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WUNDT:  Classics in the History of Psychology -- Chapter 5

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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)
Translated by Edward Bradford Titchener (1904)

Introduction

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

Our examination of the structural elements of the nervous system led us to conceive of the brain and myel, together with the nerves issuing from them, as a system of nerve-cells, inter-connected by their fibrillar runners either directly or through the contact of process with process. Our recent survey of the morphological development of the central organs lends support to this conception. We have found a series of cinereal formations which collect the fibres running centralward from the external organs and mediate their connexion with other, especially with more centrally situated grey masses. The paths of conduction that begin in the myelic columns pass upwards first in the crura and then in the corona until they penetrate the cerebral cortex. There we have the commissures, pointing to the inter-connexion of the central regions of the two halves of the brain, and the intergyral (arcuate) fibres, indicating the connexion of the various cortical zones of the same hemisphere. Hence from whichever point of view we consider the outward conformation of the central organs, we are presently met by the question as to the course taken by the various paths of nervous conduction. We know, of course, that the cell-territories stand, by virtue of the cell-processes, in the most manifold relations. We shall accordingly expect to find that the conduction-paths are nowhere strictly isolated from one another. We must suppose, in particular, that under altered functional conditions they may change their relative positions within very wide limits. But we may fully admit such a relative variability of functional co-ordination as is suggested by the neurone theory, and yet with justice raise the question of the preferred lines of conduction; -- of the lines which, under normal circumstances, are chiefly concerned to mediate determinate connexions, on the one hand, between the central regions themselves, and on the other, between the centre and the peripheral organs appended to the nervous system. This answered, we may in certain cases proceed to ask a second question, regarding the auxiliary paths or bypaths which can replace the regular lines of transmission in particular instances of interrupted conduction or of inhibition of function.[p. 151]

We distinguish two main kinds of conduction-path, according to the direction in which the processes of stimulation are transmitted: the centripetal and the centrifugal. In the former, the stimulation is set up at some point on the periphery of the body, and travels inwards, toward the central organ. In the latter, it issues from the central organ, and travels toward some region of the periphery. The physiological effects of a centripetally conducted stimulation, when they come to consciousness, are termed sensations. Frequently, however, this final effect is not produced; the excitation is reflected into a movement, without having exerted any influence upon consciousness. Nevertheless, the paths of conduction traversed in such a case are, at least in part, the same. We therefore give the name of 'sensory' to the centripetal conduction-paths at large. The physiological effects of centrifugally conducted stimulation are very various: it may find expression in movements of striated and non-striated muscles, in secretions, in heightened temperature, and in the excitation of peripheral sense-organs by internal stimuli. In what follows, we shall, however, confine our attention for the most part to the motor and the centrifugal-sensory paths, since these are the only parts of the centrifugal conduction-system that call for consideration in psychology. The muscular movements that result from the direct translation of sensory stimulation into motor excitation are termed reflex movements; those that have their proximate source in an internal stimulation within the motor spheres of the central organ we shall call automatic movements. In the reflex, i.e., centripetal is followed by centrifugal conduction; in the automatic movement, centrifugal conduction alone is directly involved.[1]

So long as the stimulation-process is confined within the continuity of determinate nerve-fibres; as occurs e.g. in the peripheral nerves, which often traverse considerable distances, it remains as a general rule isolated within each particular fibre, and does not spring across to neighbouring paths. This fact has been expressed in the law of isolated conduction. The law has usually been regarded as valid not only for the periphery, but for the conduction-paths within the central organs as well; on the ground that an external impression made upon some precisely localised part of a sensitive surface evokes a sharply defined sensation, and that a voluntary impulse directed upon a definite movement produces contraction of a circumscribed group of muscles. Really, however, these facts prove nothing more than that the processes in the principal paths are, as a rule and under normal conditions, separate and distinct. It has not been demonstrated with certainty that the stimulation is strictly confined to a single primitive fibril, even during the peripheral portion of its course. And in the central parts, any such restriction is entirely out of the question, as appears both from the general [p. 152] morphological features discussed in Chapter II., and from the phenomena of vicarious function which we shall speak of presently. The only principle that can be recognised here is a principle of preferential conduction. There is in every case a principal path, but this is supplemented by auxiliary or secondary paths.

We may avail ourselves of three distinct methods, in our examination of nervous conduction. Each one of them has certain imperfections, and must therefore be supplemented, where possible, by the other two. The first method is that of physiological experimentation; the second is that of anatomical investigation; and the third is that of pathological observation.

(I) Physiological experimentation attempts: to reach conclusions as to the course of the nervous conduction-paths in two ways: by stimulation-experiments, and by interruptions of conduction due to a division of the parts. In the former case, we look as a general rule for enhancement, in the latter for abrogation of function, in the organs connected with the stimulated or divided tissue. When we come to the investigation of the central paths, however, we find that both methods alike are attended by unusual difficulties and disadvantages. Even in the most favourable instances, when the stimulation or transsection has been entirely successful, we have established but one definite point upon a path of conduction; to ascertain its full extent, we should have to make a large number of similar experiments, from the terminal station in the brain to the point of issue of the appropriate nerves. Such a task holds out absolutely no hope of accomplishment, since the isolated stimulation or section of a conduction-path in the interior of the brain presents insuperable obstacles. There are, therefore, only two problems to which these methods can be applied with any prospect of success. We may use them to determine the course of conduction in the simplest of the central organs, the myel, and in the direct continuations of the myelic columns, the crura; and we may use them to discover the correlation of definite areas of the brain cortex with definite organs upon the periphery of the body. The answer to the former question has been attempted, for the most part, by isolated transsection of the various myelic columns; the answer to the second, by experiments upon the stimulation and extirpation of definitely limited cortical areas. Even with this limitation, however, it is difficult to secure valid results. A stimulation will almost inevitably spread from the point of attack to the surrounding part. This objection applies with especial force to the electric current, almost the only form of stimulus which fulfils the other requirements of physiological experiment, and a stimulus which the physiologist is therefore practically compelled to employ. The same thing is true of the disturbances consequent upon a division of [p. 153] substance. And if one is at last successful in securing the utmost degree of isolation of experimental interference, there will still be many cases in which the interpretation of the resulting phenomena is uncertain. The muscular contraction that follows upon a stimulation may, under certain circumstances, be due to a direct excitation of motor fibres, just as well as to a reaction upon the sense-impressions. And the derangements of function that appear as a result of transsections and extirpations always require a long period of observation before they can be accurately determined. This means that the certainty of the conclusions is, again, very largely impaired: the disturbances set up as the direct effect of operation for the most part disappear as time goes on, the explanation being that the principal path is functionally replaced by the secondary paths of which we spoke just now.

(2) The gaps left in our knowledge by the physiological experiment are largely filled out by anatomical investigation. The anatomist has followed two methods in the prosecution of his task: first, the macroscopic dissection of the hardened organ, and, later, its microscopic reduction to a series of thin sections. Of late years, the former of these two methods has fallen into disrepute, on the score that it runs the risk of substituting artificial products of the dissecting scalpel for real fibre-tracts. Carefully applied, however, it is a valuable means of orientation with regard to certain of the wider roads of brain-travel; while its critics are inclined, on their side, to underestimate the danger of error in the interpretation of microscopic appearances. And this danger is the more serious, the farther we are from the actual attainment of the ideal goal of a microscopic examination of the central organ, its complete reduction to an infinite series of sections of accurately known direction. For the rest, microscopical anatomy has been brought in recent times to a high degree of perfection by the application of the various methods of staining. The advantage of these is that they permit of the more certain differentiation of nerve-elements from the other elementary parts, and thus enable us to trace the interconnexion of the nerve-elements much farther than had before been possible.[2] Anatomical investigation is, further, very materially supplemented by embryological research. Embryology shows that the formation of the myelinic sheath in the various fibre-systems of the central organs occurs at different periods of foetal development, and thus puts it in our power to trace out separately certain paths of travel that in all likelihood are physiologically interconnected. This method, however, like the others, has its limits: the systems that develope simultaneously may [p. 154] still include numerous groups of fibres, possessing each a different functional significance.[3]

(3) Pathological observation is equally concerned with functional derangement find with anatomical change, and so in a certain measure combines the advantages of physiological and of anatomical investigation. The observations of pathological anatomy have been especially fruitful for the study of the nervous conduction-paths. Abrogation of function over a determinate functional area means that the fibres belonging to that area undergo secondary degeneration. The pathological anatomist can, therefore, appeal to a law very similar to that upon which embryological investigation is based. Unless there are extrinsic conditions present, which render an accidental concurrence of the degeneration probable, he can assume that all fibres which suffer pathological change at one and the same time are functionally related.[4] The observation of secondary degenerations is of especial value when conjoined with physiological experimentation. The joint method may follow either of two different paths. On the one hand, a severance of continuity may be effected at some point in the central or peripheral nervous system of an animal, and the consequent functional derangement observed. Then, after a considerable time has elapsed, the paths to which the secondary degeneration extended can be made out by anatomical means. On the other, a peripheral organ (eye, ear, etc.) may be extirpated in early life, and the influence observed which the abrogation of determinate functions exerts upon the development of the central nervous organs.[5] In the former case, the nerve-fibres evince the successive stages of degeneration represented in Fig. 23, p. 53. In the latter, the parts of the brain which serve as the centres for the abrogated functions sink in, and microscopic examination shows their nerve-cells in the various stages of atrophy that lead up, in the last resort, to complete disappearance (Fig. 22 B, p. 53).

The first extensive collection of material for the investigation of the microscopical structure of the central organs was furnished by the researches of STILLING. The earliest attempts to construct a structural schema of the whole cerebrospinal system and its conduction-paths by STILLING'S method, i.e. by the microscopical examination of sections, date from MEYNERT and LUYS.[6] MEYNERT, especially, rendered great services to the science; he brought to his re-[p. 155]construction of brain-structure the results of a comprehensive series of original investigations and a rare power of synthetic imagination. It is true, of course, that the schema of conduction-paths which he published was largely hypothetical, and that it has already been proved erroneous in many details. Nevertheless, it formed the point of departure for further microscopical research; so that most of the later work takes up a definite attitude to MEYNERT'S structural schema, supplementing or amending. The application of the various methods of staining, and the consequent differentiation of the nervous elements, have played an important part in this chapter of scientific enquiry. Embryological investigation depends upon the fact that the myelinic sheath is formed in the different fibre-systems at different periods of embryonic development. It is this sheath which is responsible for the coloration of the alba, so that its appearance is easily recognisable. The signs of secondary degeneration consist on the other hand, in a gradual transformation of the myelinic sheath. The tissue becomes receptive of certain colour-stains, like carmine, by which it is unaffected in the normal state. Finally, the myelinic sheath disappears altogether. At the same time, the nerve-fibres proper (neurites) change to fibres of connective tissue, interrupted by fat granules. The value of these degenerative changes for the investigation of conduction-paths lies in the fact that the progressive transformation is always confined within an interconnected fibre-system, and that the direction which it takes corresponds in all fibres with the direction of conduction (WALLER'S law); so that the degeneration of motor fibres follows a centrifugal, that of sensory fibres a centripetal course. Nevertheless, this law of WALLER, like the law of isolated conduction, appears to be valid only as regards the principal direction of the progress of degeneration. In cases where the interruption of conduction has persisted for a considerable length of time, and more particularly in young animals, the deterioration of the fibres is always traceable, to some extent, in the opposite direction as well. We have, further, besides atrophy of the nerves separated from their centres, a similar though much slower degeneration of the nerve-cells which have been thrown out of function by transsection of the neurites issuing from them. This secondary atrophy of the central elements, the initial symptoms of which are the changes represented in Fig. 22, p. 53 above, is, again, especially likely to appear in young animals. It may, however, occur in the human adult, after long persistence of a defect. Thus it has been observed that loss of the eye is followed by atrophy of the quadrigemina; nay, more, in certain cases of the kind, a secondary atrophy of certain cerebral gyres has been demonstrated.

