The Integrative Action of the Nervous System

The results bear also on the production of sensual reactions and states more complex than those of the examples taken. Hartmann, as quoted by McDougall, writes: "Only because one part of my brain has a direct communication with the other is the consciousness of the two parts unified. Could we unite the brains of two human beings by a path of communication equivalent to cerebral fibres both would no longer have two but one consciousness." There is no denying the extreme impor- tance and the vast actual extent of the spatial conjunction of cerebral elements by conductive channels in sensual and per-

ceptual reactions. Yet I cannot but think that its limitless postulation leads not so much to explanation of the high degree of unity of the individual mind as to an ultimate fallacy which Professor James has trenchantly termed that of " the pontifical cell." Pure conjunction in time without necessarily cerebral conjunction in space lies at the root of the solution of the prob- lem of the unity of mind. Since convergence of the conductors from corresponding halves of the retinae to the same field of brain-cortex does not signify physiological conjunction of right and left sense-impres- sions, can we decipher at all what it does mean? To do so does not seem difficult, and displays strikingly the different value and directness of spatial union of conducting-paths for motor synthe- sis and for psychical respectively. In animals with overlapping visual fields the lateral movements of the eyeball have a mutual relation different from the ordinary relations of the movements of a unilaterally placed organ, e. g. a limb. Especially is this the case where the overlap of the visual fields is extensive, e.g. where the ocular axes are parallel. The horizontal movements of each eyeball are balanced about the primary line of vision of the globus in its habitual resting attitude, that is, in man, straight forward. That line sensually, as shown by introspective experiment, lies in the median sagittal plane of the head (Her- ing). Hence the term ' Cyclopean * has been applied to the biunial eye of human binocular vision. Finding the median vertical plane of sight of the resting eyeball to correspond with the median sagittal plane of the body, we may assume that the motor reflexes deal with the eyeball conformably with that; otherwise there would be disaccord between the reflexes and the sensations. Therefore we must in any general consideration of the taxis of the lateral movements of the two eyeballs transfer in thought each eyeball from its own actual sagittal plane to the median sagittal plane of the head ; and this latter corre- sponds in the resting position with the sagittal median plane of the animal as a whole.

Each lateral muscle of the eyeball comes therefore to bear to the median plane of the body the same relation as does a limb on one side of the body. Thus, the external rectus muscle of the right eyeball bears the same lateral relation to the median sagittal plane of the body as does the right arm. And the internal rectus muscle of the left eyeball bears the same lateral relation to the median sagittal plane of the body as does the right arm. Now a general arrangement evident in the cerebral cortex is that the taxis of muscles lying to right of the median sagittal plane is entrusted to the left hemisphere, and vice versa. It is in accord with this that in animals with overlapping visual fields the horizontal movement of the eyes to one side should for both eyeballs be represented in 07ie and the same hemisphere. And as a fact the conjugate movement is found represented for both eyeballs together in each hemis- phere. If we regard, and it was shown above that we may do so, the median sagittal planes of both eyeballs as identical with the median sagittal plane of the head, they are identical with each other, and the scheme of cortical representation may be expressed thus : a point in the right retina and its twin point in the left demand each of them identical movement of the two eyeballs when, apart from convergence, those points excite their own replacement by the fovea {i. e, when initiating a gaze). It is obvious that the paths from the visual cortex of each side to the eyeball muscles — experiment shows such a path to exist — is connected therefore with both the right and left twin points. That is, it is a common path. The confluence of conductors from the two retinae to the same cortical field, though not uniting their retinal impressions, gives them access to a common efferent path which both must use.* At entrance to every common path Hes, as shown before, a co-ordinating mechanism. A co-ordinative mechanism is thus obtained.

This " common path " with its bilateral twin origin impinges in its turn, directly or indirectly, on the motor neurones for the lateral eye-muscles, ^^ final common paths. We have therefore to alter such a scheme as that furnished by Cajal by attaching his convergent paths to efferent paths, and * Mott has likewise independently urged that the interpretation to be placed on this convergence of paths is motor rather than sensory. Trans. Ophthalm. Soc, vol. 25, p. cii, 1905.

