Chapter VII: Part 7

It will be noted that what has been achieved by these various means of study in regard to the region of the cortex to which olfactory functions are attributed amounts at present to little more than the bare ascertainment of the existence there of nervous mechanisms connected with olfaction, and to the delimiting roughly of their extent and of their ability to influence certain movements, and in man sensations, habitually associated with exercise of the olfactory organ. As to what part the cortical mechanism has in the elaboration or association of mental processes to which olfaction contributes, no evidence worth the name seems as yet forthcoming. In this respect our knowledge, or rather our want of knowledge, of the functions of the olfactory region of the cortex, is fairly typical of that to which we have to confess in regard to the other regions of the cortex, even the best known.

_Visual Region of the Cortex._--There is a region of the cortex especially connected with vision. The _optic nerve_ and _tract_ constitute the second link in the chain of neurones joining the retina to the brain. They may therefore be regarded as the equivalent of an intraspinal tract connecting the deep ends of the afferent neurones from the skin with higher nervous centres. In the bony fishes the optic tract reaches the grey matter of the optic lobe, a part of the mid-brain, to which the so-called anterior colliculus is equivalent in the mammalian brain. In the optic lobe the axones of the neurones of the optic tract meet neurones whose axones pass in turn to the motor neurones of the muscles moving the eyeballs, and also to other motor neurones. But in these fish the optic tract has no obvious connexion with the fore-brain or with any cerebral pallium. Ascending, however, to the reptilian brain is found an additional arrangement: a small portion of the optic tract passes to grey matter in front of the optic lobe. This grey matter is the lateral geniculate body. From this geniculate body a number of neurones extend to the pallial portion of the cerebrum, for in the reptilian brain the pallium is present. The portion of pallium connected with the lateral geniculate body lies above and behind the olfactory or archipallium. It is a part of what was mentioned above as neopallium.

In the mammalian brain the portion of the optic tract which goes to the optic lobe (_ant. colliculus_ of the mammal) is dwarfed by great development of the part which goes to the geniculate body and an adjoining grey mass, the pulvinar (part of the optic thalamus). From these latter pass large bands of fibres to the occipital region of the neopallium. In mammals this visual region of the cortex is distinguished in its microscopic features from the cortex elsewhere by a layer of myelinate nerve-fibres, many of which are the axones of neurones of the geniculate body and pulvinar. Thus, whereas in the bony fishes all the third links of the conductive chain from the retina lead exclusively to the final neurones of motor centres for muscles, in the mammal the majority of the third links conduct to grey matter of the cortex cerebri.

The application of electric stimuli to the surface of the cortex does not for the greater part of the extent of the cortex evoke in higher mammalian brains any obvious effect; no muscular act is provoked. But from certain limited regions of the cortex such stimulation does evoke muscular acts, and one of these regions is that to which the neurones forming the third link of the conductive chain from the retina pass. The muscular acts thus provoked from that region are movements of the eyeballs and of the neck turning the head. In the monkey the movement is the turning of both eyeballs and the head away from the side stimulated. In short, the gaze is directed as to an object on the opposite side. The newer conductive chain traceable through the cortex does therefore, after all, like the older one through the optic lobe, lead ultimately to the motor neurones of the eye muscles and the neck, only it takes a longer course thither and is undoubtedly much more complex. What gain is effected by this new and as it were alternative and longer route, which takes a path up to the cerebral cortex and down again, we can only conjecture, but of one point we may rest well assured, namely, that a much richer inter-connexion with other arcs of the nervous system is obtained by the path that passes via the cortex. The functional difference between the old conductive circuit and the new can at present hardly indeed be stated even in outline. A natural inference might be that the phylogenetically older and less complex path is concerned with functions purely reflex-motor, not possessing sensation as an attribute. But fish, which possess only the older path, can be trained to seize bait of one colour and not of another colour, even against what appeared to be an original colour-preference in them. Such discrimination individually acquired seems to involve memory, though it may be rudimentary in kind. Where motor reaction to visual stimuli appears to involve memory--and without memory the training could hardly be effective--some germ of consciousness can hardly be denied to the visual reactions, although the reactions occurred in complete absence of a cortical path and indeed of a visual cortex altogether.