The nerve-roots leave the myel in two longitudinal series, a dorsal and a ventral. The dorsal nerve-roots, as a simple test of function by stimulation or transsection shows, are sensitive: their mechanical or electrical stimulation produces pain, and their transsection renders the corresponding cutaneous areas anaesthetic. The ventral nerve-roots are motor: their stimulation produces muscular contraction, and their transsection muscular [p. 156] disability. The fibres of the dorsal roots conduct centripetally; if they are transsected, stimulation of the central cut end will give rise to sensation, but not that of the peripheral. The fibres of the ventral roots conduct centrifugally; in their case, stimulation of the peripheral cut end will give rise to muscular contraction, but not that of the central. These facts were first discovered by CHARLES BELL, and their general statement is accordingly known as ' BELL'S law.' They prove that, at the place of origin of the nerves, the sensory and motor conduction-paths are entirely separate from each other. The same thing holds of the cranial nerves, with the addition that here, in most cases, the separation is not confined to a short distance in the neighbourhood of the place of origin, but persists either throughout the whole course of the nerves or at least over a considerable portion of their extent.[7] There can be no doubt that the union of the sensory and motor roots, to form mixed nerve-trunks, finds its explanation in the spatial distribution of the terminations of the nerve- fibres. The muscles and the overlying skin are supplied by common nerve-branches. While, therefore, the two sets of conduction-paths are functionally distinct, a spatial separation throughout their entire course occurs only in certain cranial nerves, where the terminations are comparatively near to the points of origin, but the points of origin themselves lie farther apart. Under these circumstances, a separate course involves simpler space-relations than an initial union of the sensory and motor fibres, such as we find in the trunks supplying adjacent parts of the body.

Not only the origin, but also the further peripheral course of the nerves is very largely determined by the conditions of distribution. Fibres that run to a functionally single muscle-group, or to adjacent parts of the skin, are collected into a single trunk. Hence it does not follow that the mixed nerve, formed by the junction of ventral and dorsal roots, always proceeds simply and by the shortest path to its zone of distribution. On the contrary, it frequently happens that there is an interchange of fibres between nerve and nerve, giving rise to what are called the nerve-plexuses. In explaining the occurrence of these plexuses, we must remember that the disposition of the nerve-fibres, as they issue from the central organ, meets the conditions of their peripheral distribution only in a rough and provisional manner; the arrangement is by no means perfect, and requires to he supplemented later on. The plexuses are most commonly formed, therefore. at places where there are parts of the body that need large nerve-trunks, e.g. the two pairs of limbs. Here it is evidently impossible, from the spatial [p. 157] conditions of their origin, that the nerves should leave the myel in precisely the order that is demanded by their subsequent peripheral distribution. But the plexus-formation is not only supplementary; it is, beyond question, compensative as well. The nerve-fibres that are nearest together as the nerves leave the central organs are those that are functionally related. Now functional relation does not always run parallel with spatial distribution. Thus the flexors of the upper and lower leg, e.g., are functionally related, and act in common; but those of the upper leg lie upon the ventral and those of the lower upon the dorsal side of the body, and consequently receive their nerves from different nerve-trunks, the crural and the sciatic respectively. If, then, the nerves for the flexors of the whole limb are in close proximity at their place of origin, there must he a rearrangement of fibres in the sacrolumbar plexus, in order that the two trunks may pass off in different directions. It is probable that the simpler connexions of the root-pairs are principally useful as supplementary mechanisms, while the more complicated plexus-formations are for the most part compensatory in function.

When BELL first established the law known by his name, he felt constrained by it to postulate a specific difference between sensory and motor nerves, -- a difference which found expression in this fact of the difference in direction of conduction, Physiologists for a long time afterwards gave in their adhesion to this hypothesis. There was, indeed, a prevailing tendency to refer all differences of function, e.g. those obtaining between the various sensory nerves, to conic unknown specific property of the nerve-fibres.[8] Later on, the belief gained ground that the nerves are simply in different conductors of the processes released in them by stimulation; though the only argument at first brought forward in support of it was the not very convincing external analogy of electrical conduction.[9] At the present time, we may say, with better reason, that BELL'S hypothesis of a specific conductive capacity of sensory and motor nerves is not tenable. The decisive evidence is drawn from two sources. On the one hand, the general mechanics of nerve-substance has thrown new light upon the processes of conduction in the peripheral nerve-fibre (pp. 80 ff. above). On the other, the morphological facts indicate that the difference in direction of conduction depends upon the mode of connexion of the nerve-fibres at centre and periphery. Figgs 20 and 27 (p. 50 above) gave a schematic representation of the structural relations involved in the two cases. Every [p. 158] motor fibre, as we said in describing them, is the neurite of a nerve-cell; and there is a certain principle of transmission of force which supposedly holds for all neurites alike. In accordance with this principle, the neurite is able to take up the stimulation-processes originated within the cell, or carried to it by its dendrites; but the excitatory processes of which the neurite is itself the sent, though they are conducted to the cell as a result of the general diffusion that every stimulation-process undergoes in the nerve fibre, are inhibited in the central substance of the cell (p. 99). The cells of origin of the sensory fibres always lie, on the contrary, outside of the central organ: in the invertebrates, for the most part at the periphery of the body; in the vertebrates, at any rate outside of the myel proper. Here, as we know, they form as it were little centres of their own; the spinal ganglia, situated in the intervertebral foramens. These ganglia are composed throughout of bipolar nerve-cells; that is to say, each cell sends out two, morphologically identical processes, which probably have the character of dendrites. In the lower vertebrates, the processes issue at different points -- in the fishes, at opposite sides -- of the cell. In man, the same conditions obtain in the early stages of development. As growth proceeds, however, the two processes fuse together at their point of origin, so that what were at first two distinct and separately originated processes now appear as the branches of one single process (Fig. 21, p. 50), which nevertheless retains the character of a protoplasmic process or dendrite. The two processes thus form a single neurone territory (N1), which divides into two halves. The one lies within the myel, and after giving off numerous collaterals penetrates with its terminal fibres into a second central neurone territory (N2). The other is continued in the sensory nerves, and is finally lost either in terminal arborisation among the epidermal cells, or in special end-organs, adapted for the support of the nerve-ramifications (H Fig. 21). We may therefore suppose, in agreement with what was said above regarding the diffusion of excitations carried by the dendrites (p. 42), that, where the peripheral and central processes issue separately from the cell-body, the process of stimulation is transmitted directly by the cell. When, as in man, the two processes unite to form one, the passage across, and then the transmission to higher neurones (N2), may actually take place within the fibre itself. The cell Z1 seems in this case to be cut out of the line of nervous conduction by a sort of short circuit; though this, of course, does not diminish its importance as a nutritive centre and storehouse of force. If, then, we regard the processes of conduction as conditioned in this way by the properties of the nerve-cells and the mode of termination of their processes within them, the principle of conduction in a single direction will hold only for the connexions of neurones, not for the nerves themselves; the stimulation of any nerve, at any point of its course, must, so long as its continuity [p. 159] is preserved, be followed by the production of a stimulus wave which spreads out centripetally and centrifugally at one and the same time. But, as a matter of fact, there is not the slightest reason why we should hesitate to adopt this hypothesis. It is obvious that, when a sensory nerve is cut across, the excitations carried to the peripheral cut end must disappear at the periphery without effect, i.e. without arousing any sensation in our mind, just precisely as the stimuli which act upon the central cut end of a transsected motor nerve are inhibited in the cell connected with the neurite. In both cases, it is not any property of the nerve-fibres, but the character of the nerve-cells, that is responsible for the result. We can see, more especially when we consider the different modes of origin of the cell-processes, that the nerve-cell is naturally qualified to determine the direction of conduction and to regulate the mode of transmission from one neurone territory to another. For the rest, we shall presently become acquainted with facts that speak definitely for a centrifugal conduction of certain sensory excitations (pp. 182 ff.).

We have now to investigate the farther course of the nerve-paths that lead into the myel, as they are continued in the interior of this central organ. We obtain information concerning them, in the first place, from physiological experiments upon the result of stimulation, and more especially upon the effect of transsection, of certain portions of the myel. We know that the motor roots enter the ventral, and the sensory roots the dorsal half of the myel. These experiments show that the principal lines of conduction retain the same arrangement, as they take their course upwards. The effects of outside interference with the ventral portion of the myel are predominantly motor; with the dorsal portion, predominantly sensory. At the same time, they show also that even in the myel the individual fibre-systems are interwoven in the most complicated fashion. The results of hemisection of the myel, e.g., prove that not all conduction- paths remain upon the same side of the body upon which the nerve-roots enter the myelic substance, but that some of them cross over within the myel from right to left and vice versâ. It is true that the statements of various observers as to the kind and extent of conductive disturbance after hemisection are not in complete agreement; and it is evident, also, that the relations of conduction are not identical throughout the animal kingdom. But experiments on animals and pathological observations on man have put it beyond question that the sensory fibres, at any rate, always undergo a partial decussation. Hemisection of the myel does not lead to a complete abrogation of sensation upon either half of the body. The motor paths [p. 160] appear be more variable in this respect. Experiments on animals again point to a partial decussation, though to one in which the greater part of the fibres remain upon the same side. Pathological observations, on the other hand, lead to the conclusion that in the myel of man the motor paths are uncrossed. We may, in particular, recall the well known fact that in unilateral apoplectic effusions in the brain, it is always only the one side of the body, viz. the side opposite to the apoplectic area, that is paralysed. Now there is, as we shall see presently, a complete decussation of the motor paths in the oblongata. If, then, decussation occurred to any considerable extent in the myel, the one arrangement would of necessity, so far as it went, compensate the other. We accordingly conclude that the principal motor path is situated in the ventral portion of the myel. And we are safe in affirming, similarly, that the principal sensory path lies in the dorsal portion,[sic] In the animals, it is true, we have a greater number of secondary paths, branching off to other parts of the myel, than we have in man, but even in the animals, there can be no doubt that the great majority of the fibres run their course without decussation. Impressions made upon the skin after hemisection of the myel upon the same side are not sensed, though stronger, painful stimuli will still evoke a reaction. Finally, in the lateral columns of the myel (m Fig. 66, p. 164) we have a combination of motor and sensory paths, drawn again for the most part from the fibre-systems of the same side. If the integrity of these columns be impaired, whether in man or in the animals, the resulting symptoms are in general of a mixed character.[10]