by divesting their supposed nodal cortical point of its hypo- thetical powers as a sensual Deus ex machina. And we thus meet another instance of convergence of afferent paths leading to motor synthesis, but not, or only remotely, to sensual. Seen in this light the gulf between sensation and movement looms even wider than was allowed for in the tentative suppositions which prompted the above experiments on flicker. We are thus warned against any hasty conclusion that the neural mechanisms which synthesize reflex movements illustrate in their arrangement also those concerned where sensual fusion is the phenomenon. But that does not invalidate a broad prac- tical inference which study of the nervous system in regard to motor reaction allows. This inference is that toward the solu- tion of the problems of motor taxis help is obtainable by appeal to characters evident in sensual reaction. This practical infer- ence need not in the least involve any doctrinal attitude what- ever toward the hypothesis of psycho-physical parallelism. It may proceed quite apart from that. It simply insists on the likeness of nervous reactions expressed by muscular and other effector-organs to reactions whose evidence is sensual. It in- sists on this likeness being close and fundamental enough to make each of the two classes of phenomena of use to the student of the other. A number of excellent investigators hold, on the opposite hand, that the study of the two should proceed apart even more rigidly than they do at present. Con- fusion has, it is true, been caused in both by the loose applica- tion of the terms of the one set of phenomena to the other. But to disregard the many significant similarities which exist between the two sets is, it seems to me, to throw away one of the best instruments for discovery in both. We saw how suggestive psychological data prove for classifying the various species of receptors considered as initiators of reflexes. The after- discharge of a nervous arc finds expression not only in reflex movement but in, for instance, a visual after-image. Centripetal impulses from eye-muscles reinforce visual {i. e. extero-ceptor) sensations (Macdougall) just as centripetal impulses from the leg muscles reinforce reflex movement induced from the skin

(extero-ceptor) of the foot. The " immediate spinal induction" exemphfied by reflexes has a counterpart in visual irradiation. Visual contrast, if translated into terms of reflex contraction, bears close resemblance to " successive spinal induction." The features of fatigue repeat themselves in both sets of phenomena. Receptors which initiate reflex movements adapted in regard to objects at a distance initiate as sense-organs sensations pro- jected into circumambient sensual space. Receptors which initiate reflex movements advantageous in regard to some locus of the surface of the body itself, e, g, removal of irrita- tion thence, initiate as sense-organs sensations referred to that same locus. Instances might be multiplied, but they have risen prominently in several of the foregoing lectures, and are suffi- ciently before our minds now. A practical inference from them is that physiology and psychology, instead of prosecuting their studies, as some now recommend, more strictly apart one from another than at present, will find it serviceable for each to give to the results achieved by the other even closer heed than has been customary hitherto.

Besides this similarity of time-relation and other features between the physiological and the psychical signs of neural activity, another link connects the psychological and the physi- ological for the biologist. To the physiology of pure reflexes, that is, reflexes devoid of psychical accompaniment so far as introspection can discover, psychological interest neverthe- less attaches, and on a very distinct ground. This ground of connection is seen if inquiry is followed along the animal scale in the direction from higher forms to lower rather than by the usually more favorable reverse approach. This is partly because we directly observe psychical phenonema by introspection only, that is, only in ourselves ; and the facts discovered by introspec- tion are applicable to other beings the more readily the more those beings resemble ourselves, namely, are animals ranking near to man.

Pure reflexes are admirably adapted to certain ends. They are reactions which have long proved advantageous in the phy- lum, of which the existent individual is a representative embodi- ment. Perfected during the course of ages, they have during that course attained a stability, a certainty, and an ease of per- formance beside which the stability and facility of the most ingrained habit acquired during an individual life is presumably small. But theirs is of itself a machine-like fatality. Their character in this stands revealed when the neural arcs which execute them are separated, e. g. by transection of the spinal cord, from the higher centres of the nervous system. They can be checked, it is true, as we have seen, by coUision with other reflexes as ancestral and as fatally operative as themselves (Lectures V and VI). To these ancient invariable reflexes, con- sciousness, in the ordinary meaning of the term, is not adjunct. The subject as active agent does not direct them and cannot introspect them.

