BRAIN. If by physiology of the brain we mean the study of the biological function of that organ, the state of exact knowledge regarding it is still extremely inadequate, although there exists a vast body of detailed fact. General inferences as to function drawn from morphological and phyloge netic data are therefore a permissible and welcome help although often of the nature of suggestion rather than demonstration. It is with the vertebrate brain, that the following account will be concerned; and the hu man brain constitutes the climax of the evolution of the verte brate brain at present, though not, of course, in finality. The past history of the brain traced in the vertebrate stock assists compre hension of the function of the human brain.
The brain is, however, always that part of the nervous system which is constructed upon and evolved alongside of the distance receptors. The importance of this conjunction in this matter is that it means ability on the part of the animal to react to an object when still distant and allows an interval for preparatory reactive steps, and this can go far to influence the success of its behaviour in regard to that object. The reactions initiated and guided by the distance-receptors are all steps towards final ad justments, which latter are consummations often of critical im portance for the existence of the animal (e.g., attainment of food) or of its species (e.g., fertilization). This time-interval and its series of steps, along with the vicissitudes of relation between things of changing position reacting one on another at a distance, conspire to give to the distance-receptor reflexes a multiformity and a complexity unparalleled by the reflexes initiated from other receptors. This interval affords much more copious opportunity for adjustment and side-connection as occasion demands. It gives freer play for the affixing of new-conditioned (i.e., individually acquired) reflexes to the primal inborn reflexes. Further, the time interval allows opportunity for variations of behaviour to be failures and yet recovered from, and conversely, allows greater chance for successful reaction-variants to be selectively preserved.
The other feature is not unrelated to the foregoing one. It is, that evolution, though with chequered history, has resulted in animal forms possessing successively greater dominance over their environment. Organisms are commonly spoken of as "lower" and "higher." The "lower" are usually the simpler, the "higher" the more complex; but the "lower" need not the less perfectly fulfil their primary biological requisites, preservation of self and spe cies. There are brachiopods which have without visible change maintained themselves in and upon their environment from the era of the earliest fossil-bearing rocks till today, and they are "lower" animals. Such commerce with and maintenance in the environment must be as admirably adjusted as can be any imaginable so far as concerns persistence of life. Yet, in the course of time, evolution has produced animal forms which pursue a far richer and more manifold commerce with the environment and some of these dominate the environment more variously and extensively than others, including their own ancestry, have done. In this sense they are "higher" forms." The earliest animal forms have included none of these highest, and some of the very latest are also the very "highest" forms achieved. As judged by domi nance of the environment man, although quite a recent form, is the highest as yet. The key to this evolutionary feature is fur nished largely by the evolutionary history of the brain and its functions.
The success seems partly a matter of mere increase of central ization. The receptor apparatus of the head gets increased co ordinative guidance of the body. The body tends to become a locomotor, and later a secondary prehensile train and a digestive appanage attached to the head, with, as inalienable possession, the reproductive organs. The brain in this respect merely takes with further specialized success the general role assigned to the nerv ous system from its earliest appearance and onward throughout evolutionary history, namely, the welding of the body's compo nent parts into one consolidated mechanism facing as a united en tity the changeful world about it. The work of this kind done for the "higher" animal by its brain presents the acme of animal in tegrative achievement. Hence is it that each of us, though made up of myriads of cell-lives individually feeding and breathing, and of manifoldly differing activities, constituting scores of organs, yet appears to himself a single entity, a unity experiencing and acting as one individual. That the particular bodily system which is specialized for integration, and whose sole function is integra tion, and that that portion of it where integrative function is at its highest should be the seat of mind, even from the dim mental beginning, and that mind should remain there localized and de spite all mental growth stay restricted in seat there along millions of years, on into ourselves to-day, indicate the scope and crowning importance of nervous integration and the brain.
Over and over again in the evolution of the brain there is in stanced the importance, for the process of integration, of con necting together nerve-structures which might or do react con currently but are originally unconnected. Concurrent activity of such related nerve-paths promotes actual architectonic welding of them (neuro-biotaxis, Arisus Kappers, 1908). A responsive group of neurons tend to be drawn toward their dominant stimulators.