These experiments upon severance of continuity at various parts of the myel have brought to light a somewhat complicated interlacement of the fibre-systems. One of the chief factors in the resulting formation is, undoubtedly, the cinerea which surrounds the myelocele. The presence of this grey matter also explains the change of irritability brought about in the myelic fibres by stimulation-experiments. While the peripheral nerves may be readily excited by mechanical or electrical stimuli, this is so far from being the case with the myelic fibres that many of the earlier observers declared them to be wholly irresponsive to stimulation.[11] The statement, in its extreme form, undoubtedly overshoots the mark. Excitation can always be effected by summation of stimuli, or by help of poisons, like strychnine, which enhance the central irritability. At the same time, the marked change of behaviour points clearly to the intercalation of grey matter (cf. p. 86 above). If it be asked in what way this intrusion can materially affect the processes of conduction, we reply that a path coming in from [p. 161] the periphery will be brought into connexion, by the cinerea, not with one but with many paths of central conduction. These paths will, it is true, not be all equally permeable. Some will offer more resistance than others to the passage of excitation: in certain cases inhibitory effects, of the kind with which we have become familiar as the results of certain modes of connexion of the central elements (p. 99), may destroy or modify excitations already in progress. But there will always be various secondary paths available, over and above the principal path. The experiments upon severance of continuity call our attention chiefly to the principal path; but there are several ways -- increased intensity of stimulus, enhancement of irritability, destruction of the principal path -- in which the secondary paths may be thrown into function. If the white columns are entirely cut through, at any point upon the myel, so that only a narrow bridge of cinerea remains intact, sense-impressions and motor impulses may still be transmitted, provided that they are unusually intensive. And we find, similarly, that the phenomena of disability, which appear on transsection of a portion of the white columns, disappear again, after a short interval, although the cut has not healed.[12] The existence of these secondary paths or by-paths is attested, further and more particularly, by the phenomena of transference from one conduction-path to another, phenomena which prove the presence of a connecting path between different conduction-paths. They are of three kinds: the phenomena of concomitant movement, of concomitant sensation, and of reflex movement. As all alike are of importance fur a right understanding of the functions of the central organs, especially of the myel, we shall be occupied with them in the following Chapter. We are interested in them here only in so far as they bear witness to the existence of determinate conduction-paths, preformed in the myel, but functioning only under certain special conditions. The place of transference from motor to motor, sensory to sensory, or sensory to motor paths must be sought, again, in the cinereal structures. Complete severance of the cinerea, with retention of a portion of the dorsal and ventral columns of white matter, abrogates the phenomena in question. Transferences within the motor paths, manifesting themselves in concomitant movements, may be made, without any doubt, either on the same side of the myel or from the one side to the other. Thus, the innervation of a finger-phalanx is transmitted both to other fingers of the same hand and, under certain circumstances, to the skin of other parts of the body. Bilateral concomitant movements of this kind are especially observable in movements of locomotion and in pantomimic movements. It is clear, on the other hand, that excitatory innervation within a definite path may be connected with inhibitory co-excitation of the cells of origin of another motor path. We have an instance of this state of things in the relaxation [p. 162] of tonus in the extensors that goes with excitation of the flexors of a limb.[13] This illustration, like that of co-ordinated movements, shows further that co-excitations within the motor paths may establish themselves as regular functional connexions. Transferences within the sensory paths seem, contrariwise, to be confined almost exclusively to the same half of the myel. The concomitant sensations observed after stimulation of some part of the skin are nearly always referred to cutaneous regions on the same side of the body. They are brought out most clearly by painful stimuli and by the arousal of tickling: in the latter case, more particularly when the skin is rendered unusually sensitive by enhancement of irritability. Under these conditions, stimulation of certain regions of the skin usually evokes sensations in other regions. Certain sensory parts, e.g. the external auditory meatus and the larynx, are also pre-eminently disposed for concomitant sensation.[14] We can hardly explain these facts otherwise than by the hypothesis that, on the one hand, certain preferred paths of connexion exist within the sensory conduction-paths, and that, on the other, certain sensory areas (larynx, external auditory meatus) are peculiarly susceptible to co-excitation. As regards the conditions under which conduction takes place, it is clear that concomitant movements and concomitant sensations both alike depend upon cross-conductions, that may be effected at different heights in the myel, and that differ only in direction: the motor cross-conductions extending in all directions, while the sensory, so far as we can tell, are almost exclusively unilateral, and for the most part follow the direction from below upwards. For the rest, the concomitant sensations and concomitant movements that have their ground in connexions of the myelic conduction-paths can never be certainly discriminated from those mediated by transference within the higher centres.

This statement does not apply to transferences of the third kind, reflex connexions of sensory and motor paths. The myelic reflexes may be observed for themselves alone, after the myel has been separated from the higher central parts. The conclusion to be drawn from such observation is that branch-conduction of the reflexes is effected by a large number of conduction-paths, all of which are closely interconnected. Moderate stimulation of a circumscribed area of the skin is followed, at a certain mean degree of excitability, by a reflex contraction in the muscle-group, and in that only, which is supplied by motor roots arising at the same height and on the same [p. 163] side as the stimulated sensory fibres. If stimulus or irritability be increased, the excitation passes over, first of all, to the motor root fibres that leave the myel at the same height upon the opposite side of the body. Finally, if the increase be carried still farther, it spreads with growing intensity first upward and then downward; so that in the last resort it involves the muscles of all parts of the body which draw their nerve-supply from myel and oblongata. It follows, then, that every sensory fibre is connected by a branch-conduction of the first order with the motor fibres arising on the same side and at the same height; by one of the second order, with the fibres issuing at the same height upon the opposite side; by branch-conductions of the third order, with the fibres that leave the myel higher up; and, lastly, by branch-conductions of the fourth order, with those that emerge lower down.[15] This law of the diffusion of reflexes may, however, as we shall see in the following Chapter, be modified in two ways: by variation of the place of application of the reflex stimulus, and by the simultaneous application of other sensory stimuli (cf. Chap. VI. § 2).

The conclusions which we have reached by way of physiological experimentation regarding the course of the conduction-paths in the myel are in complete agreement with the morphological facts revealed by histological examination of this organ. In particular, the arrangement of the nerve-cells and of the fibre-systems which take their origin from the cell-processes as shown in transverse and longitudinal sections, enables us to understand at once that every principal path is here accompanied by a large number of secondary paths, and that the most manifold connexions obtain between one line of conduction and another. We see, first of all, that the fibres of the ventral roots enter directly into the large nerve-cells of the ventral cornua, whose neurites they form; whereas the fibres of the dorsal roots, after their interruption by the nerve-cells of the spinal ganglia, divide upon entering the myel into ascending and descending systems, which there give off delicate branches at all points into the cinerea of the dorsal cornua. Here, therefore, as in the experiments with transsection, the white columns (l, m, n Fig. 66) appear in the role of principal paths: their ventral portions as motor, their dorsal as sensory. Secondary paths, for the conduction of unusually intensive excitations, or for the transferences required by concomitant movements, concomitant sensations and reflexes, can be mediated in a great variety of ways by the cellular and fibrillar system of the central cinerea (d, e). The interrelations of these different paths of conduction, and in particular of the two groups that in functional regard [p. 164] stand farthest apart, the motor and sensory, are then determined by their mode of connexion with their cells of origin, and with the processes which these cells give off. We thus find, in the properties of the neurone and its area of distribution as manifested within the myel, a continuation of the differences that we meet with in the primitive forms represented in Fig. 20, 21 (p. 50). Fig. 67 shows the various morphological elements in their natural connexion. Each of the large multipolar cells m of the ventral cornu has direct control of some peripheral region by means of its neurite n, which does not break up into its terminal arborisation until it reaches the terminal plate of a muscle-fibre (Fig. 20, p. 50). On the other side. the dendrites issuing from the same cell run a very short course, to enter at once into the cinerea of the ventral cornu. The dendritic reticulum stands in direct contact with the terminal fibrils of the neurite g of another nerve-cell, situated as a rule high up in the brain; so that the neurones of this motor conduction cover very extensive territories. Indeed, it is probable that in most instances the entire motor conduction involves only two neurones (N1, and N11, Fig. 20), the one of which extends from the cell n of the ventral cornu to the periphery of the body, while the other begins with some one of the fibres that run their course in the ventral or lateral column (l, m Fig. 66), and ends in a cell of the cerebral cortex. At the same time, still others of the dendrites belonging to the cells of the ventral cornua are in contact with the processes of the small cells s of the dorsal cornua, and with the small intercalatory or commissural cells c that lie scattered between ventral and dorsal cornu. In these latter connections we have, presumably, the substrate of reflex conduction. The sensory nerve-paths, on the other hand, follow a very different course. In their [p. 165] case, the spinal ganglion-cell sp forms the central point of a neurone territory, the one half of which extends by means of the peripherally directed processes h to the sensory termini of the organ of touch (Fig. 21, p. 50), while the other runs centralward in the central process f, which divides in the dorsal portion of the myel into ascending and descending branches (a, d). Both of these branches give off numerous collaterals, whose terminal ramifications stand in contact with the small cells of commissure and dorsal cornu. They themselves are finally resolved into fibrillar reticula, connected by contact with the dendrites of cells lying farther up and lower down. These structural relations seem to warrant the inference that the collaterals correspond to the various secondary paths by which transference, and especially reflex transference, is effected, and that the ascending and descending fibres constitute the principal path. The principal path of sensory conduction is, however, markedly different front the meter. As a general rule, there are several breaks in the line; the path consists of a number of neurone chains, arranged one above another. And this means, again, that the conditions of conduction in the principal path are less sharply distinguished from those in the secondary paths that begin in the collaterals. The whole morphological plan of the system of sensory conduction thus suggests a co-ordination of parts that is at once less strict and more widely variable than is the case on the motor side.

