WUNDT: Classics in the History of Psychology
-- Chapter 6
WUNDT: Classics in the History of Psychology
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Christopher D. Green
York University, Toronto, Ontario
ISSN 1492-3173
Principles of Physiological Psychology
by Wilhelm Wundt (1902)
Part I. The Bodily Substrate of the Mental Life
Chapter 1. The Organic Evolution of Mental Function
Chapter 2. Structural Elements of the Nervous System
Chapter 3. Physiological Mechanics of Nerve Substance
Chapter 4. Morphological Development of the Central Organs
Chapter 5. Course of the Paths of Nervous Conduction
Chapter 6. Physiological Function of the Central Parts
[p. 241] CHAPTER VI
The Physiological Function of the Central Parts
§ 1. Methods of Functional Analysis
EVEN if we knew the course and the interconnexions of all the paths of nervous conduction, there would still be one thing needful for an understanding of the physiological function of the central parts: a knowledge of the influence exerted upon the processes of innervation by the central substance. And there is but one possible way of determining this influence: we must attempt to ascertain the function of the central parts by means of direct observation.
Under this limitation, two roads are open to the investigator who would gain an insight into the complicated functions of the nerve centres. He can arrange the phenomena in order of their physiological significance; or, accepting the lines of division drawn by the anatomists, he can examine into the separate function of each individual central region. It is obvious that the former of these procedures is to be preferred: not only because it lays the chief emphasis upon the physiological point of view, but also because, when the investigation of the conduction paths is over and done with, a doubt must still remain whether every one of the principal parts distinguished by anatomy represents a similarly well-defined functional area. In the present state of our knowledge, however, it is impossible to carry out the physiological programme with any sort of completeness. The method can be applied, with any hope of success, only to the two lowest central organs, myel and oblongata, where the phenomena may be referred without exception to two basal physiological functions, reflex and automatic excitations: the latter oftentimes deriving directly from nutritive influences exercised by the blood. We may well suppose that these same two basal functions are the source of the physiological activities of the higher central parts. At the same time, the interrelation of the phenomena is here so complicated, and their interpretation in many cases so uncertain, that it seems wiser, for the present, to examine each individual central area for itself with a view of discovering its physiological properties. We shall, accordingly, preface our enquiry by a general discussion of reflex and automatic action, in the course of which we shall have opportunity fully to consider the functions of the lower central regions; and we shall [p. 242] then proceed to investigate the brain and its parts in regular sequence from below upwards. We may, however, omit structures which, like pons, crura and corona, are intended in the main simply for the conduction of processes of innervation, and have therefore been sufficiently dealt with in the preceding Chapter.
The methods employed in the, functional examination of the central organs are, in general, the same as those which find application in the study of the conduction paths, save that anatomical investigation, which there holds the first place, must naturally play a merely subordinate part now that we are concerned with the activities of the organs. We shall ask assistance, where possible in combination, from physiological experiment and from pathological observation; and we shall pay attention, under both headings, to symptoms of stimulation and symptoms of abrogation. The special conditions of the phenomena are such that stimulation experiments must for the most part be employed in the general study of the reflexes and of automatic excitations, whereas the functional analysis of the various departments of the brain must rely almost exclusively upon symptoms of abrogation.
§ 2. Reflex Functions
(a) -- Spinal Reflexes
The simplest mode of central function, and the mode that still approximates most nearly to a simple conduction of processes of stimulation, is the reflex movement. In so far as the reflex process is a special form of conduction, we have discussed it in the preceding Chapter. But it is more; it is a form of conduction modified in various ways by the influence of the central substance. In the first place, the reflexes are not, like the processes of stimulation, conducted in both directions in the nerve fibres, but only in the one direction, from sensory to motor path: a fact which, as we explained above, is in all probability connected with the twofold mode of origin of the nervous processes in the motor cells.[1] Secondly, the reflexes clearly show, in their dependence upon the stimuli that release them, the effects of the peculiar conditions of excitability obtaining in the grey substance. Stimuli that are weak and of brief duration fail, as a rule, to evoke a reflex movement: but the movement, once it appears, may far surpass in intensity and duration the direct muscular contraction set up by the same stimulus. Lastly, the central character of these processes is evinced by the dependence of the reflex centres upon other central areas with which they stand in connexion. Thus, it has been observed that the reflex excitability of the myel is enhanced by removal of the brain.[p. 243] It appears, therefore, that inhibitory influences are continually proceeding from the higher central organs, and lessen the irritability of the lower lying reflex centres. These are, in general, still more strongly inhibited if other sensory central parts, with which they are connected, are stimulated along with them. The reflex, e.g., released by excitation of a sensory myelic root or of its peripheral radiation is inhibited by simultaneous stimulation of the dorsal myelic columns, of quadrigemina and thalami, of another sensory root, or finally of peripheral organs within which sensory nerves are distributed. It is not improbable that the influence of the cerebral hemispheres belongs to the same group of phenomena; for this too proceeds, in all likelihood, from the terminations of the sensory conduction paths in the cortex. It has been observed that the inhibition of reflexes in mammals is especially strong if stimuli are applied directly to the centrosensory areas of the cerebral cortex.[2] The mechanism of reflex inhibition would thus appear to be the same throughout: reflexes are inhibited, when the sensory cells that should transfer their excitation to motor cells are simultaneously excited with a certain degree of intensity from other sensory areas. This inhibitory effect is, however, limited to the condition that the areas whose stimulations interfere lie at a sufficient distance from one another in space. If adjacent sensory parts, or the nerve paths corresponding to them, are stimulated, the result resembles that of summation of stimulations within the same sensory area: that is to say, the interference gives rise not to inhibition but to intensification of the excitatory processes. Lastly, reflex excitations may also suffer inhibition from central elements interpolated in their own proper path. This is the interpretation of the fact that stimuli applied to the cutaneous radiations of the sensory nerves are more effective than stimuli applied to the nerve trunks, and that contrariwise the nerve roots become more irritable after their passage through the spinal ganglion. We must suppose, that is, that the subdivision of fibres in the sense organ serves, on the one hand, to increase irritability, and that, on the other, the excitation which arrives at a spinal ganglion cell there undergoes a certain inhibition: a combination of circumstances which, naturally, brings it about that the nerve trunk possesses a relative minimum of reflex excitability.[3] The general lines to be followed, in an attempt to explain all these phenomena, are laid down by the principles of nerve mechanics that govern the reciprocal relations of excitatory and inhibitory effects, and by that morphological differentiation of the central elements which, in all probability, runs parallel with them.[4][p. 244]
There are, further, many phenomena which show, still in accordance with the general principles of nervous activity, that the individual reflex excitation aroused by a sensory stimulus does not by any means come upon the scene as interrupting a state of absolute non-excitation in the nervous elements. On the contrary, the state which we term the 'state of rest' is really a state of oscillation -- as a rule, of oscillation about a certain position of equilibrium -- in which the excitatory and inhibitory forces counteract one another. It is a state in which, on the average, there is a slight preponderance of permanent excitation, though this may be transformed, under special conditions, and more particularly under the influence of antagonistic effects, into a preponderance of permanent inhibition. In this way, the single transient reflex process is superinduced upon a reflex tonus, whose effects become apparent whenever there is interruption of the sensory paths in which the permanent innervation of reflex excitation is conducted. Thus, transsection of the sensory roots of an extremity is followed, in animals, by an atonic, quasi-paralytic state, which however neither abrogates the influence of voluntary impulses upon the atonic muscles nor prevents their action by way of concomitant movements.[5] As regards intensity and distribution, these tonic reflex excitations appear, further, to stand under the regulative influence of all the manifold conditions imposed upon the organs by the nature of their functions. Reflex stimuli, which release transient reflex movements, may accordingly produce radically different results, according to the state of the pre-existing tonus and of the relative distribution of excitatory and inhibitory forces. Thus SHERRINGTON found, in observations upon animals whose myel was cut through in the cervical region, that extensor reflexes appeared if the leg was in the position of flexion, and flexor reflexes, if it was extended. We may also appeal to this influence of the variable conditions of permanent tonic excitation upon the individual reflex movement for explanation of the fact that the law of diffusion of reflexes with increasing intensity of stimulus (discussed above, p 162) admits of exception; the general and relatively constant conditions of reflex conduction are cut across by the more variable influences arising from the reciprocal regulation of sensations and movements.[6]
(b) -- Metencephalic (Oblongata) and Mesencephalic Reflexes
The reflexes that have their seat in the oblongata are, in general, of a more complicated character than the spinal reflexes. This organ is, in particular, the seat of a number of compound reflexes, which play an important part in various physiological functions. We may mention the [p. 245] movements of inspiration and expiration, with the closely related processes of coughing, sneezing, vomiting; the muscular changes involved in the act of swallowing; the mimetic movements; vascular innervation and the movements of the heart. Many of these reflexes stand in an intimate relation of interdependence, as is indicated by the fact that their peripheral paths are oftentimes laid down in the same nerve trunks. Some of the above-named processes, such as the movements of respiration and the heart beat, result from a plurality of causes, and therefore continue after interruption of the reflex paths; in such cases, the reflex is but one of several determinants, and its influence is correspondingly restricted. Others, again, like the movements of swallowing, appear to be pure reflexes; they are abrogated by interruption of the sensory conduction to the reflex centre, even though the motor conduction to the muscles governing the movement have been left intact. All these reflexes alike, however, differ from the spinal reflexes on the point that, as a general rule, their sensory stimuli pass at once to a large number of motor paths. Many of them are essentially bilateral, and do not require the action of strong stimuli for their extension from the one to the other side of the body. Thus the respiratory movements, which are released by excitation of the pulmonary radiation of the tenth cranial nerve, involve motor roots that issue on both sides from the oblongata and from the cervical and thoracic portions of the myel. These movements furnish, at the same time, an illustration of a self-regulating reflex, which contains within it the impulse to continued rhythmical repetition. While the collapse of the lungs in expiration serves reflexly to start the movement of inspiration, their distention with inspiration serves conversely to excite the muscles of expiration. If the reflex impulse given to the expiratory muscles in inspiration is too weak to bring them into active exercise, it simply inhibits the antagonistic inspiratory muscles. This is the case in ordinary quiet breathing, in which inspiration alone, and not expiration, is connected with active muscular exertion. In the movements of swallowing, the regular sequence is apparently maintained by a different mode of self-regulation. The act of swallowing consists of movements of the larynx, pharynx and oesophagus: movements that succeed one another in regular order, on the application of a stimulus to the mucous membrane of the soft palate. The succession of movements in this instance is, perhaps, regulated in the way that stimulation of the soft palate releases, first of all, simply the movement of the palatal muscles, and that this in turn acts as a stimulus for the reflex elevation of the larynx and contraction of the pharyngeal muscles. In fine, then, it is probable that all these oblongata reflexes, whose detailed description belongs, of course, to physiology, are characterised by the combination of movements for the attainment of definite effects, -- the manner of combination being determined, oftentimes, by a [p. 246] mechanism of self-regulation, itself conditioned upon the reciprocal relation of a number of reflex mechanisms. A second noteworthy property of these reflexes is the following. The motor path of a given reflex movement sometimes stands in connexion with a second sensory path, from which, accordingly, the same movement may be aroused. Secondary sensory paths of this kind are connected, in particular, with the respiratory centres, so that the combined activity of the muscles of respiration becomes available for other purposes than those of inflation and emptying of the lungs. There is, e.g., a connexion of the sensory nerves of larynx and oesophageal mucous membrane (i.e. of the superior and, in part, of the inferior laryngeal nerves), and of the branches of the fifth cranial nerve distributed to the nose, with the centre of expiration. Stimulation of these sensory areas produces, first, inhibition of inspiration and then violent expiration. The latter is, however, preceded by a strong inspiration, the immediate consequence of the establishment of inhibition, due to the persistence of the influence of automatic excitation which we discuss below. Coughing and sneezing are, accordingly, reflexes of expiration; but they are not excited from the sensory area of pulmonary radiation of the vagus, from which the impulse to expiration ordinarily proceeds. They are distinguished by the fact that stimulation of the nasal branches of the trigeminus arouses not only the respiratory muscles but also the motor nerve of the face, the facialis, to reflex activity. The sneezing reflex consequently affords a direct transition to the mimetic reflexes of laughing, crying, sobbing, etc., in which again the muscles of the face unite in conjoint function with the muscles of respiration.[7] Further: the secondary sensory path from the expiratory centre to the mucous membrane of the air passages is paralleled by a similar path from the inspiratory centre to the cutaneous investment of the body. We are thus able to account for the movements of inspiration produced by intensive stimulation, especially cold stimulation, of the cutaneous surface.