The simultaneous components of a "reflex-figure" (Sherrington, 1906) tend to stamp in a neural pattern. The functional "reflex figure" with its simultaneously reacting parts proceeds along with (as its structural counterpart) a neural pattern which may become innate or be an individual acquirement (e.g., mammalian cortex). A keystone of the principle of integration is that the concurrent activity of related parts results in more than the simple sum of the activity of the separate component parts. Thus, in psychical integration, the single touch gives experience of itself alone. But a double simultaneous touch (e.g., compasser) gives experience of two touches, and, which is new, an untouched space between. The integration results in more than the mere sum of the components. Again, the uniocular field gives experience of some amount of "depth"; but when in combination with the other uniocular field to a binocular, there is yielded such an enhancement of the third dimension as amounts to a new result, the "depth" of stereo scopic vision. So with the visual integration illustrated by the "steps" figure, it is much more than its component lines, because whether the steps appear to "overhang" or "run up" there is always a foreground and background, i.e., the whole presents more than the parts, and constitutes a "situation" with "relations." And in the vertebrate after advent of the brain the animal's re active behaviour shows in increased measure the important qual ity of modifiability by experience, using this last term without of necessity any psychological connotation. Late in vertebrate development in a restricted number of forms, all mammalian and nearly related one to another, and relatively very recently evolved, this modifiability of behaviour has become greatly more effective. Its highest outcome appears perhaps as the rational guidance of human conduct.
It is not of course that either the fuller integration of the in dividual animal or the higher animal's wider dominance of the environment are the result entirely of the brain or of the cerebral adjunct "mind." Contributory to the latter result has been the mechanism (partly nervous) for ensuring a constant temperature environment for the tissues of the body, enabling the individual's activity to be uninterrupted by season, and largely independent of latitude; also the gestation arrangement which protects the young within the mother until a relatively late stage, providing exceptional pre-natal care for the offspring. Nevertheless the ex treme importance of the contribution by the brain is shown by the degree of dominance over the environment obtained by man as compared with that of other, even the highest other, placental mammals.
The vertebrate brain consists of a fore-and-aft series of three portions, the fore brain, mid brain and hind brain.
The excitability of the cerebellar surface by electricity has been denied, but recent studies confirm that, as formerly claimed (Fer rier), considerable areas of the surface are truly excitable by elec trical stimuli. The excitable field is palaeocerebellar (Ingvar, Bremer) and causes inhibitory relaxation of certain active pos tures, e.g., of extension-abduction of limb (Bremer, Miller and Banting, Sherrington). Destruction of the palaeocerebellar region which receives proprioceptive spinal tracts causes exaggeration of the stretch-reflexes of the limb-extensors (Bremer). Disease with cerebellar defect in man produces its most obvious detectable re sults in the field of willed muscular acts. The accuracy of execu tion of the movement is impaired by overshoot, abruptness of start and stop, ill-sustained contraction, and undue liability to fatigue. Cerebellar ataxy seems to contain the following three factors (Walshe), diminished fineness of postural adjustment, excessive intensity of postural activity, and complication of the two fore going by voluntary efforts at correction.
Obviously the function of the cerebellum is still obscure, al though it is a large organ, weighing in man more than the entire spinal cord. Proprioceptive recepts seem at the base of palaeo cerebellar function, and suggest for it an adjusting co-operation in the execution of muscular acts, the acts themselves being initiated and directed by other centres than the palaeocerebellum, probably mainly those of the mid-brain. The neocerebellar function may be inferred to be similar in character to that of the palaeocerebellum but to be adjuvant to movements of a newer physiological order (voluntary), initiated and directed by the neopallium (cerebral cortex). The neopallium in activating these movements probably activates collaterally the associated neocerebellar co-operation. The status of the cerebellum in the motor acts seems merely that of an executive instrument of them; the purpose and object of them are none of its affair. Cerebellar reactions are unconscious. Its destruction entails no loss of sensation, although cerebellar disturbance may occasion some proprioceptive misperception.
The cat retaining the mid-brain but deprived of the fore-brain reacts to sounds, although without giving indication of the direc tion whence they come. The mid-brain is in fact a large "ex change" where messages from the retina are associated with those from various other receptive nerves of the head and, via the spinal tracts, from the body (especially skin). In responding to these messages the mid-brain uses efferent paths by which it can operate upon motor centres, especially of the eyes and mouth and also of the neck and body. Severe impairments of motility and of normal posture are therefore produced by injury of the mid-brain roof, but although relatively large in lower vertebrates it becomes rel atively dwarfed in the mammalian brain. There is some evidence that in the course of vertebrate evolution along the mammalian branch, the intrinsic importance of the mid-brain as a dominating centre becomes smaller, not only relatively but absolutely. The supreme control of behaviour becomes located progressively more forward, passing from mid-brain (most fishes) to thalamus and striatum (reptiles and birds) and then to cerebral cortex (mam mals).