We have spoken so far only of the general properties of the myelic conductors, properties accruing to all nerve-fibres whose mode of origin [p. 166] and connexions conform to a certain type. In the higher regions of the myel, other conditions are at work, paving the way for that differentiation of the conduction-paths which characterises the higher central regions. Even as low down as the thoracic portion of the myel, certain funicles divide off from the three principal columns already named, the ventral, lateral, and dorsal columns (l, m, n Fig. 66). The principal paths, sensory and motor, that run their course within the length of the myel, are thus split up into several separate tracts. The significance of these new funicles can best be understood from their embryological connexions and from the course of the degenerations observed in pathological cases (pp. 154 f.). It can be shown, by both lines of evidence, that the motor division of the lateral columns ascends uncrossed in their dorsal half, in a funicle which, as seen in cross-sections, encroaches from the outside upon the cinerea of the dorsal cornu. Higher up, it passes over into the pyramids of the oblongata, and is accordingly known as the path of the pyramidal lateral column (Fig. 68). In the same way, the innermost division of the motor ventral columns, the part bordering directly upon the ventral sulcus, ascends uncrossed to the oblongata, where it too passes over into the pryamids [sic]. It is termed the path of the pyramidal ventral column, and is the only division of the pyramidal tracts to remain uncrossed in the oblongata. Of the more peripherally situated funicles of the ventral column, some take a straight course upwards, while others enter the ventral commissure and cross to the opposite side of the body. The division of the lateral column which overlies the pyramidal lateral column, at the periphery of the myel, is an uncrossed and, to judge from the conditions of its origin, a sensory path: it branches off to the cerebellum by way of the postpeduncles, and is termed the path of the cerebellar lateral column. The dorsal columns, which are exclusively sensory in function, and therefore receive from below the great majority of the fibres that enter the dorsal roots, divide in the cervical region into two funicles: the slender funicles or columns of Goll (fun. graciles), and the more outlying cuneate funicles (fun. cuneati, Fig. 68).[16][p. 167]

Oblongata and cerebellum, the parts of the brain stem that correspond developmentally to after brain and hind brain (p. 108), together with the pons that unites them, form in the brain of the higher mammals and of man a connected system of conduction paths. The system, as may be gathered from the general trend of the fibre-tracts that pass across it or decussate within it, is of importance in three principal directions. In the first place, this region furnishes the passage-way for the continuation of the sensory and motor conduction paths that come up from the myel. Secondly, it originates new nerves: the great majority of the crania nerves spring from separate grey nidi in the oblongata: and, in doing this, repeats, though in much more complicated fashion, the structural patterns which we have traced, in their comparatively simple form, in the lower central organ. Thirdly, it contains a great variety of connecting paths, them selves for the most part interrupted by deposits of nerve cells, between the various paths that lead across or arise within it; while, further, in the fibre tracts that run from the main conducting trunk to the cerebellum and back again, it possesses a secondary conduction path of very considerable extent that is interpolated in the course of the principal conduction path. It will be understood that, under these conditions, the lines of travel in the region we are now to consider, as well as in the adjoining regions of mid brain and 'tween brain, are extraordinarily complicated. A complete explication of them, in the present state of our knowledge, is altogether out of the question. But more than this: it is impossible, as things are, to put a physiological or psychological interpretation upon many of the structural features that have already been made out. The functional significance of some of the most prominent conduction paths, as e.g. the entire intercalatory system that runs to the cerebellum, is still wrapped in obscurity. Hence, in most cases, the tracing out of the fibre systems is a matter solely of anatomical interest. In physiological regard it is useful, at the best, merely as illustrating the extreme complexity of the conditions which here determine conduction. We shall therefore refer, in what follows, only to certain selected instances, adapted to give a general picture of the course of the paths of conduction in the gross; and we shall enter into some detail only in those cases which appear to be of importance for the physiological and psychophysical relations of the central processes. On the score of method, we must say also that the physiological expedient of isolating the paths by transsection of individual fibre tracts, which did good service in giving us the general bearings of the paths of conduction in the myel, can hardly come into consideration here,[p. 168] more especially in our study of the conditions of conduction in the hind and mid brain regions. Experiments of the kind are recorded not infrequently in the older physiology. The course of the paths is, however, too complicated and their origin too uncertain, to admit of any but an ambiguous result. The most that the method can give us is a point of view from which to appreciate the gross function of the organs or of certain of their parts; and we shall accordingly say nothing of the observations made by it until we reach the next Chapter. We may add that the method which has proved most fruitful for the problem of direction of conduction, apart from direct morphological analysis of the continuity of the individual fibre tracts, is the tracing of the course of degeneration in fibres separated from their centres of origin.

The simplest problem presented by our preset enquiry is that of the further course of the paths of motor and sensory conduction that come up from the myel. The two methods just mentioned furnish us with a fairly satisfactory solution, at any rate as regards the motor paths. The principal continuation of the main path of motor conduction that runs upward in the lateral and ventral columns of the myel is, as we already know: the pyramidal path (Fig. 68, p. 166; cf. Fig. 46, p. 118). The course of this path in detail has been made out, with some degree of completeness, by help of the descending degeneration which appears in it after destruction of its terminations in the brain. It is the continuation of that division of the motor principal path which lies in the myel in the dorsal portion of the lateral columns and along the inner margin of the ventral columns (Fig. 68 B). The branch of this path that belongs to the ventral columns decussates in the cervical region of the myel. Now the larger branch, from the lateral columns, also undergoes a complete decussation, clearly visible on the external surface of the oblongata (p, Fig. 47, p. 119). The central continuation of the path then runs to the cerebral cortex, without interruption by cinerea, Fig. 69 gives a schematic representation of the course of these paths, the longest and so far the best known of all lines of central conduction. After they have traversed the pons, the fibres of the pyramidal path enter the crusta (f, Fig. 56, p. 130) between lenticula and thalamus, and then trend upwards in the space between lenticula and caudatum to pass into the corona, where their principal branches constitute the fibre-masses that terminate in the region of the central gyrus and the surrounding area (VC, HC, Fig, 65, p. 145).[17] The path is thus fairly well defined. Part of it, as is proved by the paralyses following lesion of the pyramids [p. 169] and their continuation in the crus, undoubtedly subserves the conduction of voluntary impulses. In the animal kingdom, the pyramidal path affords a better measure than any other of the fibre systems collected in the brain stem of the general development of the higher central organs. In the lower vertebrates, the pyramids are altogether wanting. In the birds, they are but little developed. They steadily increase in importance in the mammalian series, up to man; while at the same time the tract from the lateral columns, which passes to the opposite side of the body in the pyramidal decussation, grows constantly larger as compared with the tract from the ventral columns, which decussates in the myel. A branch of the motor path which is forced inward by the pyramids, and which remains intact after removal of the pyramidal fibres, may be traced in part to the mesencephalon. It consists mainly of divisions of the ventral columns (mf, Fig. 70). Finally, certain of these remains of the ventral columns are collected in the interior of the rounded prominences to form the dorsolongitudinal bundle (hl, Fig. 72), which in its further course through the pons makes connexions with the pontal nidi and more especially, as it appears, with centres of origin of the oculomotor nerves and with the cerebellum.[18] We may accordingly suppose that these branches of motor conduction which run to the mesencephalon serve to mediate co-excitations in that region. The connexions of the dorsolongitudinal bundle, in particular, seem to point to connexions of the motor innervation of the eye and of the skeletal muscles, such as are involved in locomotion and in the orientation of the body in space.[p. 170]

The course of the sensory path through the oblongata has not been made out as fully as that of the motor. The main reason for this defect in our knowledge lies in the difference of structure to which we referred above. It is characteristic of sensory conduction in the myel that the path does not pass upward in unbroken continuity, but consists of a chain of neurones. This structural complexity is not only continued but increased in the oblongata, where large numbers of cells, grouped together to form separate nidi, are interposed in the line of conduction. We may suppose that these nidi serve for the most part as transmitting stations -- points at which a path, whose course has so far been single, splits up into several branches that diverge in different directions. The main divisions of the sensory path pass in this way, within the oblongata, first of all into the grey masses deposited in the slender and cuneate columns (Fig. 68, A, and Fig. 46, p. 118). Further on, the sensory path continues in a bundle lying close under the pyramids (l, Fig. 70), which appears on the ventral surface of the oblongata directly above the pyramidal decussation (p, Fig. 47, p. 119) here in its turn suffers decussation, and then passes on in the lemniscus of the crus, a structure lying in the enter and upper portion of the tegmentum. The lemniscal decussation (formerly known as the superior pyramidal decussation) thus forms yet another continuation of the decussations of myelic fibres which begin within the myel itself. Other sensory fibres (ci, Fig. 70), down from the dorsal columns, pass into the tegmentum proper, which thus brings together portions of the motor (mf) and of the sensory path. All these sensory fibres terminate in the grey masses of the region of the quadrigemina and thalami, from which, finally, further continuations of the sensory path proceed to the cerebral cortex.

The general sketch of the course of the sensory and motor paths, given in the preceding paragraphs, makes matters much simpler than they really are. There are two facts, not yet mentioned, that are chiefly responsible for the complications actually found. The one of these consists in the origination of a large number of new sensory and motor paths, which are derived front the cranial nerves, and in their further course either join the paths formed by the myelic nerves or strike out special lines of their own, the other, in the appearance of large groups of central nerve cells, which serve either as transmitting stations for the conductions comings up to the cerebrum from below, or as junctions for the important branch-conduction to the cerebellum, here opened for travel. The difficult questions concerning the origin of the cranial nerves, questions that have not yet in every case received their final answer, are of interest for psychology [p. 171] only in so far as they involve that of the paths followed by the sensory nerves. Since these belong in large part to the mesencephalic region, we may postpone their consideration until later. It will suffice for our present purpose to refer to Fig. 72 (p. 176), as an illustration of the conditions of origin of the

cerebral nerves at large. The Figure shows how the funicles of origin of these nerves spring from isolated grey masses, the nerve nidi; how they then again and again strike across the longitudinal fibre tracts; and how they finally follow the general trend of the ascending paths. Most of the fibres of origin, however, enter into still further connexions most with other nidal structures scattered throughout the oblongata. This statement applies in particular to the fibres of the oculomotor nervous system, to which we return below, when we come to discuss the conduction paths of the sense of sight, and to the mixed nerves, among which the pneumogastric, trigeminus and facialis are of especial importance by reason of their manifold functional relations. The nidi just mentioned are, we may conjecture, centres of excitation and transmission for the great functions regulated from the oblongata, -- heart beat, movements of respiration and articulation, mimetic movements. Our knowledge of the mechanics of innervation in these cases is, however, still very incomplete.[19][p. 172]

We turn now to the grey nidi of this region of the brain. We have already mentioned the nidi of the dorsal columns, interposed directly in the sensory path. Very much more complicated are the functions of the largest nidi of the oblongata, the olives (Fig. 46 B, Fig. 47, pp. 118 f.) whose principal office seems to be the giving off of branch-conductions. On the one hand, the neurites of the cells give rise to a fibre system, the further course of which is uncertain: it is supposed to connect partly with the cerebellum, partly with the lateral columns of the myel. On the other hand, the dentata give rise to two fibre systems. The first of these covers the outer surface of the olivary nidus, in the form of zonal fibres (g Fig. 48, p. 120), and then bends round into the restes and their continuations, the cerebellar peduncles (cr Fig. 70). The second issues from the interior of the nidus and crosses the median line, to decussate with the corresponding fibre-masses of the opposite side. Other fibres from the olives enter the longitudinal fibre tract that lies between them, and then run within the pons to the lemniscus of the crus (l Fig. 70); they thus appear to join the sensory principal path to the cerebrum. Putting the facts together, we may say that the olives are structures which stand in intimate relation with the branching off of conduction paths towards the cerebellum Another ganglionic nidus, lying higher up -- in man concealed by the pons, in the lower mammals projecting on its posterior border -- the trapezium or superior olive, forms, as we shall see presently, a nodal point of great importance in the conduction of the acoustic nerve.