It may, then, be taken as a matter of common occurrence that in the oblongata a given motor reflex path is connected with a number of different sensory paths. But more than this: one and the same sensory path may enter into connexion, conversely, with several reflex centres, so that its stimulation arouses coincidently various kinds of reflex movements. Here belong, e.g., the mimetic reflexes, mentioned above, in which movements of respiration are combined with facial movements. A similar arrangement of connexions is in part responsible for the interaction of respiratory movements and heart beat. The heart is supplied by two sorts of nerve paths, which affect the sequence of beats in opposite ways: accelerating nerves,[p. 247] which increase the rapidity of heart beat, and inhibitory nerves, which diminish it, or bring the organ to a complete standstill. Both may be reflexly excited; but the centre for the accelerating fibres is intimately connected with certain sensory paths that run to the heart in the spinal nerves for the last cervical and first thoracic ganglion of the sympathetic; and the centre for the inhibitory fibres with others, that take their course for the most part in the cardiac branches of the vagus. Hence stimulation of the great majority of sensory nerves, and in particular of the cutaneous, laryngeal and intestinal nerves, produces inhibition, and stimulation of the sensory fibres that enter the muscles produces acceleration of the heart beat: this latter fact explains the increased action of the heart that accompanies general muscular exertion. Similar results follow from the movements of the lungs: their inflation accelerates, their collapse reduces the frequency of heart beat. The respiratory movements are therefore regularly accompanied by fluctuations of the pulse, whose rapidity increases in inspiration and decreases in expiration. On the whole, that is, the movement of the blood is accelerated by enhancement of the movements of respiration. Again, we find the same kind of interaction between the reflex connexions of cardiac and vascular innervation. The vessels are governed, like the heart, by motor and inhibitory nerves, both of which may be reflexly excited. Stimulation of most of the sensory nerves releases the motor reflex, i.e., acts upon the nerve fibres which constrict the small arterial blood vessels and thus produce an increase of blood pressure in the larger arteries, and which are therefore termed presser fibres. The only exception to this action is in the vessels of the part of the skin to which the stimulus is applied: these vessels usually dilate, either immediately or after a brief stage of constriction, and thus occasion the hyperæmia and redness of the stimulated parts. There are, however, various sensory areas which stand, conversely, in direct reflex connexion with the inhibitory or depressor fibres of the blood vessels, and whose stimulation leads accordingly to a widespread dilatation of the smaller vessels. Here belong, in particular, certain fibres of the vagus, which radiate within the heart itself and form its sensory nerve supply: fibres which, in all probability, are exclusively devoted to this reflexly mediated interaction between cardiac and vascular innervation. For their stimulation must be effected, in the normal course of physiological function, by increased action of the heart; this, in turn, is produced by increase of blood pressure and of the amount of blood contained in the arterial system; and this, once more, can be compensated only by a dilatation of the small arteries, which permits the outflow of the blood into the veins, and thus at the same time reduces the arterial blood pressure. We see, in fine, that all these reflexes of the oblongata stand in relations of interdependence, such that the functions discharged by this central organ mutually regulate and [p. 248] support one another. An intensive cold stimulus applied to the surface of the skin produces, reflexly, a spasm of inspiration and an arrest of heart beat. But the danger which thus threatens the life of the organism is avoided, since the expansion of the lungs serves, again reflexly, to excite expiration and acceleration of cardiac movement; while at the same time the stimulation of the skin brings about, by way of yet another reflex, a constriction of the smaller arteries, and so prevents any excessive emptying of the arrested heart. In many of these cases, as in a certain number of the reflexes proceeding from the myel, the central transmissions have simply a regulatory significance. The peripheral organs are the seat of direct innervation effects, due perhaps to special ganglion cells lying within them, perhaps to the excitomotor properties of the muscle fibres themselves; and the addition of the system of spinal and oblongata reflexes can do no more than modify these effects by way of excitation or of inhibition.[8]
In all probability it is the nerve nidi of the oblongata, with their intercurrent central fibres, that we must consider as the principal reflex centres of this organ. The complicated character of the metencephalic reflexes appears to find its sufficient explanation in the anatomical conditions of these nerve nidi. They are, upon the whole, more strictly isolated than are the centres of origin of the spinal nerves. But, as an offset, certain nidi are closely connected by special central fibres both with one another and with continuations of the myelic columns. These two facts, taken together, explain the relative independence and singleness of aim of the oblongata reflexes. Myelic fibres are involved in these reflexes to a very considerable extent; and it is therefore probable that they are brought together, first of all, in some cinereal formation, and only after leaving this enter into connexion with the nerve nidi to which they are assigned. Thus, the respiratory motor fibres are, perhaps, collected in a special ganglionic nidus, which stands in connexion with the nidus of the vagus nerve. We may fairly suppose that this significance attaches to several of the grey masses scattered in the reticular substance. On the other hand, it is not probable that movements so complicated as the mimetic movements, or the movements of respiration and swallowing, possess each a single ganglionic nidus as their special reflex centre. Apart from the fact that centres of this sort, for complicated reflexes, have never been demonstrated, their existence is negatived by the nature of the movements themselves. The respiratory movements, e.g., evidently require us to posit two reflex centres, the one for inspiration, the other for expiration. Various mimetic reflexes, like laughing and crying, can he much more easily explained on [p. 249] the assumption of a reflex connexion, joining certain sensory paths at one and the same time with the respiratory centres and with determinate parts of the nidus of the facialis, than on that of an especial auxiliary ganglion, serving directly to initiate the above complex group of movements. In the same way, the movements of swallowing must be derived, like the respiratory movements, from the principle of self-regulation; we must suppose that the first movement of the entire process gives, as it is made, the reflex stimulus to the second, this the stimulus to the next following, and so on.
Of the four 'specific' sensory stimuli, two only are concerned, to any great extent, in the arousal of reflexes by way of sensory nerves: impressions of taste, and light stimuli. The former stand in reflex relation to the mimetic movements of expression; to reflexes, i.e., some of which (as we remarked above) readily combine with reflex respiratory movements, and thus lead us to infer a close connexion of the corresponding reflex centres. Light stimuli regularly evoke a twofold reflex response: first, closure of the eyelid, with a direction of the two eyes inward and upward, and secondly contraction of the pupil. Both reflexes are bilateral, though with weak excitations the movement is more pronounced upon the stimulated side. The reflexes released by way of the auditory and olfactory nerves appear in the neighbourhood of the external sense organs; if the stimuli are extensive, appropriate movements of the head may also be induced. In man, the proximate auditory reflexes are for the most part confined to contractions of the tenser tympani, which presumably accompany every sound stimulation: but in many animals reflex movements of the external ear are clearly observable.
If the stimulus is extensive, or the degree of irritability unusually high, the sphere of reflex activity may be extended beyond the limits of the direct reflex connexions. This phenomenon of diffusion is more definite and uniform for the cranial than it is for the spinal nerves. In the case of the optic nerve, e.g., the reflex to the muscles that move the eye-ball is connected, in extensive stimulation, with contraction of the corresponding muscles for movement of the head; and the facialis reflex to the orbicularis palpebrarum may be accompanied by concomitant movements of the othes mimetic muscles of the face. Reflexes touched off from the gustatory nerve fibres may cover a wider territory; they are apt to involve, not only the facial nerves, but the vagus centre as well. Stimulation of the sensory nerves of respiration is confined, as a rule, within the limits of its original reflex area. The strongest excitation of the central trunks of the pulmonary branch of the vagus has no reflex effect beyond the tetanus of inspiration. The reflex connexions of the expiratory fibres are more far-reaching. Stimulation of the sensory laryngeal nerves, and especially of their peripheral ends, is [p. 250] likely to involve the muscles of the face and of the upper extremity. But the fullest and most extended reflex relations are those of the trigeminus, the largest of the sensory cranial nerves. Stimulation of the trigeminus affects, first of all, its own motor root, which supplies the masseter muscles; and passes from this to the nerves of the face, the respiratory nerves, and finally to the whole muscular system of the body. There are two evident reasons for this range of reflex effect. First and generally, the trigeminus controls the largest sensory surface of all the sensory nerves, so that its nerve nidi also occupy a wide area, and opportunity is thus given for manifold connexions with motor centres of origin. Secondly and particularly, the position of its nidi is favourable. The superior nidi are situated, above the oblongata proper, in the pons; i.e. in the organ in which the ascending columns of alba are grouped together, by the interpolation of cinerea, to form the various bundles of the crus. We see, therefore, why it is that lesion of the oblongata and pons in the neighbourhood of the nidi of the fifth cranial nerve is followed by general reflex spasms. The result need not, of course, be attributed solely to these nidi: the stimulation in such cases may affect other sensory roots of the oblongata as well.[9]
(c) -- Purposiveness of the Reflexes. Extent of Reflex Phenomena
The reflex phenomena bear upon them the mark of purposiveness. As regards the oblongata reflexes, this characteristic appears at once from the above description of their conditions and of their orderly co-operation. But the spinal reflexes also show, for the most part, a certain degree of the same quality. Thus, if a stimulus be applied to the skin, the animal makes a movement of arm or leg that is obviously directed upon the removal of the stimulus. If the reflex becomes stronger, the arm or leg of the opposite side will make a similar movement, or the animal will jump away, apparently to escape the action of the stimulus. Only when the movements take on a convulsive character, as they do with extremely intensive stimuli or in states of unusual excitability, do they lose this expression of purposiveness. These facts have suggested the question whether the reflexes may properly he regarded as mechanical consequences of stimulation and of its diffusion in the central organ, or whether they are actions of a psychical kind, and as such presuppose, like voluntary movements, a certain amount of consciousness. Worded in this way, however, the question is evidently misleading. There can be no doubt that the arrangements in the central organ can produce purposive results with mechanical necessity; we have the same phenomenon in any perfected form of self-regulating machinery. Moreover, the oblongata reflexes are highly purposive, and nevertheless [p. 251] dependent upon definite mechanical conditions Again, there is no reason whatever why a sensory stimulus should not release a reflex movement and arouse a sensation or idea at one and the same time: so that we cannot take the absence of all conscious process as the direct criterion of a reflex movement. On the other hand, the definition of the reflex would, it is true, be indefinitely extended, and the term would cover practically the whole range of organic movement, were we to apply it to any and every movement released in the central organ by the action of sensory stimuli. Suppose, e.g., that I make a voluntary movement, in order to grasp some object that I see before me. this, which is indubitably an act of will, still falls under the general heading of a movement released by sensory stimulation. It lacks, however, and conspicuously lacks, an attribute which is specifically characteristic of the reflex; the attribute, indeed, that first gave rise to the distinction between reflex and voluntary action, and without which the distinction loses all meaning. A movement mediated in the central organ by way of response to sensory stimulation, if it is to be denominated a reflex movement, may not bear upon it the marks of psychical causation; i.e., the idea aroused by the stimulus may not constitute, for the agent's own consciousness, the motive to the external movement. My involuntary reaction to a sensed stimulation of the skin is, therefore, a reflex, so long as the sensation remains a mere accidental concomitant of the movement, so long, that is, as the movement would be made in precisely the same way without such a concomitant sensation. On the other hand, reaction is not a reflex, if I voluntarily put out my hand to seize the stimulating object that is pressing upon the skin; for in this instance the movement is conditioned, for the agent, upon the conscious process. In the individual case it may, naturally, be difficult to decide, especially if the observations are made from the outside, whether a given movement is or is not a reflex. But this practical difficulty does not justify our setting aside altogether the criterion that distinguishes the reflexes from other forms of action, and leaving out of account the fact that, while related by their purposiveness to psychically conditioned movements, they differ from them, clearly and definitely, in the lack of conscious intermediaries. It is precisely this criterion that makes the reflexes an easily distinguishable and characteristic class of organic movements. We may also mention a further aspect of reflex action, closely connected with the criterion just discussed, though naturally of less universal application: the fact that reflexes follow immediately upon the operation of sensory stimuli, while psychically conditioned movements admit of a longer or shorter interval between stimulus and movement. What holds of this holds also of other objective characteristics, as e.g. that of the possibility of choice between different means. Such criteria are not always applicable: partly, because [p. 252] the characteristics do not attach at all generally to psychically mediated movements, but partly, too, because the purposive nature of the reflexes leaves a certain amount of room for difference of interpretation.