Yet it is clear, in higher animals especially so, that reflexes are under control. Their intrinsic fatality lies under control by higher centres unless their nervous arcs are sundered from ties existing with those higher centres. In other words, the re- actions of reflex-arcs are controllable by mechanisms to whose activity consciousness is adjunct. By these higher centres, this or that reflex can be checked, or released, or modified in its re- action with such variety and seeming independence of external stimuli that the existence of a spontaneous internal process ex- pressed as " will " is the naive inference drawn. Its spring of action is not now our question ; its seat in the nervous system seems to correspond with that of processes of perceptual level. It is urgently necessary for physiology to know how this con- trol— volitional control — is operative upon reflexes, that is, how it intrudes and makes its influence felt upon the running of the reflex machinery. How is the cough, or eye-closure, or the impulse to smile suppressed? How is the convergence of the eyeballs, innately associate to visual fixation of a near ob- ject, initiated voluntarily without recourse to fixation on an object? Or how is the innate respiratory rhythm voluntarily modified to meet the passing requirements of vocal utterance? No exposition of the integrative action of the nervous system is complete, even in outline, if this control is left without considera-

tion. Reflexes ordinarily outside its pale can by training be brought within it. The actor, it is asserted, can shed tears at will, or blush or blanch. Occasional instances are recorded of power to slow the rhythm of the heart at will ; others, of power to suppress the reflex of swallowing when it has entered on its pharyngeal stage. Volitional movement can certainly become involuntary, and, conversely, involuntary movements can some- times be brought under subjection to the will. From this subjec- tion it is but a short step to acquisition of coordinations which express themselves as movements newly acquired by the indi- vidual. The controlling centres can pick out from an ancestrally given motor reaction some one part of it, so as to isolate that as a new separate movement, and by enhancement this can become a skilled adapted act added to the powers of the individual. In regard to the ring finger, the motor co-ordination ancestrally pro- vided gives extension of that finger only in company with the fingers on each side of it. We can soon train ourselves to lift the ring-finger alone without the others. The "will " dissociates the ancestral co-ordination. Similarly we can acquire the power to move a part which neither reflexly nor otherwise would seem to come within the scope of our voluntary innervation, although of course there must be motor nerve and muscle upon which our innervation can operate. Thus, we can learn to retract the pinna of the ear ; the movement is at first accompanied by other facial movement, but later with practice it becomes exe- cutable without other facial movement. A new reaction and co-ordination has been gained by the individual.

The transition from reflex action to volitional is not abrupt and sharp. Familiar instances of individual acquisition of motor co-ordination are furnished by the cases in which short, simple movements, whether reflex or not, are by practice under voli- tion combined into new sequences and become in time habitual in the sense that though able to be directed they no longer re- quire concentration of attention upon them for their execution. As I write, my mind is not preoccupied with how my fingers form the letters; my attention is fixed simply on the thought the words express. But there was a time when the formation of the

letters, as each one was written, would have occupied my whole attention. Volitional control of reflexes is a question of co-ordina- tion not explicitly before us previously in these lectures. Its analysis has not indeed proceeded far. We may premise that some extension of the same processes outlined in Lectures V and VI, as operative in simultaneous combination and in suc- cessive combination of reflexes, must be operative in this con- trol. There we saw reflexes modifying each other, and the more complex reactions being built up from simpler and more re-* stricted ones. Some extension of the same process should, in view of our inferences regarding the nature of the dominance of the brain (Lecture IX), apply here also.

It is significant that, although the reflexes controlled are so often unconscious, consciousness is adjunct to the centres which exert the control. A biologist. Professor Lloyd Morgan, has urged that *'the primary aim, object, and purpose of con- sciousness is control. Consciousness in a mere automaton is a useless and unnecessary epiphenomenon." * A somewhat similar thought rose incidentally to our lips in a previous lecture (Lec- ture IX). The pleasure-pain accompaniment of reflexes has often been interpreted as carrying that meaning. Certain it is that if we study the process by which in ourselves this control over reflex action is acquired by an individual, psychical factors loom large, and more is known of them than of the purely physiological modus operandi involved in the attainment of the control. Hence, psychological studies have been more numerous than physio- logical in this field. It is found that kinaesthetic sensations of the movement to be acquired or controlled, though helpful, are less important than the resident sensations from the part in its " resting " state. These latter, with the power to focus atten- tion upon them, appear, in a number of instances, to be a most necessary condition for the acquirement of the control. And in the monkey, voluntary control of a limb is largely lost when the limb has been rendered apaesthetic.^^^