In close association with the thalamus is an underlying struc ture, hypothalamus, among whose ascribed functions is nervous regulation of the mechanism (vascular, glandular, etc.) by which in animals of constant temperature (birds, mammals) the body temperature is maintained steady despite environmental changes, a result contributing enormously to evolutionary success. In addi tion to this so to say essentially vegetative function, the hypo thalamic region is concerned with visceral nerve-relays, putting them in touch with taste and smell (from in front) relays. It is therefore relatively large in lower vertebrates, while the thalamus, analogous with it but in an exteroceptive and somatic rather than a visceral field, gains on it in higher vertebrates. In the bird the thalamic fore-brain independently of the cerebral hemispheres seems to operate large reflex reactions to noxa, to hunger, thirst, and temperature, and possibly elaborates mental concomitants of these. But even in the highest mammals tonic activities of the sympathetic, essentially a visceral, system are traceable to this region. The nexus between sympathetic and visceral reactions on the one hand and affective and emotional mental experience on the other makes the close relation between thalamus and hypo thalamus the more significant.
The dog or cat deprived of fore-brain except thalamus does not react even when hungry to food placed before it. The muzzle has to be dipped into the food or the food put into the mouth for the feeding to occur. Food doctored with quinine, etc., is re jected. Nociceptive stimuli to skin, etc., evoke biting, barking, miaowing and withdrawal of the threatened part. Yet in all this defensive behaviour there may be little or no indication of the locality of the offending stimulus. The animal, it has been in ferred, can experience "pain"; it is not, however, able to acquire the simplest conditioned reflex. Its behaviour is confined to the stereotyped inborn reflexes. Its reflex behaviour is modifiable, however, to the extent that having hurt one foot it limps upon three legs until the hurt foot is healed. Its reactions, indicative of "pain," may possibly be pseudo-affective reflexes without psychical adjunct. But there is evidence from clinical studies (Head) that "pain" is among the reactions of the thalamus. Affective and emotional disturbances have come to be recognized as part of the syndrome of thalamic disease. The inference is therefore that the cerebral seat of mind does in some measure include—and so to say overlap upon—the thalamus even in higher mammals.
Passing to the lower mammals, the striatum is relatively smaller than in birds. Conformably with mammalian ancestry some of the striatum is ancient and olfactory and probably serves higher correlations of smell with taste and touch for locomotor and head reflexes involved in feeding. Another part, also ancient, receives exteroceptive and proprioceptive tracts (from thalamus) but no olfactory; it operates movements through an emissive path to the mid-brain. To these old parts there is a new addition of as yet undetermined function. The cat (Dusser de Barenne) with neopallium lost but striatum (and archipallium) retained, localized the direction of sounds. In man destructive disease within the striatum is thought responsible for the syndrome of paralysis agitans, characterized by tremor and tonic spasm.
From early reptilian stock came, it is held, the mammalian stock, as did independently and somewhat later in geological time the birds. In the course of bird evolution the pallium has pro gressed little or not at all, perhaps in correspondence with the avian lack of olfactory development, the pallium tracing its origin partly to nerve centres for smell. In mammals on the other hand the pallium taken over with primitive potentialities has proceeded to large and ultimately in some forms (apes and man) enormous development. Hence a progressive divergence might be expected between the courses of development of behaviour in birds and mammals respectively. The progress of bird-behaviour to its highest types might be expected to show little qualitative differ ence from behaviour of the old palae-encephalic type. Comparison of birds with bony fishes finds in both groups a great diversity of specialized forms with specialized behaviour evolved by hereditary organization, behaviour highly and rigidly stereotyped and fixed in character and little imbued with individual plasticity. In this respect both groups resemble the insects. The fish, however, re veals more individual modifiability (e.g., docility) than does the insect; and the bird on the average leads individually a more di versified life and has more power for "conditioned" moulding of innate instinctive behaviour than have fish (Herrick). Yet, there is a similarity of the behaviour of the two. Nothing essentially new in behaviour, not even in instinctive type, sunders the later group from the earlier.
Mammalian behaviour, on the other hand, in its course of evolution reveals attainment of certain additional new types of reaction, types different perhaps even radically from anything palae-encephalic. Indications of this are the replacement in some respects of "trial-and-error" learning by methods of "seeing through" or of "stopping-to-think" about a situation. Another is the "tooldom," if one may so call it, of man. And there is the specific human behaviour involving concepts and symbolic-think ing and employing complex speech. Of all this the development of the pallium is the correlative.