If we admit that these criteria are adequate to the empirical delimitation of the reflexes, as a readily distinguishable group of organic movements, we must also accept the conclusion that the central reflex area, in man and in the higher animals that resemble him, probably does not extend higher up than the mesencephalic region. In all cases where a sensory stimulus is conducted to the cerebral cortex, and there for the first time transformed into a motor impulse, the central transference appears, without exception, to involve the interpolation of psychophysical intermediaries; so that the action is presented to the agent's own consciousness as psychically conditioned. Many authors, it is true, speak of 'cortical reflexes' as of an established fact. But they are using the term reflex in a wider sense, in which any and every movement that results from sensory stimulation is denominated a reflex, whether psychical intermediaries are brought into play or not. From this point of view, the voluntary action is sometimes defined as a 'cortical reflex.' It is clear that such an expression deprives the word 'reflex' of all special significance. It is also clear that the retention of the term in its stricter meaning is extremely important; for the origin of a class of movements that are at once purely physiological and yet purposive in character is a real and distinct problem. We cannot, of course, enter upon this question of origin at the present time; we can answer it only when we come to examine the various forms of animal movements. We may, however, point out, in view of the following discussion of the functions of the different central regions, that what holds of man in this connexion does not necessarily hold of the animals. We may lay it down as a general proposition that, in man, the centre at which the idea of the reflex gives way to the idea of the psychically conditioned action is the cerebral cortex. But the law is not universally valid; not even valid for all the vertebrates. It is a result of that progressive centralisation in the ascending direction, of which we have spoken in the preceding Chapter, that the mesencephalic areas which, in man, function simply and solely as reflex centres, appear in the lower vertebrates still to be centres for psychically conditioned movements. Indeed, the facts suggest that in the lowest vertebrates, where the cerebrum as a whole is of very minor importance, even the oblongata and the myel may possibly, up to a certain point, mediate movements of this psychical kind. Lower yet, in the invertebrates, they may proceed from any one of the peripheral ganglia; and in the protozoa they evidently have their seat in the general sensorimotor protoplasmic substance of the body. The centralisation of the psychical functions in the brain, that is, goes pari passu with their decen-[p. 253]tralisation in the bodily organs; and this decentralisation corresponds to an extension of the reflex functions. Hence, in the lowest animals, all movements possess the character, not -- as is sometimes maintained in the interest of certain ingrained dogmas -- of reflexes, but rather of psychically conditioned movements.[10]
§ 3. Automatic Excitations
(a) -- Automatic Excitations in Myel and Oblongata
The phenomena of 'automatic function' are in so far parallel to the phenomena of reflex action that they are processes of a purely physiological character, and accordingly have nothing in common with processes which, like voluntary actions, recollections, etc., present themselves to us in direct experience as 'psychically conditioned.' In this purely physiological sense, the automatic functions are therefore nearly allied to the reflexes. But they differ from them in the point that the automatic stimulation processes take their origin in the nerve centres themselves, and are not released by a stimulus conducted to the centre from without. As a general rule, the motor areas that evince reflex phenomena are also susceptible of automatic excitation. The results of these automatic stimulations need not be muscular movements, or inhibitions of particular movements, but may also take the form of sensations. Hence it is not always easy to discriminate them from reflex excitations, or from the direct effects of external stimuli. For all our senses are continually affected by weak stimuli, which have their ground in the structural conditions of the sense organs themselves, and, so far as the sensory centres are concerned, these weak excitations, such e.g. as are aroused by the pressure exerted in the eye upon the retina, in the labyrinth of the ear upon the sensitive membranes, are, of course, the equivalent of stimulation from the outside. If we rule out cases of this kind; it appears that the sole source of automatic excitation is to be looked for in sudden changes in the chemical constitution of the nervous substance, caused for the most part by alteration of the blood.
As regards the myel, the effects of automatic excitation are shown most clearly by the muscles of certain organs of the nutritive system: e.g. the circular muscles of the blood vessels, whose lumen becomes enlarged after transsection of the myel,[11] and the sphincter muscles of bladder and intestine, where similar results have been observed.[12] The tonic excitations of the skeletal muscles appear, on the other hand, to be exclusively reflex in character (cf. p. 93, above), since transsection of the muscle nerves produces [p. 254] no change in muscular tension, apart from the concomitant twitch and its elastic after-effects.[13] Automatic excitations seem, however, to occur, alongside of reflex excitations, in the peripheral organs that are separated from the central organs proper and provided with independent centres, e.g. in the heart and intestinal muscles (cf. p. 248, above).
The automatic excitations that proceed from the oblongata are of especial importance. Here, too, the reflex centres appear, without exception, to be automatic centres as well. The movements that arise in them are consequently continued, after the sensory portion of the reflex path has been interrupted. Here belong the movements of respiration and heart beat, and the innervation of the blood vessels. All of these processes are connected with two centres, distinct not only in function but also in locality: the respiratory movements with centres of inspiration and expiration, the cardiac movements with centres for acceleration and inhibition of the heart beat, the vascular innervation with centres for constriction and dilatation of the blood vessels. Under such circumstances it seems to be the rule that the one centre acts reflexly while the other combines automatic with reflex functions, or even gives the preference to automatic stimuli: so the inspiratory centre in the case of respiratory movements, the centre for inhibition of heart beat in that of cardiac movements, and the centre for vaso-constriction in that of vascular innervation. It may be that the position of these nerve nidi, and the way in which their blood supply is distributed, render them especially liable to automatic excitations. The normal physiological stimulus to the production of such excitations is, in all probability, that state of the blood which is induced by arrest of breathing or, indeed, by any circumstance that prevents the elimination of the oxidised constituents of the tissues. The presence in the dyspnoeic blood of oxidation products in general, whether of the final product of combustion, carbonic acid, or of lower stages of oxidation as yet unnamed, appears accordingly to constitute it a source of nervous stimulation. The accumulation of these materials excites the inspiratory centre: an inspiration is made, which causes the lungs to distend and thus, in its turn, serves reflexly to excite the centre of expiration (p. 245). This automatic stimulation completes the circle of self-regulating functions, whereby the process of respiration is kept in perpetual activity. The first impulse is given by the change in the constitution of the blood: this acts as an internal stimulus to excite inspiration. The beginning once made, the further periodic course of the whole process continues of its own accord. The expiratory reflex excited by distention of the lung is followed, as the organ collapses, by the inspiratory reflex and at the same time, in consequence of the renewed accumulation [p. 255] of products of oxidation, by renewed automatic stimulation of the inspiratory centre.