Removal of the visual pallium in the tortoise produces little or no obvious defect in vision; but in the bird such a lesion greatly impairs the vision of the eye of the side opposite to the lesion. The impairment does not, however, amount to absolute blindness. Schrader's hawk, after removal of the pallium, reacted to movements of the mice with which it was caged. But the reactions were impaired: they lacked the sustained purpose of the normal reactions. The bird saw the mice; that was certain, for their movements across its field of vision made it turn its gaze towards them. But on their ceasing to move, the reaction on the part of the bird lapsed. Neither did their continuing to move excite the attack upon them which would have been the natural reaction on the part of the bird of prey towards its food. The bird apparently did not recognize them as prey, but saw them merely as moving objects. It saw them perhaps as things to which mental association gave no significance. Similarly, a dog after ablation of the occipital lobes of the cortex is able to see, for it avoids obstacles in its path; but if food is offered to it or the whip held up to it, it does not turn towards the food or away from the whip. It sees these things as if it saw them for the first time, but without curiosity, and as if it had no experience of their meaning. It gives no hint that it any longer understands the meaning of even familiar objects so long as these are presented to it through the sense of vision. Destruction of the visual cortex of one hemisphere alone produces in the dog impairment of vision, not as in the bird practically exclusively in the opposite eye, but in one lateral half of each eye, and that half the half opposite the hemisphere injured. Thus when the cortex destroyed is of the right cerebral hemisphere, the resultant visual defect is in the left half of the field of vision of both eyes. And this is so in man also.

In man disturbances of sensation can be better studied because it is possible to obtain from him his description of his condition. The relation of the _cortex cerebri_ to human vision can be summarized briefly as follows. The visual cortex is distinguishable in higher mammals by a thin white stripe, the stripe of Gennari, seen in its grey matter when that is sectioned. This stripe results from a layer of nerve-fibres, many of which are axones from the neurones of the lateral geniculate body and the pulvinar, the grey masses directly connected with the optic nerve-fibres. In the dog, and in such monkeys as the Macaque, the region of cortex containing this stripe traceable to optic fibres forms practically the whole occipital lobe. But in the man-like apes and in man this kind of cortex is confined to one region of the occipital lobe, namely, that of the calcarine fissure and the _cuneus_ behind that. This region of cortex thus delimited in man is one of Flechsig's areas of earlier myelinization. It is also one of his areas possessing projection fibres; and this last fact agrees with the yielding by this area, when under electrical stimulation, of movements indicating that impulses have been discharged from it into the motor neurones of the muscles of the eyes and neck. Evidence from cases of disease show that destruction of the cortex of the upper lip of the calcarine fissure, say in the right half of the brain, causes in man impairment in the upper right-hand quadrant of both retinae: destruction of the lower lip of the fissure causes impairment in the lower right-hand quadrants. Destruction of the calcarine region of one hemisphere produces therefore "crossed hemianopia," that is, loss of the opposite half of the field of vision. But in this hemianopia the region of central vision is always spared. That is, the piece of visual field which corresponds with the yellow spot of the retina is not affected in either eye, unless the calcarine regions of both hemispheres are destroyed. This central point of vision is connected therefore not with one side of the brain only but with both.

The impairment of sight is more severe in men than in lower animals. Where the destruction of the visuo-sensory cortex in one calcarine region is complete, a candle-flame offered in the hemianopic field cannot even be perceived. It may hardly excite a reflex contraction of the pupil. In such cases the visual defect amounts to blindness. But this is a greater defect than is found in the dog even after entire removal of both occipital lobes. The dog still avoids obstacles as it walks. Its defect is rather, as said above, a complete loss of interest in the visual images of things. But a dog or monkey after loss of the visual cortex hesitates more and avoids obstacles less well in a familiar place than it does when entirely blind from loss of the peripheral organ of vision. In man extensive destruction of the visual cortex has as one of its symptoms loss of memory of localities, thus, of the paths of a garden, of the position of furniture, and of accustomed objects in the patient's own room. This loss of memory of position does not extend to spatial relations ordinarily appreciated by touch, such as parts of the patient's own person or clothing. There is nothing like this in the symptoms following blindness by loss of the eye itself. Those who lose their sight by disease of the retina retain good memorial pictures of positions and directions appreciated primarily by vision.

Cases of disease are on record in which loss of visual memory has occurred without hemianopia. Visual hallucinations referred to the hemianopic side have been observed. This suggests that the function of visual memory in regard to certain kinds of percepts must belong to localities of cortex different from those pertaining to other visual percepts. The area of cortex characterized by the stripe of Gennari occupies in man, as mentioned, the calcarine and cuneate region. It is surrounded by a cortical field which, though intimately connected with it by manifold conducting fibres, &c., is yet on various grounds distinct from it. This field of cortex surrounding the visuo-sensory of the calcarine-cuneate region is a far newer part of the neopallium than the region it surrounds. Both in the individual (Flechsig) and in the phylum (Bolton, Campbell, Mott) its development occurs far later than that of the visuo-sensory which it surrounds. Flechsig finds that it has no "projection" fibres, that is, that it receives none of the optic radiations from the lower visual centres and gives no centrifugal fibres in the reverse direction. This field encompassing the visuo-sensory region differs from the latter in its microscopic structure by absence of the lower layer of stellate cells and by the presence in it of a third or deep layer of pyramidal cells (Mott). Its fibres are on the average smaller than are those of the visuo-sensory (W.A. Campbell). This zonal field is small in the lower apes, and hardly discoverable in the dog. In the anthropoid apes it is much larger. In man it is relatively much larger still. The impairment of visual memory and visual understanding in regard to direction and locality is said to be observed in man only when the injury of the cortex includes not only the calcarine-cuneate region but a wide area of the occipital lobe. From this it is argued that the zonal field is concerned with memories and recognitions of a kind based on visual perceptions. It has therefore been termed the _visuo-psychic_ area. It is one of Flechsig's "association-areas" of the cortex.