But it is never reached by any receptive nerve immediately; it is reached only through relay-systems which climb to it via suc cessive correlating-mechanisms. The recept-patterns which enter the neopallium (cerebral cortex) are therefore always greatly changed from those furnished to the first receiving stations by the groups of receptor nerves themselves. Yet, as is clear in the more primitive mammals just as the thalamus shows some sub division into regions individually concerned with recepts pre dominantly of one kind of source, retinal, auditory, cutaneous, etc., so in the neopallium connected with the underlying thalamus there are individual territories which receive patterns composed predominantly of recepts traceable to one kind—be it this, or that—of receptive source. There is thus some "localization of function" in the neopallium in correlation with some at least of the sets of receptor organs, or, psychologically put, some at least of the modalities of sense. Vision is an instance.
One of these steps is judged to have been the emancipation from an aquatic existence achieved partially by amphibia with conversion of paired fins into limbs, and development of lungs for air-breathing. The body's greater need for support in air than in water made of the limb a jointed motor prop for locomotion with movement of diverse direction and with fingers and toes for clasping and other use. The limb and the evolving nervous sys tem conspired so to say to draw advantage from this. The con quest of the land completed by the reptilian was accompanied by relative increase of the fore-brain. The land we may regard as a habitat of more varied difficulty and opportunity. Yet, launched from that stock, the primitive mammal was completely equipped for a land existence. Its fore-brain as judged from primitive existent forms was able, and grew more so, to learn with fewer repetitions and better retention; not only so, but its warm con stant body-temperature provided for cerebral and other activities uninterrupted by seasonal abeyance. One great branch of this stock, developing a mechanism (placenta) of nutrition and pro tection for the young within the mother's body (ultimately in the human case nine months long), entered into active and successful competition with other land-forms, and indeed upon great corn petition within itself. We learn, by comparison of the fossil mem bers of this great group with its present members of similar kind, that even with regard to allied forms the cerebral neopallium has become relatively much larger since the early Tertiary period. That is to say that in this group the modern individual has rel atively more neopallium than had its ancient ancestor of like form and body bulk.
In a good deal of this work it is now customary to apply the term reflex to the acts of the normal animal, e.g., dog. Hitherto the application of the term reflex in physiology and in medicine has been to reactions, through the nervous system, which either in man are known to be, or in animals there is cogent reason to believe are, unaccompanied by mental experience. The extension of the term "reflex" to such an act on the part of one's dog as its coming when called by name may be taken to mean not that the observer denies that mental action attaches to the dog's be haviour, but that the observer explicitly disregards them, and is studying the behaviour solely as neuro-muscular bodily reaction, hoping thus to study the brain, much on the lines on which the spinal cord may be studied.
Reflexes.—All purely spinal and hind- and mid-brain reflexes of the placental mammal seem to be innate. They are trans mitted by heredity and are the common property of the species, often of the genus. They include not only such simple acts as mastication, swallowing, the blink-reflex, the knee jerk, the scratch-reflex, crude sexual acts, etc., but standing, stepping, the falling cat's "righting" reflex, etc. They may develop only in adult maturity, but they are innate. Built up into chain-reflexes they make the basis of much instinctive behaviour.
Besides this type of reflex reaction the individual animal is able to develop other responses operating through its central nervous system. Thus, when the skin of one foot is subjected to a hurtful electric shock, retraction of the foot ensues. If along with or just preceding this, some other stimulus, e.g., a sound, be given, then after a number of regular repetitions of this concurrence, the "sound" itself evokes the retraction of the foot. The response to such an associated stimulus is called an "associated" (Bechterew) or "conditioned" (Pavlov) or "individual" (Beritoff) reflex. The "associated reflex" is (dog) a response for which the neopallium is a sine qua non. In entire absence of the neopallium (cerebral cortex) the dog is incapable of acquiring any such responses and loses any such as it has already acquired. A dog's normal every day behaviour is largely composed of such responses which the common happenings and experiences of its life as an individual from puppyhood onward have taught it. By loss of the pallium this stock of reactions is woefully reduced ; it could not maintain even its base existence. Its behaviour is cut down to a few rigidly fixed reactions. This is exemplified by Pavlov's decorticated dogs not able even to feed unless food were placed in the mouth ; and by Dusser de Barenne's cats, although there one of them, in which the archipallium had been spared, still found its food by smell, and fed itself.