We may suppose that the same changes in the composition of the blood condition the automatic innervation of the inhibitory centre for the heart and of the presser centre for the blood vessels. It is ordinarily assumed that the excitations in these two cases are not, as they are in the case of respiration; subject to a rhythmical rise and fall, in consequence of the self-regulation of the process of stimulation, but hold throughout to a uniform level of intensity. This is inferred from the facts that severance of the inhibitory nerves of the heart, the vagus trunks, produces a persistent acceleration of the heart beat, and that severance of the vascular nerves effects a permanent dilatation of the small arteries. But these facts are not incompatible with the theory that the automatic excitation in both cases oscillates between certain upper and lower limits. There are, in reality, numerous phenomena that tell in favour of such a theory: e.g., the alternate constrictions and dilatations that may be observed in the arteries, and that usually disappear after transsection of the nerves; or the connexion between rapidity of pulse and respiration, a connexion which, as we have seen, depends in part upon the changes of volume in the lung and is therefore explicable in reflex terms, but in part also suggests a different origin, seeing that a long-continued arrest of breathing, whether it occur in the position of inspiration or in that of expiration, arrests the heart as well. Moreover, in death by suffocation we always find, besides intensive excitation of the inspiratory muscles, constriction of the blood-vessels and inhibition of the heart beat. We may accordingly conjecture that the automatic excitation of all these oblongata centres depends upon analogous changes in the constitution of the blood. The observed differences may very well have their ground in the relations of the peripheral nerve terminations; for the inspiratory centre stands in connexion with ordinary motor nerves, whereas heart and blood vessels are characterised by the independence of their peripheral innervations. The heart continues to pulsate, though with change of rhythm when separated from all nerves whatsoever; and the vascular wall remains capable, under the same conditions, of alternate constrictions and dilatations. The causes which determine these peripheral excitations are, in all probability, similar to those which regulate the respiratory innervation in the myel and, like the latter, are compounded of automatic and reflex processes while the rhythmical function of the heart and the equilibrium between excitation and inhibition in the vessels are also maintained by some self-regulative mechanism. That is to say, the innervations of lungs, heart and blood vessels are, probably, in so far related to one another that the automatic excitations from which they spring may be referred to one and the same source of origin. The centres for these [p. 256] movements appear to offer especially favourable conditions for the action of the internal stimuli; for no other central area reacts so sensitively to fluctuations in the composition of the blood. In other quarters of the central nervous system, we may suppose, the influence of the blood becomes effective only if the blood supply has been modified from these centres of respiratory, cardiac and vascular innervation, and the changes thus set up form a source of central stimulation. Thus, excitations of the vascular centre, which inhibit the circulation of blood in the brain, are probably, in many instances, the cause of general muscular convulsions. Under such circumstances, the external symptoms are, for the most part, initiated in the pons; sometimes, perhaps, in a more anteriorly situated motor brain-region.[14] The dyspnoeic blood may, however, occasion muscular convulsions of the same kind, though less widely diffused, by stimulation of the myel.[15]
(b) -- Automatic Excitations in the Brain Cortex
Of the parts lying beyond the pons, the centrosensory and centromotor regions of the brain cortex seem to be the principal centres from which, under the appropriate conditions, automatic excitations may proceed. In their case, however, we are never in presence of purely automatic processes, in the physiological sense defined above. The relations of the cerebral cortex to the psychical functions are such that the automatic excitations are connected, in every instance, with conscious processes, -- processes that may: in general, be subsumed under the rubric of psychical association, and that refer us to psychophysical conditions of a very complicated kind. Nevertheless, the part played by automatic stimulation is far from unimportant. It serves to modify the excitability of the cerebral cortex; and the state of cortical excitability largely determines the appearance and course of these psychophysical processes. Among its results, we must mention, in the first place, those phenomena of stimulation that may almost be termed the normal accompaniments of sleep. They show themselves usually, and oftentimes exclusively, as sensory excitations. So arises the customary, sensory form of the dream, in which automatic enhancement of excitability in the sensory centres produces -- always, probably, under the influence of external sense stimuli -- ideas of hallucinatory character. Sometimes motor excitations are also involved: muscular movements occur, ordinarily in the mechanisms of speech, more rarely in the locomotor apparatus, and combine with the phenomena of sensory excitation to form a more or less coherent series of ideas and actions. In all these phenomena, sensory and motor alike, the automatic change of excitability is simply [p. 257] the foundation, upon which the complex psychophysical conditions of the dream consciousness and its outward manifestations are built up. The point of departure of these central changes, which follow the oncoming of sleep, is again to be sought, most probably, in the innervation centres of the oblongata. MOSSO has shown, by observation of cases in which a portion of the skull had been removed, that at the moment of falling asleep the flow of blood to the brain is reduced; and, further, that the supply may, in most instances, be temporarily increased by the application of external sense stimuli, even if these are too weak to arouse the sleeper.[16] The general reduction of the blood flow is, in all probability, the cause of the marked diminution in the excitability of the brain centres, and of the corresponding obscuration of consciousness, that characterise the approach of sleep. Very soon, however, this inhibition of the central functions spreads still further, involving to a certain extent the centres of respiration and heart beat; so that the phenomena of dyspnoea not infrequently make their appearance during sleep. The enhanced excitability of particular central elements of the brain cortex, vouched for by the phantasms of dreaming, may accordingly be ascribed to the direct excitatory influence upon the cortex of the dyspnoeic modification of the blood. It is also possible, in view of the reciprocal relations sustained by the various central areas, that stimulations accidentally set up in a given region of the cortex will produce the more intensive result, the greater the degree of latent excitation in the adjacent parts.[17]
Similar excitations of the cerebral cortex may occur in the waking state; but they are then invariably the result of pathological changes. Here, again, investigation frequently refers us to an abnormal state of the circulation as their ultimate condition. The abnormality may be of local origin, proceeding from the vessels of the meninges or of the brain itself. Local lesions, in particular, set up in the neighbourhood of the sensory centres, are ordinarily attended by corresponding hallucinations. These, however, may also be due to general disturbances of circulation, which appear sometimes as the consequence, sometimes as the cause of psychical derangement;[18] for changes in the innervation of heart and vessels are frequently observed in cases of mental disease.[19] Now all the chronic forms of insanity are connected with more or less serious modifications of [p. 258] the brain cortex; and diffuse affections of the vascular membrane with which the cortex is invested are the most frequent causes of acute psychical disorder. But the phenomena of stimulation accompanying such disorder closely resemble those that normally appear in sleep. They belong, as the latter also belong, partly to the sensory, partly to the motor sphere. The sensory excitation manifests itself in sensations and ideas of the different senses, oftentimes equal in intensity to those that can be caused by external impressions, and therefore indistinguishable from them. These hallucinations are accompanied by changes in the subjective sensations, muscular and organic, upon which the affective disposition largely depends. Motor stimulations show themselves in the form of imperative actions, which are likely to impress the observer by their unwonted energy. Here too, however, as in dreams and dream movements, the enhancement of excitability due to automatic stimulation is combined with further psychophysical processes, which are responsible for the specific contents of the phenomena.[20]
§ 4. Functions of the Mesencephalon and Diencephalon
(a) -- Functions of the Mesencephalon and Diencephalon in the Lower Vertebrates
It is evident from mere inspection, and without recourse to histological methods, that the mesencephalic and diencephalic region, which in man and the higher mammals, more especially in the nearly related primates, cannot compare with the mass of the overarching cerebral hemispheres, forms in the lower vertebrates the most highly developed part of the central organ. Even in the birds and the lower mammals, where the prosencephalon has already attained a considerable size, its relative development is still greater than that of the superior parts (cf. Fig. 54, p. 128). These salient facts of the gross anatomy of the brain are paralleled throughout by functional differences; so that it is far more dangerous in the case of mesencephalon and diencephalon than it is in that of myel and oblongata to argue from symptoms observed in the lower animals to the organisation of the higher, and in particular of man. Yet another difficulty in the way of a functional analysis of this region, whether in the animals or in man, lies in the circumstance that experimental interference and pathological disturbance rarely affect a definite and definitely circumscribed area, but are apt to spread to adjacent parts, -- experimental interference, more especially, involving the crural and coronal fibres that pass upward below and between the thalami and quadrigemina. Hence most of the results of the earlier [p. 259] experiments upon the transsection of these centres leave us uncertain whether the motor derangements observed were really the consequence of the destruction of the parts themselves, or not rather of the interruption of the neighbouring conduction paths.[21] Indeed, the whole method was at fault. The symptoms of stimulation and abrogation do good service in the investigation of conduction paths, and especially of their beginnings in the myel and of their terminations in the cerebral cortex. But in the present case, where the separation of the parts under examination from their surroundings presents extreme difficulty, they can hardly be employed with any prospect of success. As the stimulus method is here, for obvious reasons, practically out of the question, physiology has accordingly come more and more to substitute for the direct an indirect form of the method of abrogation. Instead of asking what functions remain intact after removal of the mesencephalic and diencephalic centres which he has under investigation, the modern physiologist inverts the question, and asks what functions are still left when all the prosencephalic parts that lie above and beyond them have been cut away. He then makes a series of similar observations upon animals of the same species in which the entire central organ has been removed with the exception of oblongata and myel; and, by recording the difference of result in the two cases, is able to reach a conclusion with regard to the functional significance of the intermediate central region. This method was employed long since by FLOURENS upon birds, -- though employed, at first, rather with a view to the determination of the importance of the prosencephalon itself, whose extirpation it involved.[22] It was then applied, systematically, by GOLTZ, in work upon the frog;[23] and has been used by CHRISTIANI [24] and, still more recently by GOLTZ [25] again for mammals, and finally by J. STEINER [26] for vertebrates of all classes. It evidently guarantees a somewhat more reliable result, if not for each individual centre included in the mesencephalic and diencephalic region, at least for this region as a whole.
The observations taken on the lines here laid down prove that the functional importance of the mesencephalic and diencephalic centres through-[p. 260] out the vertebrate series keeps practically even pace with the development of the parts as revealed by gross anatomy. This development is not uniform for the two regions: in the lower orders, the mesencephalon (bigemina or optic lobes) has the preponderance, and the diencephalon (thalamus) is relatively insignificant. Thus, in the entire class of the fishes, with the exception of Amphioxus lanceolatus which stops short at the myel, the mesencephalon appears as the dominant central organ. So long as it remains uninjured, the essential psychical functions are hardly modified. In particular, the animals react quite normally to optical and tactual impressions, and move spontaneously and appropriately. Smell alone is abrogated: the olfactory nerves are, naturally, removed with extirpation of the prosencephalon: and the inception of nourishment, in so far as it is governed by impressions of smell, may in consequence be more or less seriously deranged.[27] Passing to the amphibia, we find at once a marked difference of behaviour in animals whose cerebrum has been removed. One function is, unquestionably, retained by them, and must therefore depend for its effectiveness upon the integrity of the mesencephalon: the function of progression, and the regulation of co-ordinated movements of the whole body. The decerebrised frog sits upright, like the uninjured animal; if made to change its place by the action of cutaneous stimuli, it avoids obstacles laid in its path; and so on. It presents but a single abnormality: that, at first, it neither moves nor takes food of its own accord.[28] At the same time, its behaviour shows two noteworthy features. On the one hand, the functional separation of mesencephalon and diencephalon is becoming clearer; on the other hand, we observe the influence of practice upon the formation of new habits. If the diencephalon is intact, the frog, as SCHRADER remarked, slowly recovers: it begins to catch flies again of its own accord, and continues to improve until at last it is altogether indistinguishable from a normal animal.[29] A bird deprived of the prosencephalon behaves in very much the same way. It, too, as FLOURENS observed many years ago, at first remains motionless: it stands upright, breathes regularly, swallows if it is fed, and reacts to stimuli by co-ordinated movements of flight; but it makes no movements of its own initiative. Here again, however, there is a gradual change of behaviour, if the animal is kept alive for any length of time: it makes restless movements from side to side, avoids obstacles as it moves, and so forth.[30] CHRISTIANI, who was the first to make observations on mammals, found that the rabbit, after removal of all the parts of the brain anterior to the mesencephalon and diencephalon, is similarly capable of reacting appropriately to light stimuli, of avoiding [p. 261] obstacles when stimulated to movements of escape, and of occasionally executing what appear to be spontaneous movements.[31] Finally, a still more thorough-going restoration of function was seen by GOLTZ, in the case of dogs that he had kept alive for a considerable period of time after complete extirpation of the cerebrum.[32] As usual, the animals were entirely passive in the interval immediately following the operation: only the vegetative functions (heart beat, breathing, movements of swallowing upon the introduction of food into the gullet) went on without disturbance from the beginning. The progress of time brought with it, however, a much more complete recovery of active function; and at last the animals moved about in an almost normal manner, reacted to tactual stimuli by barking, got on their feet again if they had fallen down, alternated between sleep and waking, and could be aroused from sleep by sound stimuli. Smell had, it is true, been entirely abrogated with the extirpation of the olfactorius; nevertheless, the dogs fed of their own accord when food was held against their muzzles. Bad-tasting morsels they spat out again. On the other hand, there was never any expression of pleasurable feeling, of attachment, and never any act that could be interpreted as a sign of personal recognition. These were permanently lost.
