerably from shock after transection of the cord in the hinder cervical or thoracic region, when it has recovered is but little, and but briefly, depressed by a second transection made behind the previous. In this case also trauma does not, there- fore, account for the spinal shock. The shock following the trauma is proportioned not to the mere wound, but to the num- ber and character of the descending nerve-paths through which the lesion breaks. Porter's ^^^ * well-known experiment on the respiratory intraspinal path from the bulb to the phrenic neu- rones points to the same conclusion.

There remains the further question as to whether spinal " shock " is a phenomenon of inhibition. A reflex during its depression by spinal shock does not present the features it shows when reduced by inhibition so much as features resem- FiGURE 68. — Scratch-reflex under " spinal shock." Spinal transection six weeks previously. The reflex was elicited by vigorous mechanical stimulation, electrical stimulation being, as is usual under shock, unable to evoke it. The time of application of the stimulus is marked by the signal line, which also records the time in fifths of seconds. The reflex is slow to appear, feeble and irregular, and lapses during the continuance of the stimulation. The small waves on the base line are due to the vigorous rubbing necessary to evoke the reflex at all, and are simply mechanically conveyed to the limb attached to the myograph.