By means of the pallium any agent which acts on a receptor can by training become a signal evoking a particular movement or a secretion. The training required is that the agent act several times concurrently with the act of movement or secretion, or im mediately before it, or, which is much less favourable, just after it (Beritoff). The movement or secretion to which the extraneous stimulus becomes attached as a signal is called the ground-act (Beritoff) or "unconditioned reflex" (Pavlov). In using the lat ter term it must be remembered that the protective movement in response to a hurtful stimulus which has for instance been much used as a ground-act, for which various neutral stimuli can, by training the individual, acquire the value of signals, is not itself entirely a reflex in the usual physiological sense of that term.
It is true that in the protective ground-act there is a kernel of reaction evocable even when only the spinal cord remains and therefore purely reflex. But to this in the protective act evoked by a similar stimulus in the intact animal there is much added. In this latter case cerebral and cortical reactions are superadded to the strictly reflex. The ground-act is therefore a behaviour-re sponse far more complex than that which is usually denoted physiologically a reflex. If we face the full biological situation it includes, instead of consisting merely of a pure physiological reflex, a large psychical reaction as well. It must, so to say, reverberate through wide regions of the pallium (cerebral cortex). Similarly, the secretion of the saliva in response to food in the mouth has been greatly used as a ground-act (Pavlov) and in terming it an unconditioned reflex we must remember that al though secretion of saliva can after severe curtailment of the nervous system be obtained as a pure reflex, it yet, as obtained in the feeding response of the intact animal, is but one component of an immense reaction with emotional and other mental ac companiments inevitably involving wide regions of the pallium. These ground-acts as reactions to the essential stimulus, e.g., food in mouth for salivation, are innate inherited reactions, although possessing cortical extensions. The attaching them to other stimuli, by training in the individual, constitutes the so-called "con ditioned reflex" of Pavlov, the "individually-acquired reflex" of Beritoff, and also involves the pallium. An individually-acquired reflex has been proved by the experiments of Pavlov to be in every case a response involving some part of the pallium. It is like the ground-act cortical, and it can be itself the ground-re sponse for a further individually-acquired (i.e., "conditioned") reflex, which latter again is of course cortical. This last is "secondary" by Pavlov, or by Beritoff the individually-acquired reflex of higher order.
At commencement of the acquiring of an individual reflex, the response tends to be evoked not only by exact repetition of the particular stimulus, but also by other stimuli broadly resembling it though not necessarily very closely. Anrep gives interesting figures of this. The individual reflex is said then to be in the generalized stage. Further training brings greater precision, in the sense that the response occurs only to more precise repetitions of the specific stimulus. This process is "differentiation." By this means it has been ascertained that the brain of the dog can dis criminate between notes only one-eighth of a tone apart ; and can hear notes of much higher pitch than the highest audible to man. It exhibits discrimination between figure patterns of relatively slight difference, e.g., a thicker and a thinner capital T. It fails, however, to discriminate colours although well discriminating different luminosities.
The "individually-acquired reflex" is termed a "deferred re flex" when the beginning of the conditioned stimulus considerably precedes that of the ground-response, although continued until that of the ground-response has begun. The individual reflex so ac quired brings its effect, e.g., salivation, only after the conditioned stimulus has endured for a time practically corresponding with that employed in the repetitions giving the training. The name "trace-reflex" is given to individual reflexes in which the con ditioned stimulus is allowed to lapse before the stimulus for the ground-response begins. The "trace-reflex" "differentiates" its stimulus relatively slowly and poorly; it is also less durable. It is noteworthy that although nociceptive stimuli have inborn defensive reflexes at command, such stimuli are difficult for in duction of individual acquired reflexes—the training is long and uncertain, although sometimes successful (Pavlov). This sug gests paucity of nociceptive afferent connections with the cortex in spite of probable wealth of connections with the thalamus. If some unusual stimulus is employed concurrently with an established "conditioned" stimulus and in face of this concur rence the ground-stimulus is omitted, the individual acquired re flex occurs only weakly or not at all. The stronger this foreign stimulus the greater its inhibitory effect. This inhibitory result wears off under repetition of the same "foreign" stimulus. It has been shown in Pavlov's laboratory that the degree of inhibition exerted by the "foreign" stimulus is directly proportional to the intensity of the "investigatory reflex" which it excites. By "in vestigatory-reflex" (Beritoff's "orientation reflex"—the reaction agrees with Head's "vigilance"), is denoted an attitudinizing of the head which is excited by all sorts of stimuli but on repetition of the same stimulus rapidly tends to die out. It may accompany various individual reflexes (Beritoff). It is destroyed by de struction of the pallium. It has been noted that incidental ac tivity of a natural innate skin-reflex, e.g., the scratch-reflex, may suffice to inhibit an acquired reflex belonging to quite another distant skin-region, e.g., of forefoot (Beritoff).