From these results we must conclude that the mesencephalic and diencephalic region plays a very considerable part in the whole vertebrate series up to the carnivores. It contains a group of important central stations for the colligation of sense impressions with their appropriate movements, -- stations which, like the reflex mechanisms of the myel, continue to function after their severance from the higher central parts. But more than this: its integrity is the condition of the integrity of the simpler psychical functions. The mental loss that the animal suffers by operation is twofold. It loses, on the one hand, the functions connected with determinate sensory nerves that are involved in the lesion caused by removal of the prosencephalon: so the reactions to smell impressions. It loses, on the other hand, the functions which presuppose a manifold connexion of present impressions with past experiences: so the recognition of persons, the feelings of attraction and repulsion, of joy, etc. Some authors, it is true, disregarding the results obtained from decerebrised dogs, and relying on observations made upon anencephalic monsters, localised the feelings and emotions, in man as well as in animals, in the mesencephalic and diencephalic region. But they are guilty of an obvious error in reasoning. They ascribe the response to gustatory stimuli -- mimetic reflexes, which in these pathological cases are left intact -- to concomitant feelings. It is, of course, no more allowable to argue in this way than it would be to interpret any other reflex movement,[p. 262] on account of its apparent purposiveness, as of necessity a conscious and voluntary action.
We see, then, that these middle brain regions are, for the animals in general, something more than centres of complicated reflexes. In the light of the phenomena described above, we must consider them also as centres for the simpler psychically conditioned functions. A more detailed comparison shows, now, that as regards the time of their appearance these phenomena present very striking differences. In the lowest vertebrates, the fishes, removal of the prosencephalon produces no marked change of any kind in the psychical behaviour of the animals. At a somewhat higher level, in amphibia, reptiles and birds, there is, at first, an interruption of the psychical functions; and those that remain intact, since there is no trace of any lasting after-effect and but very slight indication of adaptation to new conditions, might if needs were be interpreted as complicated reflexes. It is, however, evident that in course of time the animal makes a fairly complete functional recovery. The same picture, only with its lines more strongly drawn, may stand, finally, for the mammal. The lapse of psychical functions after the operation is here more pronounced; a longer period is required for recovery; the permanent mental defect is more clearly observable. Nevertheless, the injury is compensated within wide limits. If we are rightly to interpret these phenomena, we must of course remember that all the functions which are permanently lost in the higher mammals -- recognition, expressions of pleasure and attachment -- do not exist at all in the, lower animals. Now this gradual return of the expressions of mental life, in animals endowed with a fairly well developed prosencephalon, admits, if looked at simply by itself, of two explanations. It may be that the operation gives rise to some sort of inhibitory effects, perhaps conditioned upon the injury to the parts, which must be gradually overcome; or it may be that the uninjured remnant of the brain gradually takes on a share of the functions discharged in the normal organism by the prosencephalon. According as we incline to the one or the other of these interpretations, will our estimate of the functional significance of the mesencenphalic and diencephalic regions vary. If we accept the former, these parts will be responsible, throughout the vertebrate series up to the carnivores, for a very considerable proportion of the functions of the brain at large; the animal's recovery will mean, for them, simply a restoration of their original rights. If we accept the latter, their functional activity after recovery will be abnormally increased, because partly vicarious. Now it cannot be denied that there are many facts which tell in favour of the effect of operation, as at least a joint factor in the general result. Radical interferences by operation, and especially interferences with the central organs, are known to affect the functions for a certain period of time. Still, it is [p. 263] hardly probable that the effect of operation is the determining factor in the case before us. The contrast between the proximate effects of the loss and the subsequent state of the animal is too striking and too uniform. Besides, there would be nothing to explain the graduation of phenomena in the animal kingdom: the fact, e.g., that in the frog -- to say nothing of the mammals -- a considerable period of time elapses before complete compensation is observable, whereas in the fishes recovery sets in at once. And lastly, there are numerous phenomena, drawn from all kinds of sources, which prone that injury or loss to the central parts, whether in man or in the animals, may within very wide limits be offset by the vicarious functioning of uninjured organs. We shall see presently that this law of functional substitution is indispensable, if we are to explain the reciprocal relations of the various cerebral areas. It is, then, only natural, in the absence of evidence to the contrary, that we should posit its validity in the present instance for the interrelations of the various parts of the brain. We may add that the general possibility of such vicarious function is inherent in the nature of the quadrigemina and thalami, as intermediate stations upon the direct lines of conduction between the peripheral organs and the cerebral cortex, -- stations in which all the sensory paths, and a large proportion of the motor, are interrupted by the interrelation of neurone chains. Putting all these facts together, we arrive at the following genetic conclusions, as a general point of view from which the various phenomena observable in the vertebrate series may be classified and explained. At the lowest stages of brain development, the mesencephalic and diencephalic region -- especially the former, since the diencephalon is as yet comparatively insignificant -- appears as the principal central organ. Subordinate to this, on the one side, are oblongata and myel. Adjoining it, on the other, as an appendicular structure, is the prosencephalon; originally, we must suppose, an outcome of the separate development of the olfactorius. As we proceed upwards through the vertebrate series, further representations of the conduction paths brought together in the bigemina gradually make their appearance, as superior centres, in the prosencephalon. In proportion as the latter advances, the mesencephalon and the diencephalon -- the latter conditioned in its own development upon the formation of the prosencephalon -- take on the function of intermediate centres, where excitations from the periphery touch off complicated reflexes, and excitations from the prosencephalon evoke reactions depending upon a more extended colligation of impressions. Nevertheless, the possibility remains, up to the higher stages of development, that on the removal of the superior regulatory mechanisms the lower centres may gradually recover some measure of the autonomy of which at a lower level they were in complete possession. Hence it may well happen that in the normal interplay of the central organs, -- so long, that is, as they stand [p. 264] under the dominance of the prosencephalic region, -- these parts of the brain may have no other function than that of complex reflex centres; but that in the absence of the higher regulatory organs they may once more assume the character of independent centres, whose co-operation involves the appearance of psychical functions.
We conclude, therefore, that this part of the brain possesses, under all circumstances, the importance of a centre whose office it is to bring into connexion the principal organs of sense and of movement. Such a view of its functions is, upon the whole, confirmed by the symptoms which ordinarily follow upon its direct removal or impairment. The most striking of these, upon the sensory side, is the blindness which, in mammals, in accordance with the course of the opticus paths, is correlated in particular with the pregemina, including the pregeniculum. Disturbances of movement appear, on the other hand, provided that the lesion does not extend beyond the quadrigemina, to be confined, at least in the mammals, to the muscles of the eyes; the general muscular system of the body is unaffected.[33] If, however, the diencephalon is injured, the general motor derangement is very pronounced. It consists, where the injury is unilateral, in peculiar imperative movements, in which the animal, instead of going straight forwards, turns round in a circle. These circus movements (Reitbahnbewegung, mouvement de manège) are also observed after injury to other parts of the brain, more especially the crura and the cerebellar hemispheres, and after unilateral extirpation of the semicircular canals of the internal ear. In the lower vertebrates, e.g. the frog, the circus movements are invariably made towards the uninjured side. In the invertebrates, too, the principal ganglia behave, in this matter of motor disturbance and its direction, in precisely the same way as the mesencephalon of the lower vertebrates.[34] In the mammals, on the other hand, the rule is that movement is directed towards the injured side, if the anterior portion of the thalamus has been divided, towards the uninjured side, if the section has been made in its posterior portion. Abnormalities have also been observed in the tonicity of the muscles of the body, so that the animal when at rest is not extended, but bends upon itself, the direction of curvature corresponding to that of the circus movements.[35] These movements may take on various forms, according to the special conditions of the injury: they may appear as rolling movements about the longitudinal axis of the body, as 'clock hand' movements, or finally as circus movements proper. They are, we may suppose, occasioned in all cases by an asymmetrical innervation, which however may itself be due to a number of causes: to unilateral increase [p. 265] or decrease of motor innervation, or to the asymmetrical release of reflex movements connected with disturbances of sensitivity. Which of these conditions, or what combination of them, is actually at work cannot, at present, be certainly determined.
The operation never fails to set up this motor derangement. In the higher mammals, we find, further, symptoms of abrogation or diminution of cutaneous sensitivity upon the uninjured side of the body. Such symptoms are always ambiguous, and the results are correspondingly doubtful.[36] On the whole, however, if we look at the phenomena in their entirety, and take into consideration at the same time the defects observed shortly after removal of the prosencephalon and the known facts regarding the course of the conduction paths, we may conclude that the mesencephalic and diencephalic region constitutes, in all the higher vertebrates, an important intermediate station on the road front the deeper lying centres to the prosencephalon; a station for the release, on the one hand, of compound reflexes, more especially of reflexes to visual and auditory stimuli, in which the prosencephalon is not concerned; and, on the other, of centrifugal excitations from the cerebrum, whose components are here co-ordinated in such a way as best to subserve the needs of the organism. In view of the anatomical relations and of the results of experiments with partial extirpation, it is probable that the postgemina represent, in the main, intermediate stations for the acoustic area; the pregemina and pregenicula similar stations for the sense of sight; and the thalami proper stations for the extensive area of the sense of touch. We thus have, in this whole region, a group of nodal points for the function of all the sense departments (with the exception of smell) and of the movements correlated with them. It follows that the mesencephalon and diencephalon, in proportion to their degree of development as compared with that of the prosencephalon, are able between them, after the lapse of the prosencephalic functions, to undertake in their own right the unitary regulation of the processes of the animal life; although certain functions, conditioned exclusively upon the cerebrum, are of course permanently lost. This physiological status implies a concomitant development of psychical processes: the persistence of impressions of sense for a certain time after the cessation of stimulus, the formation of complex perceptions, mediated by associative processes, and the conduct of movements in accordance with impressions received in the more remote past. In other words, centres that originally subserved reflex action and the transmission of impulses have, under pressure of novel conditions, become transformed into independent centres of direction. They are still centres [p. 266] of the second order; but their lesser functional value, as compared with the higher centres whose substitutes they are, depends essentially upon the degree of development attained by these higher centres themselves.