The conduction paths that branch off from the oblongata to the cerebellum, and there turn back again to join the caudex in its course through the pons, bear a striking external resemblance to a shunt interposed in the main current of an electrical conduction. And it seems, as a matter of fact, that this obvious comparison fairly represents the actual relations of the nerve paths, as they are shown schematically in Fig. 71. The sensory and motor principal paths, just described, have also been included in this diagram, in order that the reader may obtain a rough idea of their relation to the branch path leading to the cerebellum. The mammalian cerebellum contains, as we have already said, two formations of cinerea: the one appearing in the ganglionic nidi, the other in the cortical layer investing the entire surface of the organ (pp. 121 f.). Our present knowledge of the relations between the fibres that enter into and issue from the cerebellum and these grey masses may be summarised as follows (cf. Fig. 48, p. 120). The fibres of the restes are deflected round the dentatum, more especially over its anterior margin. They do not appear to connect with the cinerea of the nidus, but radiate from its upper surface towards the cortex, where [p. 173] they terminate and are lost. From the cortex itself comes a system of transverse fibres; which cut across the more longitudinal radiations of the restes, and draw together in stout fascicles to form the medipeduncles (brachia of the pons). The interior of the dentata gives rise, further, to the funicles which pass into the prepeduncles (crura ad cerebrum). And, finally, there is a connexion between the dentata and the cerebellar cortex. This path, together with the radiation of the restis and the medipeduncle, occupies the outer division of the alba, while the innermost portion is constituted by the prepeduncle. It is therefore probable that all the fibres running through the postpeduncles of the cerebellum from the oblongata have their termination in the cortex. The cortex itself gives rise to two fibre systems: the one passes directly over into the medipeduncles, the other appears first of all to connect the cortex with the dentatum, which then gives off the vertically ascending fibres of the prepeduncles. These run upwards, with the continuations of the myelic columns, converging as they proceed; just anteriorly to the upper end of the pons they reach the middle line, and undergo decussation. Besides the two divisions of this system of ascending fibres, we find, lastly, further radiations, whose fibres subserve the interconnexion of more or less remote cortical areas. Some of the longer lines cross from the one, side to the other in the vermis.

The further course of the paths leading from the cerebellum to the cerebrum is as follows. The path which is continued in the medipeduncles appears, first of all, to terminate in grey masses in the anterior region of the pons. From these masses arise new, vertically ascending fibre, some of which can be traced to the anterior brain ganglia, the lenticula and striatum, while others proceed directly to the anterior regions of the cerebral cortex. The fibres collected in the prepeduncles find their proximate termination in the rubrum of the lemniscus (hb Fig. 56, p. 130). A small number of the fibres issuing from this point probably enter the thalami; but the greater portion pass to the internal capsule of the lenticula, and thence in the corona to the cerebral cortex, ending in the regions posterior to the central gyre, and more especially in the precuneus. The valvula (vm Fig. 48, p. 120), which joins the prepeduncles at the beginning of their course, serves in all probability to supplement the connexions of the cerebellum with the brain ganglia, by mediating a conduction to the quadrigemina.

We must believe, in view of these results of anatomical investigation, that the concurrence of conduction paths in the cerebellum is extremely complicated. Let us consider these paths as a branch conduction, interposed in the course of the direct conduction from myel to cerebrum as mediated by oblongata and pons. We have two divisions, a lower and an upper. The lower division of the branch conduction carries sensory fibres from [p. 174] the dorsal and ventral columns (olivary path of the dorsal columns, and cerebellar path of the lateral columns), which connect the myel with the cerebellum; and motor funicles, which branch within the pons to enter the restes. The upper division makes two principal connexions, by way of the medipeduncles: the one with the cerebral cortex direct, the other with the anterior brain ganglia (lenticula and striatum). At the same time, there is a connexion, mediated by prepeduncles and valvula, with the posterior brain ganglia (thalami and quadrigemina). The most extensive of these conductions, that to the cerebral cortex effected by the medipeduncles, radiates out to all parts of this organ, but is principally directed forwards to the frontal brain and the adjacent regions.

The schema given in Fig. 71 shows the main features of this conduction system. The reader will recognise, first of all, the pyramidal path, with its crossed branch from the lateral and its uncrossed branch from the ventral columns, running directly between myel and cerebral cortex (p1p2, p). He will next notice the other motor paths, derived from the ventral columns, and interrupted in the mesencephalic region by masses of cinerea. Some of these paths are continued in a new neurone chain, and extend to the cerebral cortex; other fibres of the same system probably terminate in the mesencephalic region itself (vv'). A considerable division of the sensory path (gg') drawn from the dorsal columns, passes in the lemniscal decussation (k2) to the opposite side: part of it is lost in the [p. 175] grey masses of the pons, part continues in fibre tracts which, interrupted by grey nidi, run to the anterior brain regions and so finally to the cortex. There is also an uncrossed sensory path (cc), derived from the dorsal and lateral columns, which passes into the tegmentum of the crus and finds its proximate terminus in the tegmental grey nidi. Another path, also sensory in origin, is the uncrossed branch conduction (cs) carried to the cerebellum from the restes in the postpeduncles; it terminates in the cerebellar cortex, for the most part in the vermis. Finally, there is a crossed conduction (f), issuing from the grey nidi of the olives, which, unlike the former, enters into the nidal structures (N) of the cerebellum. These are all incoming paths. The outgoing lines, leading to the cerebrum, are two in number: the prepeduncles, which start from the cerebellar nidus, and may be traced partly into the prosencephalic ganglia, partly to the cerebral cortex (e'); and the fibres of the medipeduncles (bb'), which run direct from the cerebellar cortex to the cerebrum. These latter enter, first, into the grey nidi of the pons, and are by them brought into connexion, in some measure, with the brain ganglia, but most extensively with the cerebral cortex, and in that principally with the frontal region. The system is completed by the paths of connexion between nidal structures and cortex (rr) which belong exclusively to the cerebellum.

The general relations of these incoming and outgoing paths suggest that the cerebellum brings into connexion with one another conductions of different functional significance, This inference finds further support in the peculiar structure of the cerebellar cortex. The characteristic constituents of this region are, as we saw above (Fig. 15, p. 44) the cells of PURKINJE, easily distinguished by their large size and the manifold arborisation and reticulation of their protoplasmic processes. If, now, the cerebellar cortex serves to connect fibres of different function, sensory and motor, as is suggested by the relations of the incoming and outgoing paths, it is clear that we may look upon these cells of PURKINJE as elementary centres of connexion between functionally different fibre elements. We should then have to assume, on the analogy of the large cells in the ventral cornua of the myel, that the dendrites mediate centripetal, the neurites centrifugal conductions: in other words, that the chief office of the former is to take up the excitations carried in the postpeduncles, while the latter collect to form the paths of conduction that continue in the medipeduncles to the cerebrum and there, as it appears, are chiefly connected with the centres of innervation of the prosencephalon.

The pons is chiefly important as receiving the paths to be carried up from cerebellum to cerebrum, and associating them to the vertical ascending fibres of the crus. Its development in the animal kingdom thus keeps even pace with the development of all these paths of conduction, and [p. 176] especially of the pyramids and medipeduncles. The fibres that cross over from the one side to the other in the median line of the pons (at R Fig. 72) are decussating fibres belonging in part to the direct continuations of the myelic columns through the pons, in part to the medipeduncles of the cerebellum. The decussation of these latter has been established by pathological observations: atrophy of a cerebral lobe is ordinarily attended or followed by a wasting away of the opposite half of the cerebellum. The fibres of the medipeduncles, probably without exception, pass through internodes of grey matter before they are deflected into the vertical paths; and small grey nidi are also strewn in the path of the directly ascending prepeduncles (ba Fig. 72). These presently decussate, and come to art end in the rubrum of the tegmentum. In this way, by collection of the myelic columns that come up from below, and of the continuations from the cerebellum that join them front above and from the side, there forms within the pons that entire fibre tract which connects the lower-lying nerve centres with the structures of the cerebrum, -- the crus. At the same time, the pons is broken root bundles of certain cranial nerves, which take their origin higher up. The nidi of origin of these nerves are situated partly upon the cinereal floor of the highest portion of the fossa rhomboidalis (metacele), partly in the neighbourhood of the Sylvian aqueduct (mesocele), which forms a continuation of the central canal.[p. 177]

As a result of its cleavage by cinerea and by the cross fibres of the medipeduncles, the crus divides into two parts, distinguishable in the gross anatomy of the brain, and known as crusta and tegmentum. A third division, the lemniscus, belongs to the tegmentum so far as regards the direction of its course, but in all other respects is clearly differentiated from it. Neither of the two principal parts constitutes a complete functional unit; on the contrary, each of them includes conduction paths of very diverse character. Nevertheless, the twofold division of the crus seems to represent a first, even if a rough classification of the numerous paths of conduction to the cerebrum. Thus the inferior portion or crusta (p--p' Fig. 72) is principally made up of the continuations of pyramidi, remains of the dorsal columns, and medipeduncles. Its outermost portion carries that continuation of the dorsal columns which passes in the lemniscal decussation to the opposite side of the body (k2 Fig. 71). The intercalatum (substantia nigra of SÖMMERING: Sn Fig. 73) is a ganglionic nidus, belonging to the conduction paths of the crusta, which separates crusta from tegmentum. The portion of the crus which lies above the intercalatum, the tegmentum (v'--hl Fig. 72), is at first composed of the remains of the lateral and dorsal columns, and of a part of the remains of the ventral columns. In its further course, beyond the point at which the rubrum appears in cross sections of the tegmentum (R Fig. 73), these are reinforced by the prepeduncles (mf, hi, cr Fig. 70). Finally, the lemniscus, which we have recognised as a separate subdivision of the tegmentum (sl--sl' Fig. 72), also carries fibres from the dorsal columns, as well as fibres from the ventral columns and the cerebellum. Taking the origin of all these tracts into consideration, we may designate the crusta as that part of the crus which, so far as it derives directly from the myel, is especially devoted to the conveyance of motor paths; the tegmentum and lemniscus are of mixed, and mainly, as it seems, of sensory origin. At every point, however, these direct continuations of the myelic systems are augmented by intercentral paths, the conductions from the cerebellum. In this way, as may be seen from Fig. 72, which shows a cross section taken approximately through the middle of the organ, the structure of the pons becomes extraordinarily complex. We may add that it contains, crowded together in a comparatively small space, the whole number of conduction paths, many of which in their later course are widely divergent. It is, therefore, a remarkable coincidence that, besides the epiphysis, which is not a nervous centre at all (see p, 124, above), the pons should have been regarded with especial favour by the metaphysical psychology of past times as the probable 'seat of the mind.' HERBART himself accepts this view. If, on the contrary, one were asked to lay one's finger upon a part of the brain that by its complexity of structure and the number of elements it compresses into a small space [p. 178] should illustrate the composite character of the physical substrate of the mental life, and therewith show the absurdity of any attempt to discover a simple seat of mind, one could hardly hope to make a happier choice.