Experimental Investigations.—The effect of variously situate partial destructions of the cerebral cortex upon acquired reflexes has been studied (Pavlov) . The destruction of no one single even large field of cortex precludes totally or permanently all acquisition of "individual reflexes." Pavlov regards this as conclusive against the teaching that one special field for "associ ation" exists in the cortex. The extirpation of a part of the cortex of whatever region temporarily upsets the "conditioned" reflexes, and with selective disturbance of them. After bilateral extir pation of the occipital region (visual) the dog never again directs its movements by sight, and fails to distinguish size and distance of objects. Full and bilateral destruction of this region in man causes total blindness; but the dog after bilateral extirpation of even much additional to both occipital regions still discriminates light from dark, and good "conditioned" reflexes were acquired with light difference as stimulus. Bilateral extirpation of the temporal lobes (acoustic) precluded acquired reflexes to sounds, but only for a time ; conditioned reflexes to single sounds and even with some discrimination of tone-sequences could later be established. Yet after loss of the temporal lobes the dogs, and many observers have found it so, ceased permanently to respond to their names. Bilateral destruction of the parietal and "motor area" regions, especially impaired "acquired reflexes" trained on touch and proprioception respectively, and especially in the limbs. After bilateral removal of the frontal lobes formerly-acquired visual and auditory "conditioned reflexes" returned and new ones could be established soon after the operation (Babkin) . There ensued a persistent supersensitivity of the skin.
In sum, localized damage to cortical areas affects "acquired" reflexes selectively according to the species of receptor of their stimulus (psychologically, their sense-modality) and to the topo graphical seat of the lesion in the field of the cortex. Pavlov con cludes that the motor area of the cortex is merely the area of proprioceptive receptors. He denies to the cortex any special association area. "The cerebral cortex should be regarded as the essential organ for the maintenance and establishment of con ditioned reflexes." It may be added, on the basis of older experi ments, that total destruction of the pallium of only one hemisphere in the dog impairs its behaviour relatively little.
Instinctive behaviour has been defined (Lloyd-Morgan) as "that which is, on its first occurrence (in the individual) independent of prior experience; which tends to the well-being of the in dividual and the preservation of the race; which is similarly per formed by all the members of the same more or less restricted group of animals, and which may be subject to subsequent modi fication under guidance of experience." It is an innate behaviour trend which a certain more or less complex group of stimuli of external and internal origin can call forth.
The experiments of Ceni show that while in the hen the train of behaviour corresponding with incubatory and maternal rear ing of the young is not obviously disturbed by destruction of the whole pallium, in the bitch on the contrary not only does de struction of the pallium in toto cause all maternal instinctive be haviour to disappear, but that bilateral destruction even confined to the frontal region does so. Inherited behaviour as well as in dividual acquired behaviour is therefore laid up in the cerebral cortex of the dog.
With those on the dog may be compared observations on the rat (Lashley) in regard to ability to learn and retention of learnt behaviour e.g., for a maze or the entrance-fastening to a food box, and the effect upon them of pallial destructions. With learning of a simple maze acquisition occurs at normal rate after bilateral destruction of any one-third of the cortex, and similarly is re tained. With a test in which two alleys to food are offered, one lit the other dark, the food being attainable always by the lit, never by the dark one, bilateral destruction of the occipital third (visual) of the pallium abrogates totally the successful behaviour already learned, but offers no impediment to acquiring it as quickly as in the original training. With a more complex test for entrance to a food-box the learnt solution of it and its reacquisition is disturbed, but not completely abolished, by bilateral destruction of either the frontal or occipital regions.
Turning from mammals lower than the dog to others higher, the older experiments on the monkey, while showing greater im pairment of motor behaviour than in the dog, gave evidence of considerable powers of recuperation. In monkeys, the destruction of the precentral gyrus "the motor area" (perhaps proprioceptive) of both hemispheres, the animals having previously been trained in habits of manipulation, revealed on subsidence of the temporary paralysis perfect retention of these habits (Lashley). Nor did even the paralysis recur on destruction of the striatum subsequent to recovery from the diplegia. Even in the man-like apes the temporary arm-paralysis caused by destruction of the cortical "excitable" area for the arm produces no permanent impairment of individually-acquired motor habits executable by that arm. The peeling and other manipulations in eating a banana, the taking and holding of a cup of water to drink from (Graham Brown and Sherrington), the learnt "hand-shake" with a visitor (Leyton and Sherrington) are all recovered. Further, the recovery is not frustrated or undone by additional removal of the arm area of the opposite hemisphere or of the post-central gyrus of the same side.