For a long time, the physiology of the mesencephalic and diencephalic region suffered from a misconception. It was insistently held that the functions of these parts were not only analogous, but in the main actually equivalent throughout the vertebrate kingdom; so that, in particular, what held of the animals must also hold of man. The older method, of experiments with direct abrogation, was not competent to remove this error. The necessary change of view has been brought about, gradually, by extirpation experiments on the prosencephalon itself; experiments which, as we remarked above, have really attained in this way a different purpose from that upon which they were originally directed. Physiologists, from FLOURENS to GOLTZ, made these experiments with the primary intention of deriving from the resulting symptoms of abrogation a more exact knowledge of the function of the prosencephalon. But it became more and more evident -- especially, as it happened, in the course of investigations pursued by GOLTZ and his pupils -- that this direct end could be accomplished but very imperfectly, if at all, on account of the direct and indirect consequences that follow the operations, and that make the comparison of the injured with the normal animal anything rather than a problem in simple subtraction. At the same time, it became evident that all such experiments yield most important information regarding the functions of which the uninjured brain remnant is capable. Extirpation experiments are, therefore, still valued by the modern investigator, but they are valued for a different reason. They are not expected to reveal anything of moment concerning the functions of the parts destroyed, but rather to illustrate the possible functions of those that are left intact. It need, however, hardly be said, after the discussion in the text, that these functions are not to be identified forthright with those discharged by the same parts in the normal interplay of the organs. In this connexion, the differences that we find throughout the animal kingdom, the very differences that were formerly overlooked, are of great significance. First and foremost, the differences between the various vertebrate classes, but secondarily and within certain limits -- despite the radically divergent position of the central organs, from the genetic point of view -- the differences among the invertebrates as well, have in many instances shed light upon the far more complicated conditions obtaining in the brains of the higher mammals. The impulse to such comparison came in the first instance from morphology. On the side of physiology, it is the especial merit of J. STEINER to have shown, by his experiments on fishes and on the frog, supplemented by later work on reptiles and invertebrates, how extremely variable is the role assigned to the mesencephalon in the vertebrate series. Setting out from the spinal functions of Amphioxus, which, as we know, has no other central organ than the myel, STEINER has, further, attempted a general theory of the mesencephalic functions at large. But here, unfortunately, his foundation is uncertain, and the structure erected upon it still less secure. In the annelids, he says, the individual metameres and the corresponding terms in the series of ventral ganglia are all equivalent, so that any portion of the worm is, in its own right, just as capable of movement and, apparently, of sensation, as is the whole animal. Amphioxus is, now, to be regarded in the same way: its myel consists simply of a series of [p. 267] equivalent terms, not subordinated to any higher centre. Then, at the next stage, represented by the primitive fish, the shark, and at stages of progressive advance, represented by the other fishes, the myel is brought under the central superintendence of the mesencephalon, which henceforth remains, throughout the vertebrate series, the real directing centre: the prosencephalon is to be considered as merely supplementary. It is true that, in the mammals, the prosencephalon attains a marked preponderance; nevertheless, the mesencephalon and diencephalon contain the centres for the regulation of the whole system of bodily movements, and therefore still hold the part of the central organ proper. In this sense, STEINER defines a 'brain' as "the universal centre of movement, in connexion with the functions of at least one of the higher sensory nerves." The criterion of an 'universal centre of movement' consists, for him, in the occurrence of unilateral forced movements after injury to the one side of the organ. If, then, there is no part of the central organs at which these circus movements can be released, there cannot either be any unitary centre of direction within the nervous system; the entire central organ must consist of a number of equivalent metameres.[37] Now it is plain that the point of departure for all these theoretical considerations is furnished by the annelids. The bodily segments of these animals appear, when divided off, to represent independent vital units, every whit as capable of continued spontaneous movement as was the original, uninjured worm. The annelids, moreover, do not execute circus movements after removal of the dorsal ganglion of the one side. But the only inference that can be drawn from the latter fact is, surely, this: that the occurrence of forced movements is, in all probability, not an universal criterion of the presence of a directing centre from which lower centres are controlled. And further: we have no right to assume the complete functional independence and equivalence of all the terms in the series of ventral ganglia, unless the individual segments move just as independently while they are still connected with the total annelid body as they do after their separation. This, however, is not the case; in the uninjured animal all the metameres move in exact co-ordination with one another. We must, therefore, conclude that the whole chain of ganglia normally functions as an unitary system, of which, if we may judge from the anatomical relations, the dorsal ganglion is the directive centre. In just the same way the myel in Amphioxus is apparently controlled by its most anterior portion as in some sort the equivalent of the brain of the craniota (see p. 252 above). The word 'brain' is, in the first instance, an expression taken over by science from popular parlance, and on that account is, as ordinarily employed, extremely difficult of definition. If we are, nevertheless, to make the attempt, and are not to break with the general application of the term, we must say that the vertebrate brain is not a separate central organ, but rather the complex of all those central organs which share in the direction of the animal functions. In this sense, oblongata, mesencephalon and diencephalon, prosencephalon and cerebellum have equal claims to the name. If, on the other hand, we are to restrict the term 'brain' specifically to the central parts that are able in their own right to maintain the animal life -- though perhaps on a reduced footing -- after the removal of the rest, then the oblongata still has, at any rate, as good a claim as the mesencephalon and diencephalon. The point is that the brain, taken as a whole, is not a centre, but a complex of centres, and of centres so related to one another that, if one of [p. 268] them is lost, a portion of its functions can, as a general rule, be taken over by another.
Experiments on completely decerebrised animals, especially the higher mammals, are of extreme importance, not only for the functions of the mesencephalic and diencephalic region, but also for the more general question of the functional representation of higher by lower centres. We therefore append here a somewhat detailed account of the phenomena observed by GOLTZ in the decerebrised dog that, of all operated on by him, longest survived the operation.[38] The animal was deprived of its left hemisphere in two experiments, performed on the 27th of June and the 13th of November, 1889; the entire right hemisphere was removed on the 17th of June, 1890. It was killed, with a view to post mortem examination, on the 31st of December, 1891. The general result of the autopsy was confirmatory; the cerebral hemispheres had been completely done away with, in part directly by the operations, in part indirectly by subsequent softening of the tissue. The animal had thus lived for more than eighteen months after the final operation. Immediately afterwards, it had been entirely motionless; but the capacity of spontaneous movement returned as early as the third day. The dog moved to and fro in the room, and was able to avoid obstacles laid in its way, without having first run against them. Placed on a smooth floor, it would slip up, but recover itself at once and of its own accord. If its toes were forced into an unnatural position, it corrected the displacement immediately as it began to walk, and stepped with the sole of the foot in the normal way. It lifted its leg, without falling in, from a hole that had been prepared for the purpose of the experiment. It once sustained an accidental injury to one hind paw, and thereafter, until the wound was healed, held up the injured leg in walking, precisely as a normal dog would do. The sense of touch was blunted; but the animal reacted to tactual stimuli of some intensity, though the localisation of the point stimulated remained, it is true, fairly uncertain. If, e.g., the left hind foot were seized, it would snap to the left, but generally in the air, without reaching the hand that held it. The auditory sensitivity was also greatly reduced; nevertheless, the animal could be aroused from sleep by intensive sound impressions. Gustatory stimuli were sensed. Meat dipped in milk and held before its mouth was seized and chewed up; meat dipped in a solution of quinine was taken, but spat out again, with wry movements of the mouth. The sense of smell was, of course, entirely abrogated: the olfactory nerves had been destroyed in the operations. At first, therefore, the dog took nourishment only when food was placed in its mouth. Later on, it became accustomed to seize and gulp down bits of meat, and to drink milk, as soon as its muzzle was brought in contact with them. It ate and drank of the solid and liquid food thus offered until its appetite was satisfied; it would then lie down and go to sleep. The functions of the sense of sight were shown -- in addition to the avoidance of obstacles, mentioned above -- in the reaction of the pupils to light stimuli. On the other hand, the animal was wholly insensitive to threatening gestures and movements, and to other animals presented for its notice. Consistently with this behaviour, it remained till its last day dull and apathetic. There was no question of any real 'cognition' and 'recognition' of the objects about it. The only expressions of feeling were snarling and biting when intensive stimulation was applied to the skin, and a tendency to restlessness under the influence of hunger. Nevertheless, the avoidance of [p. 269] obstacles shows an adaptation of movement to the varying conditions of sense impressions; and the same fact is brought out still more clearly in the following experiment. Two long boards were put together to form a blind passage-way, about twice as long as the animal itself, and so narrow that it could not turn round. When the dog was introduced into this passage-way, it first walked to the farther end, and ran against the wall. For some time, it reared up vainly against this obstacle; but presently it began to back out, and finally, by this crablike movement, reached the open. Of all experiments on decerebrised animals, this is, without doubt, the experiment whose result seems to approach most nearly to what is termed an 'expression of intelligence.' Nevertheless, it is plain that in this case, as in the others, the adaptation of the reacting movements to the sensory stimuli are still confined within limits where it is out of place to speak of any real 'reflection' -- of a choice between different possibilities. The symptoms themselves, considered solely by themselves, might, naturally, be interpreted as voluntary actions. But it is another question whether the whole context in which the phenomena appeared permits of such an interpretation. And this question must, surely, be answered in the negative, for the same reason that we decline, e.g., to ascribe the avoidance of obstacles to a true 'cognition' of the objects, -- the cognition in this instance being disproved by other symptoms. If, however, we rule out expressions of intelligence and voluntary actions, in the strict meanings of those terms, this attitude must not, of course, be construed as a denial that the actions of the decerebrised animal are, in part, conscious processes. On the contrary, it must be regarded as, at the least, extremely probable that they may be interpreted as conscious and, in this sense, not merely as purely mechanical reflexes. We cannot, however, enter upon this question with any fulness until we come to our psychological discussion of the idea of 'consciousness' (cf. Part V., Ch. xviii., below).
(b) -- Functions of the Mesencephalon and Diencephalon in Man
In man, and indeed in all the other primates, who in this regard stand upon practically the same level as man, the preponderance of the prosencephalon, which becomes the more marked the higher we ascend in the vertebrate series, has reached a limit where the centres of mid brain and 'tween brain retain least of their original relative independence. This statement is justified both by the relations of the conduction paths and by the nature of the disturbances produced by pathological defects. We cannot, it is true, -- and the reasons are obvious, -- expect to find human cases that shall reproduce the conditions of total extirpation of the prosencephalon, with permanent retention of function in the middle brain regions. But in cases of restricted lesion of the quadrigemina and thalami, it would seem that a restitution of functions by way of vicarious representation in co-ordinated or superior parts may occur very extensively in the human brain. At the same time, the close connexion of the pregemina with the visual functions is evidenced by the derangement of ocular movements that accompanies injury to these parts; while disturbances of visual sensitivity in man appear, for the most part, only when the geniculum is involved. In individual cases -- and the [p. 270] result accords with what we know of the course of the conduction paths -- auditory disturbances have been observed after injury to the postgemina. Lesions of the thalami, as might, again, be expected from the anatomical facts and from the results of experiments on animals, are followed by anæsthesia or by motor disturbances or by both combined. Sometimes, it is true, affections of the thalami run their course without any sign of disturbance whatsoever:[39] a fact that testifies to the wide range of vicarious functioning possible, in this particular instance, within the human brain, and that constitutes a marked quantitative difference between man and the animals, in which the phenomena of abrogation are much more intensive. A second and still more striking difference is this: that the symptoms which in experiments on animals, from the fishes up to the mammals, are set up with the greatest uniformity by unilateral lesions of this area -- the imperative circular movements -- are represented in man, at the best, only by such reduced and vestigial forms as a permanent deflection of the eyes or an unilateral execution of mimetic movements.[40] The determining factors in this result are apparently two: on the one hand, the voluntary suppression of the symptoms, and, on the other, the greater scope of the automatic regulations and functional substitutions that, in the human brain, counteract the disturbances in question. Both factors indicate that, while the basal functions of this region of the human brain correspond to those discharged by the same region throughout the animal series, still its relative importance, as compared with the superior centres, has now become less. Compound reflex centres for the principal sense departments, sight, hearing and touch; and comprehensive regulatory centres for the motor excitations issuing from the higher parts of the brain: these the mesencephalon and diencephalon have remained. But other regulatory mechanisms, and the independent processes of release within the prosencephalon, have increased in importance alongside of them: so that their assumption in man of such psychical function as has been observed to persist in the dog alter removal of the prosencephalic parts can hardly be regarded as probable.