If we look at the series of cerebral ganglia, we see at once that those of mesencephalon and diencephalon, the quadrigemina and the thalami, serve as intermediate stations on the line of conduction: peripherally, they receive sensory and motor fibres; centralwards, they stand in connexion with the cerebral cortex. They lie, as their function requires, directly upon the crura, whose fibre masses partly run beneath them straight to the prosencephalon, partly curve upwards to enter into the grey nidi of the ganglia. There is a difference, however: the thalamus takes up comparatively few fibres from below, and sends out very considerable bundles to the cerebral cortex; the quadrigemina do just the reverse. Both ganglia, as we shall see in detail later, are of especial importance as nodal points in the optic conduction. Fig. 73 shows a section taken through the middle region of this whole area, and will assist in some degree towards an understanding of the structural relations.

The position of the prosencephalic ganglia, the striata with their two subdivisions, caudatum and lenticula, is more obscure. The incoming and outgoing fibres tell us but little of their function. Both divisions receive fibres front the periphery, derived for the most part from the diencephalic and mesencephalic ganglia. The crural fibres, on the other hand, pass below and between the prosencephalic ganglia, without entering them (Fig. 74). The grey masses of the ganglia send no further reinforcements [p. 179] to the coronal radiation. It would appear, then, that these structures are terminal stations of conduction, analogous to the cerebral cortex, and not intermediate stations like the thalami and quadrigemina.[20]

An important place is filled in the system of conductions that falls within the region we are now considering (prosencephalon, diencephalon, mesencephalon) by the paths of the sensory nerves. Fortunately, these are among the conductions that have so far been most fully investigated, and whose functional significance is at the same time relatively easiest of interpretation. In view

of their great importance for psychology, we shall, therefore, depict their principal features in some little detail. We begin with the conduction paths of the nerves of taste and smell; putting these together not so much because the peripheral sense organs are closely related, both in spatial position and in function, as rather because the two lines of conduction may in a certain sense be regarded as prototypes of the much more complicated conditions that obtain in the cases of sight and hearing. The [p. 180] gustatory path approaches very closely to the type familiar to us in the sensory paths of the general sense. When the conducting fibres have left the central organ, they pass only once more, and then in the near neighbourhood of the centre, through bipolar

cells, analogous to the cells of the spinal ganglia; they then break up at the periphery of the organ in a reticulum, which is distributed between non-nervous epithelial elements. The olfactory path, on the other hand, is a pronounced instance of the second type of sensory conduction, characterised by the outward displacement of central nerve cells to the peripheral organ, which accordingly represents, in all essential particulars, a portion of the central organ: cf. above, p. 47.

There is, however, a further point, in which the path of the gustatory nerves differs from those of the other nerves of special sense. The gustatory fibres, in consequence, we may suppose, of their distribution over a functional area of some considerable extent, run their course in two distinct nerve trunks: those destined for the anterior portion of this area in the lingualis (L Fig. 75), and those intended for the posterior portion in the glosso-[p. 181]pharyngeus (G). This division appears, however, to be simply external. Both of the gustatory nerve paths take their origin from the same masses of nidal cinerea on the floor of the metacele. At first, however, the gustatory fibres that run to the anterior portion of the tongue join the facialis, at the genu of which (F) they pass through the cells of a small special ganglion. Thenceforward they are continued in the chorda tympani (Ch) side by side with the lingual branch of the trigeminus. The glossopharyngeus, on the other hand, which supplies the posterior portion of the tongue, passes through its own ganglion. At the periphery, as we shall see when we come to consider the peripheral sense apparatus the two nerves break up into terminal fibrils, which end in and among the taste breakers, without, as it appears, coming into contact with other than epithelial terminal structures. The course of the fibres, as shown synoptically in Fig. 75, accordingly corresponds in all details with a general sensory conduction, such as is represented in Figg. 21 and 67 (pp. 50, 165) for the myelic nerves: the ganglia VII. and IX. may be regarded as analogues of the spinal ganglia.

The paths of the olfactory nerves follow a radically different course. Their point of origin lies furthest forward of all the sensory nerves, so that they border directly upon certain cortical regions of the cerebrum. This is the reason that the olfactorius, from the outset, is not a single nerve, but appears in the form of numerous delicate threads, which issue direct from a part of the brain that belongs to the cortex, the olfactory bulb (Fig. 52, p. 125). Conduction begins at the periphery in the cells of [p. 182] the olfactory mucous membrane (A Fig. 76), which are set between epithelial cells, and have themselves the character of nerve cells that send their neurites centralward. These neurites, in their course to the olfactory bulb, break up into delicate fibrils, which for the most part come into contact with the dendrites of small nerve cells: the two sets of processes together forming a compact ball of tissue (a, b). Each of these cells, in its turn, sends out a principal process, which passes into one of the large nerve cells of the bulb (C). Here we must place the proximate cortical station of the olfactory path. The dendrites issuing laterally from the cell bodies represent, in all probability, ramose secondary conductions; while the main path of centripetal conduction is continued, in the direction of the arrows, in the neurites, which leave the cell upon the opposite side, and pass into the olfactory tract. This, the principal path, accordingly extends over a peripheral and a central neurone territory. There is, now, a second group of central olfactory cells which, if we may judge from their connexions and the direction of their processes, are probably to be regarded as nodal points of a system of centrifugal conduction. These cells (D) send out a single peripherally directed neurite, which breaks up within the glomeruli in a delicate reticulum of terminal fibrils (c). The olfactory path thus shows a marked divergence from the type of sensory conduction represented by the cutaneous nerves. The peripheral organ itself appears as a peripherally situated portion of the cerebral cortex, and the olfactory fibres, by a natural consequence, resemble central rather than peripheral nerve fibres. Another novel feature is introduced in the probable existence of a secondary path of centrifugal conduction,. And, lastly, we must note the central connexion of the olfactory region; of the two sides by the precommissure (ca Fig. 53, p. 127). The connexion is presumably to be interpreted as an olfactory decussation, by which the centripetal paths are carried to the opposite hemisphere, and the neurones D c are also enabled to mediate co-excitation, in the centrifugal direction, of the peripheral cells A of the opposite side of the body.

In man and the higher vertebrates, the cochlea of the auditory organ is, in all probability, the only part of the labyrinth of the ear that subserves auditory sensation. If we may judge from the character of the cochlear nerve terminations, the peripheral starting-point of the acoustic conduction conforms in all respects to the conduction type represented by the cutaneous nerves. The terminal fibrils of the acusticus extend among the epithelial and connective tissue structures of the basilar membrane (cf. Ch. VIII., § 4, below), and then, in the auditory canal of the cochlea (S Fig. 77) traverse groups of bipolar ganglion cells (g) which resemble the cells of the spinal ganglia [p. 183] and are termed in common the spiral ganglion. The cells of this ganglion, which accordingly corresponds to an outlying spinal ganglion, give off neurites which run centralward, and finally break up into

terminal arborisations within various accumulations of cinerea, more especially in two large nidi in the region of the metacele, with the processes of whose cells they are thus brought in contact. The nidi in question are a somewhat smaller anterior nidus, the anterior acoustic nidus (V A) and a larger posterior nidus, the tuberculum acusticum (Ta). Both of these ganglia send off a small number of fibres, which pass upwards on the same side (uf), and a larger number, which ascend on the opposite side of the body (kf). The [p. 184] former run partly to the postgemina, and partly, having joined the lemniscus, direct to the cerebral cortex direct to the cerebral cortex. There is also, in all probability a branch path from the tuberculum acusticum, which proceeds with the postpeduncles to the cerebellum (Cb). The great majority of the fibre tracts issuing from the two nidal masses cross, however, to the opposite side, either directly or by way of the superior olives (Ol); in each of which the conduction is transferred to new neurone territories. After decussation, they continue in the same direction as the uncrossed fibres: some to the postgemina (UV), some in the lemniscus to the acoustic area of the cerebral cortex (H). Still other neurites (r'r'), which take their origin from cells in the superior olives, follow a shorter road, running crossed or uncrossed, to nidi of motor nerves. This latter path must accordingly be regarded as a reflex path. The branch conduction to the postgemina is continued, with interruption by their cell masses, to the pregemina (OV), from which again a centrifugal fibre system (rr) runs to motor nidi, and more especially to the nidi of the oculomotor nerves. It would seem, therefore, that this quadrigeminal path is also, in part, a reflex path; though the quadrigemina serve at the same time as a transmitting station, from which a further centripetal conduction is continued to the anterior brain ganglia. To the sensory conductions already described must be added, finally, a path between the same cell stations in the quadrigemina (OV, UV) and the acoustic nidi (VA, Ta), which, if we may judge from the peripheral direction of its neurites, conducts in the opposite direction to that marked in the Figure, and world therefore represent a centrifugal path. Like the centripetal path which it accompanies, it consists of a large number of crossed and a small number of uncrossed fibres. It is indicated by the downward pointing arrows. We thus have, in summary, the following paths of conduction: (1) the primary path, conforming in type to the path of the spinal nerves, which runs from the peripheral end-fibres of the acoustic nerve to the ganglion spirale (the equivalent of a spinal ganglion) and thence to the acoustic nidi (VA, Ta) in the oblongata; (2) the principal centripetal path, beginning in these nidi, which divides into a smaller uncrossed and a larger crossed bundle and runs, with interruption by the superior olives or by other masses of nidal cinerea, to the cerebral cortex; (3) a branch path, beginning in the same nidi of the oblongata, which also divides into crossed and uncrossed portions, and runs to the quadrigemina and thence in all probability to the anterior brain ganglia; (4) reflex paths, which lead across to motor nidi as low as the superior olives and as high as the pregemina, and which include, more particularly the paths of the oculomotor nerves and of the facial muscles concerned in the movements of speech; (5) a branch path to the cerebellum, which again begins in the primary acoustic nidi; and, finally, (6) a centrifugal sensory [p. 185] path, which issues from the quadrigeminal nidi, and is associated in its peripheral course with the corresponding centripetal path.[21]

This list shows us how extraordinarily complex is the network of relations into which the auditory organ is brought by its central paths. Apart from its twofold crossed and uncrossed connexion with the cerebral cortex, the following facts should be noted as of especial significance. First, there is a reflex path connecting the acoustic centres with the points of origin of muscular nerves, and among them with the centres for the movements of articulation and for the movements of the eyes, which latter are extremely important in the spatial orientation of the body. Secondly, we find that the conduction system, like that of the olfactory nerve, includes centrifugal paths, whose office is, perhaps, to transmit the excitations of the auditory organ of the opposite side, or other sensory excitations that find their nodal points in the mesencephalic region, in the form of concomitant sensation.