A biological inference arises at this point. We have admitted that the organs to which psychosis is adjunct, namely, the brain, and especially in higher vertebrates the cerebral hemispheres, supply the surest touchstone to rank in the scale of animal creation. That is to admit, in other words, that development of these organs constitutes, on the whole, the best criterion to the success of an animal form in the competition which lies at the root of animal evolution. These organs, we have just seen, are the organs of nervous control ; and that control is ex- ercised mainly in the perfecting and readjusting of manoeuvres of ancient heritage. The way in which we ourselves acquire a new skilled movement, the means by which we get more pre- cission and speed in the use of a tool, the handling of an instru- ment, or marksmanship with a weapon, is by a process of learning in which nervous organs of control modify the activities of reflex centres, themselves already perfected for other though kindred actions. Our process of learning is accompanied by conscious effort These nervous organs of control form, therefore, a special instrument of adaptation and of readjustment of reaction to better suit requirements which may be new. New adaptations whence the individual may reap benefit are thus attained. The more complex an organism, the more points of contact it has with the environment, and the more frequently will it need readjustment amid an environment of shifting relationships. These nervous organs of control being organs of adjustment will be more prominent the further the animal scale is followed up- ward to its crowning species, man. And these organs which give adjustability to the running of the reflex machinery, as such, seem themselves — perhaps, by reason of their constant relative newness — to be among the most plastic in the body.

In man and the species near him, these organs are most de- veloped, and their mechanisms are cerebral. These cerebral mechanisms constitute the clearest criterion of evolutional suc- cess. In these types it is cerebral function which best compasses that modification of old and that development of new reaction, which perfects the adaptation of the individual to the environ- ment. The relatively high development in man of this organ for individual adjustment of reactions makes him the most successful

animal on earth's surface at the present epoch. No doubt the greater part of all this adjustment of reaction will, in his case, as he stands now, come under intellectual activity. In him reason enables the individual profitably to forecast the future, and to act the more suitably to meet it, from memory of the past. Mere experience can, however, apart from reason, mould nervous reac- tions in so far as they are plastic. The *' bahmmg'* of a reflex exhibits this faculty in germ. In the humble spheres of nervous activity, such as alone fall within the scope of these lectures, simple sensori-motor experience seems to count for more than reason in the actual process of acquiring new motor co-ordina- tions. Of course reason, directing effort, counts in the selection of the field of operation of motor experience. But the inefficacy, as a means to arrive at a new motor correlation, of instruction merely verbal, or of ideas constructed without motor experience, is common knowledge. To learn skating or racquets by simple cogitation or visual observation is, of course, impossible. Here mere sensori-motor experience is more valuable than any course of reasoning can be. Hence the training for a new skilled motor manoeuvre must be simply ad hoCy and is of itself no training for another motor co-ordination, — apart from the well- known mutual influence of training on symmetrical parts of the body. Yet, in high animal types, the connection between skilled movements and the so-called ** motor *' region of the cortex cerebri, and the defect in these which injury of that region en- tails, countenances the belief that the " experience " involved in this training, though not rational, is cerebral. The compensation of co-ordi native defects which the cerebrum accomplishes after cerebellar or labyrinthine lesions, points to a similar conclusion.

And we must remember that though the mere sensori-motor experience counts for so much in the mastering of a new move- ment, they are perceptual, and in man rational, processes which initiate, maintain, and guide effort toward acquirement of an act which is new. We thus, from the biological standpoint, see the cerebrum, and especially the cerebral cortex, as the latest and highest ex- pression of a nervous mechanism which may be described as the organ of^ mid for ^ the adaptation of nervous reactions. The cere- brum, built upon the distance-receptors and entrusted with reac- tions which fall in an anticipatory interval so as to be precurrent (Lect. IX), comes, with its projici^nce of sensation and the psychical powers unfolded from that germ of advantage, to be the orgdin par excellence for the readjustment and the perfecting of the nervous reactions of the animal as a whole, so as to im- prove and extend their suitability to, and advantage over, the environment. These adjustments, though not transmitted to the offspring, yet in higher animals form the most potent in- ternal condition for enabling the species to maintain and increase in sum its dominance over the environment in which it is im- mersed. A certain measure of such dominance is its ancestral heritage ; on this is based its innate right to success in the com- petition for existence. But the factors and elements of that competition change in detail as the history of the earth pro- ceeds. The creature has to be partially readjusted if it is to hold its own in the struggle. Only by continual modification of its ancestral powers to suit the present can it fulfil that which its destiny, if it is to succeed, requires from it as its life's purpose, namely, the extension of its dominance over its environment. For this conquest its cerebrum is its best weapon. It is then around the cerebrum, its physiological and psychological attri- butes, that the main interest of biology must ultimately turn.

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