(c) -- Striatum and Lenticula
Striatum and lenticula belong, morphologically, to the prosencephalon (pp. 128 f.). But little is known of their function. They appear, however, to be cortical areas, sunk into the substance of the hemisphere, and specifically correlated with the mesencephalic and diencephalic ganglia. This view is suggested by the extent of their fibre connexions, more especially with the thalami (Fig. 74, p. 179). It is borne out, further, by the phenomena observed in experiments on animals, and in cases of lesion in [p. 271] man, in which these structures are involved. The phenomena consist always of paralytic symptoms or, when excitatory influences are at work, of exaggeration of movement. Here again, however, and particularly in the case of man, the phenomena of abrogation are most pronounced when the lesion has been rapidly produced: slow growing tumours may, under certain circumstances, run their course without giving rise to any symptom whatever. NOTHNAGEL found, further, that mechanical or chemical stimulation of the striatum of the rabbit occasioned hurried running movements.[41] MAGENDIE observed the same result after complete removal of the striatum.[42] Anæsthesia, on the contrary, does not appear to be a consequence of injury to these structures.[43] For the rest, the intensive disturbances that ordinarily follow upon sudden lesions of the striatum are not beyond suspicion; they may be due to implication of the pyramidal paths ascending in the capsula to the cerebral cortex. Besides these relations to the mid brain and 'tween brain, the anatomical facts indicate a further connexion with the cerebellum. As a matter of fact, atrophy of the striatum, and especially of the lenticula, has been observed in cases of congenital failure of the cerebellum.[44]
§ 5. Functions of the Cerebellum
The functions of the cerebellum form one of the most obscure chapters in the physiology of the central organs. The obscurity is intelligible, when we remember the extensive connexions of the cerebellum with numerous other central arts, -- with the oblongata, with the mesencephalon and diencephalon, and above all with the cerebral cortex. For, on the one hand, these connexions make it difficult to determine whether destruction of the other brain centres involves abrogation of the corresponding cerebellar functions. And, on the other, we are left equally in doubt whether the disturbances observed in cases of lesion or defect of the cerebellum are not due, in part at least, to the indirect implication of other parts of the brain with which it stands in connexion. To these is added the further difficulty, that the cerebellar derangements appear to be peculiarly easy of compensation by the enhancement or substitution of function in other central parts. We have, therefore, as many reasons to overestimate as we have to underestimate the importance of this organ; and our uncertainty is not a little increased by the ambiguity of symptoms, which characterises all the phenomena of central abrogation, but is especially marked in this particular case.[p. 272]
These symptoms themselves consist, for the most part, in motor disturbances. Complete extirpation of the cerebellum in animals renders all movements vacillating and uncertain, staggering or tremulous, though the influence of the will upon the individual muscle groups is not destroyed. Transsection of various parts of the cerebellum, as well as of the cerebellar peduncles, -- whose radiations are, for that matter, involved in all deep-going injuries to the organ, -- is ordinarily followed by unilateral motor derangement. If the section passes through the most anterior portion of the vermis, the animals fall forwards; in spontaneous movements, the body is bent over anteriorly, always ready to fall and fall again. If it passes through the posterior portion of the vermis, the body is bent backwards, and there is a tendency to backward movements. If the one pileum is injured or removed, the animal falls towards the opposite side, owing to unilateral contraction of the corresponding muscles; violent movements of rotation about the long axis of the body are apt to follow. There occur also, at the moment of operation, convulsive movements of the eyes, usually succeeded by a permanent detection. These abrogation symptoms agree, upon the whole, with the phenomena of stimulation observed with electrical excitation of various parts of the cerebellar cortex. Both alike are, without exception, same-sided, in contradistinction to the consequences of cerebral injury, which appear upon the opposite side of the body. The stimulation phenomena consist in spasmodic movements of the head, the vertebral column, and the eyes.[45]
As regards man, clinical experience is in accord with the results of the observations on animals. Motor disturbances are, again, the most constant symptom. They consist, chiefly, in an uncertain and vacillating gait, sometimes also in similar movements of the head and eyes. The arms appear to be less seriously involved; and it is but seldom that we observe, in man, those violent rotatory movements that, in the animals, accompany unilateral lesions of the pilea or the medipeduncles. For the rest, the motor disturbances in man are most intensive when the vermis is the seat of injury; while affections of the pileum of either side, especially if the change is merely local, may run their course without symptoms of any kind. Serious derangement occurs, seemingly, only with complete functional disability of the pilea, or in the rare cases of atrophy of the entire organ. Under such circumstances, however, the symptoms are not confined to motor dis-[p. 273]turbances; they become exceedingly complicated, and interpretation is correspondingly difficult.[46] Disturbances of cutaneous sensibility do not appear to result from affections that remain limited to the cerebellum, not even from total atrophy of the organ.[47] On the other hand, a characteristic subjective symptom, more frequently connected with disease of the human cerebellum than with other central disorders, is the dizziness that accompanies the motor disturbances. It is therefore probable that the attacks of dizziness induced in the healthy subject by the passage of a strong galvanic current through the occiput are due, in part at least, to its influence upon the cerebellum.[48] And for the same reason we may suspect that this organ is involved in the dizziness produced by certain toxic agencies.[49] Now there are, in general, two conditions under which the phenomena of dizziness may be manifested: first, the functional derangement of certain peripheral sensory apparatus, whose impressions mediate the arousal of sensations that generate the idea of the static equilibrium of the body during rest and motion; and, secondly, such functional disorders of central areas as are in any way calculated to alter the normal relation subsisting between sense impressions and movements or ideas of movement. We shall presently become familiar with a sensory apparatus of the former kind in the ampullæ and canals of the labyrinth of the ear.[50] On the other hand, we appear to have in the cerebellum not the sole, but certainly the most frequent central seat of symptoms of dizziness. When we remember how near together are the labyrinth of the ear and this central organ, we can readily understand that the two forms of disturbance of equilibrium are difficult to discriminate. Besides, we have every reason to believe that they are functionally connected: the vestibular nerve, that supplies the vestibule and canals with sensory fibres, sends a large number of representatives to the cerebellum.[51] These relations to the vestibular division of the labyrinth are, perhaps, our best means of accounting for the influence of the cerebellum [p. 274] upon bodily movements. We know that all the other sense departments, and more especially those that mediate our spatial apprehension of sensory impressions, the senses of sight and touch, find abundant representation in it. And we find that where dizziness is set up by the action of definitely demonstrable subjective or objective causes, these may ordinarily be traced back to one general condition: disturbance of the normal correlation of sense impressions and bodily movements. Again, however, this disturbance may, in the individual case, be brought about, centrally and peripherally, in a great variety of ways. A man may be made dizzy by walking on the ice, if he is not accustomed to it. The uncertainty of vision that goes with amblyopia or strabismus, or that may he induced in a normal-sighted person by covering the one eye, is not infrequently attended by dizziness. The symptoms are still more evident in the walking movements of patients whose tactual sensations are dulled or destroyed by a degeneration of the dorsal columns of the myel. In such cases, the resistance of the ground is not sensed in the accustomed way: the patients lose their equilibrium; they stagger, and try to save themselves from a fall by balancing with the arms.[52] These phenomena show, at the same time, the indispensableness of the co-ordination of sense impression and movement for the correct execution not only of involuntary, but also of voluntary movements. In the latter, too, it is as a rule only the end to be attained that is clearly conscious; the means whereby this end is reached are entrusted to the automatic working of a motor mechanism, where movement interlocks with movement in the right order and to the right purpose. Each separate act in a compound voluntary action reveals, accordingly a precise adaptation to the impressions that we receive from our own body and from external objects. But since the voluntary action is directed exclusively upon the end to he attained, the sense impressions that regulate the movements do not, ordinarily, take any part in the idea of movement. Even the sudden lapse of the regulatory impression is, in most instances, perceived only indirectly, by way of the consequent motor disturbance and the subjective phenomena dependent upon it.
Disturbances of movement due to central causes may now, in general, be brought about in four different ways. They may (1) be paralytic phenomena, i.e. they may be occasioned by a partial abrogation of voluntary movements. They may (2) appear as purely anæsthesic symptoms. They may (3) consist of disturbances of motor co-ordination. Or they may (4) result from disturbance of the normal relation obtaining between sensations and the movements depending upon them. The first of these possibilities is ruled out at once, since paralytic symptoms do not occur after removal [p. 275] of the cerebellum or of separate parts of it; besides, dizziness is never observed in the train of purely motor disabilities. The second seems to promise better. Indeed, it has to a certain extent found acceptance; some authors have conjectured that the cerebellum is an organ of what is termed the 'muscular sense.'[53] But this view can hardly be reconciled with the fact that in cases of atrophy of the cerebellum in man, and after total extirpation of the organ in animals, the capacity of active movements of locomotion is still retained; the movements may be vacillating and uncertain, but they nevertheless allow us to posit a certain degree of sensation in the locomotor muscles. The abrogation of other sensations is equally out of the question. The third interpretation of the cerebellum, as centre of motor co-ordination, was first put forward by FLOURENS,[54] whose views have held their own, down to the most recent times, among physiologists and clinicians. But, first, this definition is too indeterminate to characterise the specific form of co-ordination mediated by the cerebellum. There is no single central motor area, from the myel upwards, that is not the seat of some sort of motor co-ordination. Secondly, the phenomena of dizziness also tell against FLOURENS' interpretation. They indicate that some kind of sensory disturbance is always involved along with the motor. We are thus forced to the conclusion that the fourth of the above hypotheses is the most probable: the hypothesis that inhibition of function in the cerebellum interferes with the action of those sensory impressions that exercise a direct regulatory influence upon the motor innervation proceeding from the cerebrum.