We remark, in conclusion, that the acoustic nerve proper, which comes from the cochlea, is connected over a part of its peripheral course with the nerve that comes from the vestibule and canals. This, the vestibular nerve, is a branch of the eighth cranial, and is commonly accounted, like the cochlear, to the acoustic nerve. In its central course, however, it appears to follow a different road. It passes through special nidal structures, and finally, as its secondary degenerations prove, terminates in separate areas of the cerebral cortex. [22]

The principal difference between the optic and acoustic conductions is that the optic surface itself, like the olfactory surface, is an outlying portion of the central organ, displaced to the periphery of the body. It is natural, therefore, that the optic fibres too, when they emerge from the retina, should at once appear, as by far the great majority of them do, in the character of central nerve fibres. The cells that give visual sensation its specific quality, the rods and cones (S and Z Fig. 78) -- usually termed, on this account, visual cells -- are sensory epithelia which, like the gustatory cells, are connected only by contact with the terminal fibrils of the optic conduction. In the retinal layers that cover them are several strata of nerve cells, easily divisible by their marked differences of form into two main groups: the large multipolar ganglion cells (G2), which my be regarded, from the relations of their neurites and dendrites, as proximate points of departure for the optic conduction running centripetally from the retina to the brain; and bipolar ganglion cells (G1) to which may be added stellate intercalary cells, found far forward in the neighbourhood of [p. 186] the elements S and Z, and not represented in the Figure. These last two classes constitute together a neurone territory, intervening between the last termina: fibrils of the peripheral optic conduction and the large ganglion cells (G2), which may be considered as the extreme peripheral member of the centripetal optic conduction. Between its limits we find, further, terminal arborisations of neurites (e), derived not from cells of the retina itself but from more central regions, -- probably from the pregemina, since these, as we shall see in a moment, form important nodal points in the optic conduction at large. There is thus a further point of resemblance between the outlying central area represented in the retina and the olfactory surface: here as there, the structural relations indicate the existence of a centrifugal secondary path, running alongside of the centripetal running alongside of the centripetal principle path.[23][p. 187]

The fibres collected in the optic nerve conduct, then, for the most part centripetally; though there is, in all probability, a small admixture of centrifugal conductors. Following its course, we come upon the decussation of the optic nerve, the chiasma, where a distribution is made of the optic fibres, to the paths running further towards the central organ, that is obviously of extreme importance for the co-operation of the two eyes in binocular vision. It is instructive, in this regard, to trace the phylogenetic stages through which the mode of distribution in the human chiasma has gradually been attained. In the lower vertebrates, up to the birds, there is a complete decussation of the paths, the right half of the brain receiving only the left optic path, and conversely. In the mammalian series, from the lower orders onwards, direct paths play a larger and larger part alongside of the crossed fibres; until finally, in man, the distribution has become practically equal; so that the one (the temporal) half of the retina passes into the optic tract of the opposite side, and the other (the nasal) into the tract of the same side.[24] We owe our knowledge of this fact less to direct anatomical investigation, which finds great difficulty in the tracing of the detailed course of the optic nerve, than to pathological observations of the partial loss of sight resulting from destruction of the visual centre of one hemisphere or from the pressure of tumours upon the optic tract of one side. The main results of these observations are brought together, in schematic form, in Fig. 79. It will be seen that the corresponding halves of retina and optic nerve are cross-hatched in the same direction. The temporal halves of both retinas have a crossed (tt), the nasal halves a direct path (nn). Before decussation, the crossed path lies on the outside, the uncrossed on the inside of the optic nerve; after decussation, the crossed changes to the inside, the uncrossed to the outside of the optic tract. In contradistinction to the retinal halves which thus receive only a one-sided representation in the brain, the central area of the visual surface, or macula lutea, where the retinal elements are set most thickly, is favoured with a bilateral representation. Destruction of the central optic fibres of one side is accordingly followed by half-blindness (hemianopsia) or limitation of the field of vision to one-half of each retina (hemiopia), with the exception of the area of direct vision around the fixation point, which becomes blind only when the central disturbance affects both sides of the brain.[25] We return later (pp. 229 ff.) to the relations which this peculiar mode of distribution sustains to the function of vision.

The conduction of the optic tract of either half of the brain is thus composed of temporal paths from the opposite retina, nasal paths from the [p. 188] retina of the same side, and macular paths from both retinas. It divides again, on both sides -- as is shown schematically in Fig. 78, where abstraction is made from the decussations which we have just been discussing -- into

two paths: the one of which runs first of all to the pregenicula (AK), while the other passes to the pregemina (OV). The two paths, appear to have no connexion with each other, despite the proximity of quadrigemina and genicula (see Fig. 48, p. 120). In the same way, the optic path to the pregeminum runs direct to this through the prebrachia, without coming into contact with the postgeminum. The first of these two paths, that which travels to the pregeniculum, forms the direct optic radiation (ss) to the cortex or the occipital lobe. It passes over, in the grey nidi of the pregeminum, into a new neurone territory, whose neurites break up into terminal fibrils in the brain cortex. On the other hand, the large pyramidal cells of the cortex send out neurites, which apparently join the coronal [p. 189] fibres that enter the pregemina, and thus constitute a centrifugal conduction extending to that point (c'f'), The pregemina themselves, which contain terminal arborisations of fibres, first beginnings of neurites from ganglion cells, and various forms of intercalary cells, and which send out fibres both to the peripheral sense organ and to the nidi of the oculomotor nerves, appear accordingly as intermediary stations of great complexity, while, on the one side, they receive the central conduction coining from the brain cortex, they serve, on the other, towards the periphery, as points of departure for sensory and motor fibres, -- the sensory paths conducting in part centripetally to them, and in part centrifugally away from them. If, then, we abstract from the facts of decussation, described above, we may say that the fibres of the optic tract make up the following paths: (1) the centripetal sensory principal path (ss), which in the large multipolar ganglion cells of the retina (G2) receives the excitations pouring in from the periphery; has an intermediary station, again interrupted by ganglion cells, in the pregeniculum (AK); and finally reaches the cerebral cortex in the optic radiation of the corona: (2) a centripetal sensory mesencephalic path (cp), which runs to the pregemina; here, in its turn, enters a new neurone territory; an finally, as it would appear, passes into the centrifugal paths (rr) that go to the nidi of the oculomotor nerves -- the whole path cprr thus representing a reflex path, which connects retina and oculomotor nerves in the mesencephalon: and lastly: (3) a path which, if we may judge from the mode of connexion of its elements, conducts centrifugally, and which divides into two parts: a central branch c'f', running front optic cortex mesencephalon, and terminating in the pregemina; and a peripheral branch cf, beginning in the pregemina and [p. 190] ending, as we saw above, in the retina. In view of all these connexions we can readily understand that reactions to light impressions can be released in the mesencephalon, without any participation of the principal path: released as reflexes to the oculomotor system, by way of the transferences effected in the quadrigemina, and as reflexes to other muscles of the body, by way of the other connexions. The paths which, from the direction of their neurone connexions, must in all probability be regarded as centrifugal conductors may be supposed to serve, on the one hand, as the vehicle of direct reactions of central excitations to the nervous structures at the periphery, and, on the other, as lines of transmission, by help of which excitations in the one retina mediate centrally coexcitations of the other.[26]

We have now traced, as accurately as may be, the course of the fibre systems that run to the cerebral cortex, whether directly from the crura, or indirectly from the cerebellum and the brain ganglia. We have made use, in this enquiry, both of the results of anatomical investigation and of the degenerations set up by severance of the fibres from their centres of function. But we have not been able to say anything at all definite of the final distribution of the central fibre systems in the cortex itself. As a matter of fact, there are still certain mazes of interlacing fibres to which anatomists have not yet found the clue; and our two methods fail us, when we seek to determine by their aid the precise relations in which the various regions of the cerebral cortex stand to the deeper lying nerve centres and to the peripheral parts of the body. We therefore ask assistance at this point from two other sources, physiological experiment and pathological observation. The former supplies us with a certain correlation, in the animal brain, between definite cortical areas and the various motor and sensory functions of the peripheral organs. The latter attempts the same problem for the human brain, by a comparison of the functional derangements recorded during life with the results of post-mortem examination. The conclusions drawn from experiments on animals may be transferred to man only, of course, in so far as they answer the general question [p. 191] of the representation of the bodily organs in the cerebral cortex. When we attempt to map out, on the human brain, the terminal areas of the various paths of conduction, we have to rely solely upon pathological observations. These possess the further advantage that they allow us to make more certain tests of the behaviour of sensation than do the experiments, upon animals. On the other hand, they have the disadvantage that circumscribed lesions of the cortex and pallium are of comparatively rare occurrence, so that the collection of data proceeds but slowly.