But these are relatively restricted lesions and in the man-like apes, as in man, objective study solely by means of motor-be haviour apart from speech of the effects of damage to this or that field of neopallium (cerebral cortex) becomes less and less ade quate to the complexity of the phenomena if all reference to psychical accompaniment be eschewed. The fully developed pallium is mainly so to say a mental organ. To a spectator other wise unacquainted with the play, "Hamlet" in dumb-show would convey but meagrely that play's contents. The experiments of Thorndike and others go to show that lower mammals give little evidence for their possession of images in the form of ideas ; or in their learning of mazes, door-fastenings, etc., of doing so even by imitation. There is, it is found, some power of imitation, though not so much as has been generally supposed, in monkeys; and a questionable existence of image-ideas. A situation before which a cat is helpless a monkey will grasp. A chimpanzee will solve a situation by making use of some object present at hand as an implement ; will recall the position of an object it has seen placed in hiding a day previous; will in some cases fetch an object, remembered though out of sight, to serve as implement suitable for solving a newly-arisen situation (Yerkes) ; and has been ob served to pause in a waiting attitude trying, as it were, to "see" how to attain indirectly an objective unattainable directly, some what as a man "stops to think." It is inferred that the man-like apes form and retain memory-images not essentially dissimilar from man's memory-images.
It would seem that no gross lesion of the neopallium occurs without inflicting a certain degree of lasting disturbance upon mental reactions. It may be that that impairment will, by im proved analysis of the conditions, be found to be essentially of the same kind for all regions of the pallium. Be that as it may, it is already certain that disturbances predominantly in this or that sphere of sense are related regularly to spatially separate areas of the human cortex; and that impairment of the perform ance of "willed" movements by the muscles, especially of limbs and face in the opposite side, results from damage of a particu lar pallial region, the pre-central gyrus in man just as in the ape, and more severely. Speech per se without any paralysis of the motor mechanisms of its production is affected very commonly by injury of the cerebral cortex. The manner and degree of the dis turbance of speech so produced differs greatly in differently placed structural lesions. Their study, although difficult, affords perhaps the best opportunity for analytic examination of the mental func tions attaching to the neopallium in man. They form a theme too large and also too special to be entered upon adequately here.
One character of these movements is that they tend to spread. Thus when the point for the thumb is stimulated the movement will begin in the thumb, then under continuance of the stimulus, may spread to the fingers, then to the wrist, the elbow and shoul der, and even to the face and leg as well, so that the musculature of all one side of the body may thus be simultaneously convulsed. This spread is called the "march," because it resembles a feature, termed by Hughlings Jackson, the "march," in the epileptic seizure. Strong and prolonged stimulation of a "motor cortical" point is apt to be followed by a clonic (convulsive) spasm re sembling that of the true epileptic seizure. The representation of certain fields of the musculature of the body is more liberal than that of others. Variety of movement rather than power of movement seems to demand extent of cortex. The cortical area for the thumb (gorilla and chimpanzee) is larger than those for the whole of the abdomen and chest combined. The cor tical area for the tongue (anthropoids) is larger than that for the whole of the neck. Only in very few cases is the movement bilateral, i.e., both right and left, from unilateral cortical stimula tion. One of these rare instances is that of the vocal cords which bilaterally adduct (phonation). Another is that of the eyelids, which blink for both eyes. A condition for obtaining the motor responses from this "motor" field of the pallium is that the nar cosis under which the animal is necessarily placed at the time of experimenting must not be too heavy. It is known from ob servations in man by the surgeon (Harvey Cushing and others) that no pain or indeed other sensation attaches to electric stimu lation of the "motor cortex." All that is felt, even in the fully conscious person, is some perception of the movement which is evoked. If the narcosis go beyond inducing sleep of a natural depth, no visible response to stimulation, however strong, is ob tainable from the pallium, although spinal reflexes, e.g., the knee jerk, are still readily elicitable by their appropriate stimuli.