The acceptance of this hypothesis removes various difficulties. Thus, we can explain at once how it comes about that the disturbances produced by lesions of the cerebellum resemble the symptoms due to partial anæsthesia, and yet differ from them on the important point that abrogation of sensations never makes its appearance among the cerebellar phenomena. Where all conscious sensations persist, the only impressions that can be supposed to lapse are those that act upon movement directly and without previous translation into conscious sensations. Voluntary movements as such are as little affected as sensations; even after complete destruction of the cerebellum, the will retains its right of control over each individual muscle. This explains, again, how it is that the disturbances set up by injury to the cerebellum may gradually be compensated. Compensation takes place in this way, that the movements are regulated afresh by the conscious sensations that persist unimpaired. But a certain clumsiness and uncertainty never disappear. It is evident, as one watches, that the [p. 276] movements must always proceed from a sort of reflection. The immediacy and certainty of movement shown by the uninjured animal are either lost or, if they may in some measure be regained, must be acquired slowly and gradually, as the result of a long continued course of renewed practice. Here too, therefore, the principle of the manifold representation of the bodily organs in the brain is seen in opposition. The cerebellum appears to be intended for the direct regulation of voluntary movements by sense impressions. If this hypothesis be correct, it will, accordingly, be the central organ in which the bodily movements incited from the cerebrum are brought into harmony with the position of the animal body in space. This conception agrees sufficiently well with our anatomical knowledge of the course of the lines of conduction, incoming and outgoing. In the postpeduncles the cerebellum receives a representation of the general sensory path, reinforced, in all probability, by fibres from the optic nerve and the most anterior sensory cranial nerves which run in the valvula and the prepeduncles. Its connexion anteriorly is effected by the prepeduncles and medipeduncles, by which it is united partly to the anterior brain ganglia, partly to the most diverse regions of the cerebral cortex.[55] Finally, the extensive representations of the auditory nerve in the cerebellum (Fig. 77, p. 183) may be brought under the same point of view. For if the cerebellum deflects at all that sensory secondary path whose office it is to conduct impressions that influence voluntary movement directly, and not indirectly, by way of conscious sensations, then we shall certainly expect to find that this same path contains a representation of the eighth cranial nerve. The acusticus is precisely the sensory nerve that gives certain objective sense impressions a specific relation to movement; our movements adapt themselves involuntarily, in a corresponding rhythm, to rhythmical impressions of sound.
The question of the functions of the cerebellum cannot be answered, at the present time, with any degree of finality. The one point upon which physiologists are fairly unanimous is that this organ is set off in relative independence, anatomically and functionally, from the other parts of the central organ, and more especially from the cerebrum: so that no single function -- in particular, therefore, neither sensation nor movement -- is wholly abrogated even after its complete elimination, though profound derangements are produced in the co-ordinations of function. But this very fact of relative independence, which in man and the higher animals must be connected with a position of high functional importance, -- a position attested, in any case, by the structure and volume [p. 277] of the organ -- renders the exact determination of the nature of the 'co-ordinations' or 'regulations' effected by the cerebellum a matter of extreme difficulty; and it is not altogether surprising that a good many of the physiologies are still satisfied to stop short at these indefinite terms, -- terms that apply more or less to every central organ, and are therefore tolerably non-committal in the particular case. This position has been attacked, and rightly attacked, by LUCIANI. Aiming from the first at a definiteness of statement that should match the preceding indefiniteness, LUCIANI undertook to analyse the phenomena, so far as possible, along all their various lines, and thus to refer them to distinct groups of symptoms. He has thus been led to distinguish three principal symptoms of abrogation, which he regards as characteristic of cerebellar lesions: asthenia, atony, and astasia. The movements lack their normal energy (asthenia); the tonus of the muscles is lowered (atony); and the movements are uncertain and incoherent (astasia).[56] It has been objected, with some justice, to this characterisation, that the symptoms which it discriminates are, in part at least, closely interconnected: atonia and asthenia, e.g., always occur together.[57] But if the three terms are considered simply as collective expressions for certain partial states, they may be accepted as really denoting the essential features of the cerebellar symptoms. For the interpretation of the phenomena, however, the emphasis must fall, without any question, upon that member of the triad which is at once the most characteristic and also, unfortunately, the most complicated, upon 'astasia.' LUCIANI seems here, in some measure, to have missed the true perspective; he lays most weight upon the first two symptoms, -- which, no doubt, admit of a simpler interpretation -- asthenia and atony. As a result of this mistake, he is inclined to regard the cerebellum as primarily an apparatus for the production of nervous force, an 'auxiliary' or 'intensificatory system' for the whole cerebrospinal organ, which is not the seat of any specific or peculiar functions, but reinforces the functional activity of the entire nervous system. In support of this view, he adduces the trophic disturbances that appear, in course of time, more especially after complete extirpation of the cerebellum, and that ordinarily take the form of muscular atrophy, cutaneous inflammations, decubitus, etc. Now these disturbances, as well as the striking lack of motor energy that perhaps stands in a certain relation to them, are unquestionably very important symptoms. But the possibility still remains that the 'atony' and 'astasia' of movement are interconnected phenomena, in which a part is played by the influence of sensory impressions. We saw, when we were discussing the myel, that the phenomena of tonus are straitly conditioned upon the continued effect of such impressions (p. 93). And trophic disturbances, of the kind observed after extirpation of the cerebellum, appear in all cases of permanent derangement of innervation; they result from the disability of sensory as well as of motor nerves; and they appear always to involve the co-operation of direct trophic influences, exerted by the nerve centres, and of indirect, which have their source in the abrogation of functions. LUCIANI lays special stress upon the fact, established by his observations, that dogs whose cerebellum has been destroyed are still able, when thrown into the water, to make the normal movements of swimming. But this experiment merely confirms, in a very complete way, the fact that all cutaneous impressions can be sensed, and all locomotor movements voluntarily [p. 278] performed, without assistance from the cerebellum. Swimming is precisely the form of movement that may, under certain circumstances, bring into action a continuous voluntary regulation, compensating any inco-ordinations that have arisen involuntarily, for the reason that an intermission of movement means in its case the danger of drowning. The animal that constantly staggers as it attempts to walk or run is, in swimming, compelled at every movement to maintain itself above water by a maximal effort of will.
The view here taken of the cerebellar functions is in all essential points the same as that developed by the author in the first edition of this work.[58] It finds striking confirmation in the statements made by KAHLER and PICK, from the pathological standpoint, concerning the relation of other forms of 'ataxia,' as it is termed, to the cerebellar symptoms.[59] HITZIG, too, in his interpretation of cerebellar dizziness, seems to take up a very similar position.[60] In any attempt at explanation of this symptom, and, indeed, of the abrogation phenomena at large, especial attention must, in the author's opinion, be paid to the two facts brought out just now: that, in the case of voluntary impulses proceeding from the cerebrum, the individual terms in the series of purposive co-ordinations and regulations of the movements always succeed one another, under ordinary conditions, in independence of the will, i.e. automatically; and that they must always, on the other hand, take their direction from the sensory impressions received by the organism.
The impressions conveyed to the central organs may, according to circumstances, be clearly or obscurely conscious, -- may, in many instances, fail to come to consciousness at all. But, at any rate, it is not in consciousness that they are transformed into the motor impulses whose direction they determine. From this point of view we might, perhaps, characterise the cerebellum outright as an auxiliary organ which relieves the cerebrum of a large number of secondary functions: functions that were originally practised under the continuous control of the will, and that in consequence can always be partially resumed by the cerebrum itself. As for the first stage of practice, it may have occurred here, as in many other cases, either in the course of the individual lifetime or in the previous life history of the species, which has left its permanent traces, if anywhere, certainly in the organisation of the central parts. To ascribe to the cerebellum itself any share in conscious functions, or to endow it, as some have done, with a separate consciousness of the second order, a 'subconsciousness' is, as in the light of these arguments it seems to the author, entirely unwarranted. For the fact before us is that the cerebellum has developed into a centre of sensorimotor regulation, and that in the course of this development the individual co-ordinations of the separate acts of movement with the impressions of sense, all purposive and all subordinate to the ultimate end of the voluntary action, have gradually been withdrawn from consciousness. And there is, upon the whole, only one way in which this process can be envisaged we must suppose that, under the influence of definitely directed cerebral innervations, there has developed a central mechanism, automatic in unction, whose office it is to transmit the first, and only the first, discharging impulses to an auxiliary centre; end that this auxiliary centre is endowed with self-regulating apparatus, again [p. 279] automatic in function, which adapt each several movement to the sense impressions coming in at the particular moment. These impressions may, of course, either come to consciousness by the way or remain unconscious: the former, if the conditions favour their special conduction to the sensory centre, the latter, if they are against it, or if the conduction is somewhere inhibited: for the self-regulations as such the matter is indifferent. On the other hand, it may very well happen, as a consequence of the direct conveyance of sensations to the cerebrum and of its response to them, that disturbances in the cerebellar mechanism of the sensorimotor self-regulations are presently compensated. Such compensation will, in particular, always be possible where the lesions are simply partial, so that a new course of practice may be entered upon and novel co-ordinations established. Where, on the contrary, the entire cerebellum is thrown out, a large draft upon the cerebral functions will suffice to hold the disturbances in check and so to mitigate the symptoms: but we can, it is true, expect nothing more.
We suppose, then, that these self-regulations of the voluntary movements are in some way mediated by the cerebellum. If, now, we are asked to give an account of them in detail, me must reply that the question is very difficult to answer, all the more since there is still much obscurity surrounding the directions and terminations of the conduction paths that meet within the organ. The anatomical relations suggest, and we may accept the suggestion as a provisional hypothesis, that the cerebellum, on the one hand, receives centripetal paths, derived from every sensitive portion of the body, and, on the other, sends out intracentral (as regards the organ itself, centrifugal) paths to every centromotor region of the cerebral cortex. We may imagine, accordingly, that the sensory components functioning in a movement, more especially sensations of touch and movement, are in the cerebellum united into a single resultant; and that this is then conducted onwards to the cerebral cortex, and makes connexion with the centromotor processes of discharge which are there in course. Thus, the regular sequence of walking movements is at every stage dependent upon the condition that the sensory impressions produced at each step by the movement itself are repeated in uniform succession. Suppose, now, that such a rhythmical sequence is summated to form a resultant which connects automatically with the voluntary impulses; and suppose that it remains unchanged so long as its components persist without change, while it varies at once when and as its components vary. We should then have, physiologically, a mechanism of self-regulation which at one and the same time reinforces and relieves the centromotor functions of the cerebral centres; and we should be able, psychologically, to explain by appeal to it the automatic, unconscious character of these self-regulations of our movements, which still leaves room for voluntary corrections and novel courses of practice.[61]
Over and above its influence upon the bodily movements, -- of whose reality there can be no doubt, however various may be the interpretations put upon it, -- the cerebellum has at times been accredited with functions of an entirely different order. Thus, the disturbances of intelligence observed in cases where the organ is lacking, combined perhaps with the anatomical fact that in the medipeduncles the cerebellum has extensive connexions with the prosencephalon, has persuaded several authors to attribute to it a share in what are called [p. 280] the 'intellectual' functions. Apart, however, from these isolated observations, which may very probably be explained by concomitant affections of other parts of the brain, the hypothesis has no facts that are at all definite to support it. The view held by GALL and his pupils, that the cerebellum stands in relation to the sexual functions, is hardly held by any physiologist at the present day. The uncritical way in which GALL himself, and still more the phrenologists who followed him, -- COMBE, for instance, -- heaped together quotations from older authors, records of cases that had not been properly investigated, and observations in which the suspicion of self-delusion forces itself irresistibly upon the reader, -- the whole forming a mass of evidential matter that should be impressive solely by its bulk, -- would of itself forbid our deviating any attention to their writings, even if we did not find upon every page the mark of inveterate prepossession.[62] It should be mentioned, on the other side, that, now and again, observers who cannot be accused