Experiments on animals fall into two main classes: stimulation experiments and abrogation experiments. Under the latter heading, we include all experiments which are intended to abrogate, temporarily or permanently, the function of some cortical area. In stimulation experiments, the symptoms to be observed are phenomena of movement, twitches or contractures in the muscles; abrogation experiments bring about abrogation or disturbance of movements or sensations. Both forms of experiment are of value for the definition of the terminal areas of the motor paths; for the sensory areas, we must have recourse in most cases to abrogation experiments. There are, however, many regions of the cerebral cortex which form the terminal areas of intercentral paths from the cerebellum and brain ganglia, paths which are connected only in a very complicated and roundabout way with the lines of sensory or motor conduction, or with both. We shall, therefore, expect a priori that not every experimental or pathological change, induced over a limited area, will be followed by noticeable symptoms; and that, even where such symptoms appear, they will not, as a rule, consist in simple phenomena of irritability and disability such as arise from the excitation or transsection of a peripheral nerve. This expectation is amply confirmed by experience. At many points, stimulation may be applied without producing any symptoms whatsoever. Where it does produce a result, the muscular excitations often have the character of co-ordinated movements. The symptoms of abrogation, on the other hand, are for the most part simple disturbances of movement or impairments of sense perception; it occurs but seldom, and in general only where the lesion is of considerable extent, that there is complete abrogation of function, sensory or motor. It is well, therefore, in speaking of experiments upon the cerebral cortex, to use expressions, that in some way indicate this ambiguity of result. We shall accordingly distinguish between centromotor cortical areas, whose stimulation produces movements of certain muscles or muscle groups, and whose extirpation is followed by a derangement of these movements, and centrosensory areas, whose removal brings in its train symptoms of loss or defect upon the sensory side.[27] These terms must [p. 192] not, however, be interpreted at the present stage of the enquiry as implying any hypothesis whether of the significance of the phenomena of stimulation and abrogation or of the function of the cortical areas to which they are applied. The only question to be discussed here is that of the termination of the paths of conduction in the cerebral cortex; and all that we require to know, in order to answer it, is the functional relation obtaining between the various regions of the cortex and the peripheral organs. How these functional relations are to be conceived, and in what manner the different cortical areas co-operate with one another and with the lower central parts, -- these are questions that we do not yet need to consider. There is, however, one point, so important for the right understanding of the conditions of conduction that it should, perhaps, be expressly mentioned in this place; a point that follows directly from the extreme complexity of interrelation which we have found to prevail in the central parts. It is this: that, for anything we know, there may exist several centromotor areas for one and the same movement, and several centrosensory areas for one and the same sense organ; and that there may quite well be parts of the cortex which unite in themselves centromotor and centrosensory functions. Suppose, then, that we are able to demonstrate certain results of stimulation and abrogation. They will simply indicate that the particular area of the cortex stands in some sort of relation with the conduction paths of the corresponding muscular or sensory region. The nature of the relation can be conjectured only after a comprehensive survey has been made of the whole body of central functions. All questions of this kind must, therefore, be postponed until the following Chapter.

The extreme complication of the course of the conduction paths, and the unusually complex conditions that govern the central functions, in face of which the formation of a critical judgment becomes a matter of serious difficulty, make us realise all the more keenly the comparative crudeness and inadequacy of all, even the most careful, experimental methods. In stimulation experiments, it is never possible to confine the stimulus effect within such narrow limits as is desirable, if we are to establish the relations of conduction obtaining between distinct cortical areas. Moreover, the central substance, as we have seen, has its own peculiar laws of excitability, which make negative results practically worthless as data from which to draw conclusions. Physiologists are therefore inclining more and more to attribute the higher value to abrogation experiments. But here, again [p. 193] there are difficulties, as regards both the performance of the experiments and the interpretation of their results. The shock given by the operation to the whole central organ is usually so violent, that the immediate symptoms cannot be referred to any definite cause; they may be due to functional disturbance in parts of the brain widely remote from the point of injury. Hence almost all observers have gradually been led to agree that the animals must be kept alive for a considerable period of time, and that only the later, and more especially the chronic symptoms may be made the basis of inference. Even so, however, various sources of error are still possible. Thus, as GOLTZ pointed out, inhibitory influences may continue to be exerted, either upon the entire central organ or upon distant regions, particularly if but a short interval has elapsed after the operation. Or, if a longer time has passed, the injured part may have been functionally replaced by other cortical areas: numerous pathological observations on man have put the efficacy of such vicarious function beyond the reach of doubt. Or, finally, as LUCIANI remarked, the cortical lesion may, on the contrary, set up a secondary degeneration of deeper lying brain centres, so that the abrogation of functions may be extended far beyond its original scope. In view of these difficulties, which mean that the experimental result may be obscured by sources of error of the most various kinds and of opposed directions, it is obvious that conclusions in which we are to place any measure of confidence, must be drawn without exception from a large number of accordant observations, made with due regard to all the factors that might affect the issue. And when these precautions have been taken, it is still inevitable that the conclusions, in many cases, attain to nothing higher than a certain degree of probability. In particular, they will as a general rule fail to carry conviction, until they are confirmed by pathological observations upon the human subject.

Centromotor areas in the cerebral cortex may easily be demonstrated, as HITZIG and FRITSCH were the first to show, by experiments with electrical or mechanical stimuli. The simplicity of the structural plan of the carnivore brain (Fig. 61, p. 138) makes it comparatively easy to rediscover the irritable points, when they have once been found. In Fig. 80 there are marked upon the brain of the dog the principal points about which the statements of the different observers are in general agreement.[28] Besides these superficial areas, there appear to be other cortical regions in the same neighbourhood, lying concealed in the depth of the crucial fissure, which [p. 194] are mechanically excitable: their exact localisation is, however, impossible, owing to their inacessible [sic] position.[29] The motor areas are all situated over the anterior portion of the brain, between the olfactory gyre and the Sylvian fissure. With stimuli of moderate intensity, the effect of stimulation is produced on the opposite side; bilateral symptoms are observed only in the case of movements in which there is a regular functional connexion of the two halves of the body, e. g. in ocular movements, movements of chewing, etc. With stronger stimuli, the effect is confined as a rule to the muscles of the same side of the body. The stimulable areas are seldom more than a few millimetres in extent, and excitation of points lying between them is, if the stimuli are weak, unaccompanied by any visible effect. If the stimulus is made more intensive, or is frequently repeated, contractions may, it is true, be set up from these originally indifferent points; but it is possible that such results are due to diffusion of currents (in electrical stimulation) or to an enhancement of excitability brought about by the preceding stimulation. There can, indeed, be no doubt that repetition of stimulus is able to induce this enhancement for it is often found that, under such conditions, the excitation spreads to other motor areas, so that the animal is finally thrown into general spasms, the phenomena of what is called cortical epilepsy.[30] For the rest, the contractions set up by cortical stimulation are always distinguished from those released by electrical stimulation of the coronal fibres by a much longer duration of their latent period, the expression of that retardation of the stimulation processes which is of universal occurrence in the central elements.[31]

The phenomena of abrogation, observed after extirpation of definite portions of the cerebral cortex of the dog, differ in two respects from the [p. 195] results of the experiments with stimulation. In the first place, they show that the removal of a stimulable area is usually followed by disturbances of movement in other groups of muscles, which were not excited by stimulation of the same area. Thus, extirpation of the area d in Fig. 80 is likely to produce paralytic symptoms in the fore leg as well as paralysis of the hind leg, and, conversely, extirpation of the area c is attended by a partial paralysis of the hind leg; again, destruction of the centres of neck and trunk aa' involves both the extremities; and so on. At the same time, the paralysis of the stimulable areas is always more complete than that of the areas sympathetically affected. In the second place, the extirpation of parts of the cortex that are irresponsive to stimulation may also give rise to phenomena of paralysis; and this statement holds not only of points of the cortex lying between the stimulable areas, within the zone of excitability, but also of more remote regions. It can thus be demonstrated that the entire anterior portion of the parietal lobe, and even the superior portion of the temporal region as well, are in the dog centromotor in function. Only the occipital and the larger, inferior portion of the temporal region can be removed without producing symptoms of abrogation on the motor side. Fig. 81 gives a graphic representation of these facts. The sphere of centromotor abrogation is dotted over; the size and number of the points in any given area indicate the intensity of the phenomena of abrogation appearing (always on the opposite side of the body) after extirpation of that particular zone.[32] The character of these disturbances, and more especially the regularity with which definite muscle groups are affected by the extirpation of definite parts of the cortex, render it improbable that the results obtained from non-stimulable areas are the outcome of transitory inhibitions, propagated as simple sequelæ of the operation from the point of injury to other, uninjured parts. We may more reasonably explain the differences between the phenomena of stimulation and those of abrogation [p. 196] by supposing that the excitable zones stand in closer relation to the peripheral conduction paths than do the others, whose centromotor influence can be demonstrated only by way of the inhibition of function which follows upon their removal. For the rest, it is a significant fact for the theory of these phenomena of centromotor abrogation that they do not consist by any means in complete muscular paralyses. In general, there is inhibition of voluntary movement only: the muscles involved will still contract reflexwise upon stimulation of the appropriate points upon the skin, and may be thrown into sympathetic activity by the movement of other muscle groups. Further, all symptoms of abrogation, save where very considerable portions of the cortical investment of both hemispheres have been removed, are impermanent and transitory; the animals will, as a rule, behave, after the lapse of days or months, in a perfectly normal way, and the restoration occurs the more quickly, the smaller the extent of the cortical area destroyed.[33]

The demonstration of the centrosensory areas, if it is to be accurate and reliable, must, as we said above, be undertaken by help of the phenomena of abrogation. This limitation of method, and more especially the uncertainty which attaches to sensory symptoms, place serious obstacles in the path of investigation. There are, however, two points in which the disturbances of sensation set up by extirpations of the cortex in the dog appear to resemble the motor paralyses which we have already passed under review. First, the cortical regions correlated with the various sense departments are, evidently, not well-marked and circumscribed; they always cover large areas of the brain surface, and even seem to overlap. Secondly: the disturbances, here as before, do not consist in any permanent abrogation of function. If the injury is restricted to a comparatively small area, they may be entirely compensated. If it affects a larger portion of the cortex, there will, it is true, be permanent sensory derangements, but they will express themselves rather in an incorrect apprehension of sense impressions than in absolute insensitivity to stimulus. Thus, dogs whose visual centre has been entirely removed will still avoid obstructions, and others, whose auditory centre has been extirpated, will react to sudden sound impressions, although they can no longer recognise familiar objects or the words of their master. They take a piece of white paper, laid in their path, for an obstacle which they must go round; or confuse bits of cork with pieces of meat, if the two have been mixed together.[34] All these phenomena indicate that the functions of perception have in such [p. 197] cases been abrogated or disturbed, but that the removal of the centrosensory areas is by no means and in no sense the equivalent of destruction of the peripheral sense organs. There is, further, one respect in which the terminations of the sensory conduction paths differ from those of the motor: while the derangements of movement point to a total decussation of the motor nerves, the disturbances of sensation, or at least of the special senses, are bilateral, and accordingly suggest that the fibres of the sensory paths undergo only a partial decussation in their course from periphery to centre. Figg. 82, 83 and 84 show roughly the extent of the visual, auditory and olfactory areas in the cortex of the dog, as determined by the method of abrogation. The frequency of the dots indicates, again, the relative intensity of the disturbances which follow upon extirpation of the area in question; the black dots correspond to crossed, the hatche