It was at one time thought that the response on application of electrical currents to this cerebral surface was due to stimulation, not of the cortex itself, but of bundles of nerve-fibres under the cortex. The distinction if existent would not be of much signifi cance, because such fibres must issue from the cortex; that it is, however, some element in the cortex itself which is excited may be regarded as established. Probably the element in question is the large nerve-cell, of which numbers are scattered throughout this excitable field. Each such large cell sends a long thread-like fibre down far beyond the confines of the forebrain itself. These cells get severally into touch with the primary motor nerve-cells in the various segments of the head and body. They form to gether a direct path from motor cortex to the spinal cord, etc., the pyramidal tract.
When this excitable field of the cortex was first investigated it was thought by some that it might prove to be as the immediate starting-place of a path executive of "willed" cuts of movement. The immediate and severe paralysis of "willed" movement, which, in man and the monkey, results from destruction of this excitable field of cortex, supported such a view. But there follows in course of no long time a remarkable "restitution" of the "willed" move ments, even in the man-like apes. And this is not due to vicarious functions on the part of the corresponding area of the opposite half of the pallium (Graham Brown and Sherrington) or of the underlying striatum (Lashley). The inference is that other fields of cortex than the so-called motor and other routes than the pyramidal tract are capable of carrying out willed acts.
That the movements excited from the "motor cortex" are produced via the fibres of the pyramidal tract seems clear ; they are precluded by severance of that tract below the cortex. But that they resemble truly closely "willed" acts of movement is un likely on several grounds : Severance of the afferent spinal roots supplying a limb although it does not impair the motor sup ply of the muscles, etc., in the least, disturbs the willed move ments of the limb, very greatly indeed, rendering them so inac curate and wild as to be worse than useless. The animal, e.g., monkey, soon relinquishes use of the "deafferented" limb alto gether. Electrical stimulation of the field of the motor cortex corresponding with the deafferented limb nevertheless evokes in it all the movements normally so elicitable, and with no detected departure from the normal. The willed movements are grossly disturbed; yet the motor responses of the "motor" cortex re mains practically unaffected. (2) Degeneration experiments show that the spinal terminals of the fibres (pyramidal tract) from the motor cortex are scattered actually among the ultimate motor cells themselves. The motor cortex presumably, therefore, makes direct synaptic junction with the final motor cell which directly innervates the muscle. This simplicity of connection of the "motor" cortex with the muscle could hardly provide for the complexity of a "willed" movement. But it accords with the further fact that under stimulation of the "motor" cortex the rate of rhythm of response of the muscle follows the rhythmic stimulation of the cortex pari passu even up to 18o per sec. (Cooper and Denny-Brown). Also the time interval between de livery of the electrical stimulus to the motor cortex and the re sponse by the muscle is only 13a, which is less than the latent period for many spinal reflexes (Cooper and Denny-Brown). (3) Recent observations indicate that the electrical and myo graphic behaviour of the muscles under motor-cortex stimulation denotes conflict of excitatory with inhibitory influence, simulta neously exerted on the same muscle. The clonic after-action so characteristic of motor-cortex excitation seems traceable to alter nating excitation and inhibition (Cooper and Denny-Brown). All this renders it unlikely that the "motor" cortex and the pyram idal tract descending from it to play upon the motor nerve-cells yield of themselves, at least when excited artificially (i.e., elec trically in experiment) movements truly resembling "willed" movements.
Two patients offered opportunity while in a fully conscious state for elicitation of movements of the right hand by electrical stimulation of the motor cortex (Cushing). As reported from their own introspection the reaction was attended by no sensation other than a secondary awareness of changed position of hand and fingers. With the anthropoid ape an impressive observation repeatedly noted is the seeming entire ignorance on the part of the animal, on its awakening from a "motor cortex" ablation ex periment, of any disability precluding its performance of its willed acts as usual. Surprise at the failure of the limb to exe cute what it intended seemed indubitably the animal's mental attitude, and not merely for the first few minutes but for many hours. The animal was slow to realize the limb's inability. It was often many hours before repeated and various failures to exe cute ordinary acts for climbing, feeding, satisfying its curiosity, etc., gradually impressed upon the animal that the usual services were no longer to be expected from the limb. Even after this lesson seemed to have been learnt an emergency would call forth a new attempt and surprise at failure as though the former experi ence has been for the moment again forgotten. The impression conveyed is that the fore-running idea of the act intended is present and as definitely and promptly developed as usual. The surprise seems to argue unfulfilled expectation, and defect in the motor execution rather than in the mental execution of the act, raising the question whether the function of the part of the cortex ("motor") ablated in such cases be not indeed infra-mental.
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