Adjoining the antero-lateral border of the just-described _visuo-psychic area_ lies another region separate from it and yet related to it. This area is even later in its course of development than is the visuo-psychic. It is one of Flechsig's "terminal fields," and its fibres are among the last to ripen in the whole cortex. This terminal field is large in man. It runs forward in the parietal lobe above and in the temporal lobe below. Its wide extent explains, in the opinion of Mott, the displacement of the visuo-sensory field from the outer aspect of the hemisphere in the lower monkeys to the median aspect in man. To this terminal field all the more interest attaches because it includes the angular gyrus, which authorities hold to be concerned with the visual memory of words. Study of diseased conditions of speech has shown that the power to understand _written_ words may be lost or severely impaired although the words may be perfectly distinct to the sight and although the power to understand _heard_ words remains good. This condition is asserted by many physicians to be referable to destruction of part of the angular gyrus. Close beneath the cortex of the angular gyrus runs a large tract of long fibres which pass from the visual cortex (see above) to the auditory cortex (see below) in the superior temporal gyrus and to the lower part of the frontal lobe. This lower part of the frontal lobe is believed--and has long been believed--to be concerned intimately with the production of the movements of speech. A difficulty besetting the investigation of the function of the angular gyrus is the fact that lesion of the cortex there is likely to implicate the underlying tract of fibres in its damage. It cannot be considered to have been as yet clearly ascertained whether the condition of want of recognition of seen words--"word-blindness"--is due to cortical injury apart from subcortical, to the angular gyrus itself apart from the underlying tract. Word-blindness seems, in the right-handed, to resemble the aphasia believed to be connected with the lower part of the frontal lobe, in that it ensues upon lesions of the left hemisphere, not of the right. In left-handed persons, on the contrary, it seems to attach to the right hemisphere.

_Auditory Region of the Cortex._--Besides the two great organs of distance-receptors, namely, the nose and eye, whose cerebral apparatus for sensation has just been mentioned, those of a third great distance-receptor have to be considered. The agents of stimulation of the two former are respectively chemical (olfactory) and radiant (visual); the mode of stimulation of the third is mechanical, and the sensations obtained by it are termed auditory. Their cerebral localization is very imperfectly ascertained. Electric stimuli applied to a part of the uppermost temporal gyrus excites movements of the ears and eyes in the dog. Destruction of the same region when executed on both hemispheres is argued by several observers to impair the sense of hearing. To this region of cortex fibres have been traced from the lower centres connected with the nerve-fibres coming from the cochlea of the ear. From each cochlear nerve a path has been traced which passes to the _insulae_ and the above-mentioned _temporal_ region of cortex of both the cerebral hemispheres. The insula is a deeper-seated area of cortex adjoining the uppermost temporal convolution. To it Flechsig's chronological studies also impute a connexion with the nerves of the ear. Early myelinization of fibres, presence of ascending and descending "projection" tracts to and from lower centres outside the cortex, calibre of fibres, microscopic characters of its cortical cells, all those kinds of indirect items of evidence that obtain for the visual cortex likewise mark out this insular-temporal area as connected fairly directly with a special sense-organ, as in fact a sensory field of the cortex; and the suspicion is that it is auditory. Clinical observation supports the view in a striking way, but one requiring, in the opinion of some, further confirmation. It is widely believed that destruction of the upper and middle part of the uppermost temporal convolution produces "word-deafness," that is, an inability to recognize familiar words when heard, although the words are recognized when seen.

More precise information regarding this auditory region of the cortex has recently been obtained by the experiments of Kalischer. These show that after removal of this region from both sides of the brain in the dog the animal shows great defect in answering to the call of its master. Whereas prior to the operation the animal will prick its ears and attend at once to the lightest call, it requires after the removal of the auditory regions great loudness and insistence of calling to make it attend and react as it did. This is the more striking in view of other experimental results obtained. Kalischer trained a number of his dogs not to take meat offered them except at the sound of a particular note given by an organ pipe or a harmonium. The dogs rapidly learned not to take the food on the sounding of notes of other pitch than the one taught them as the permissive signal. This reaction on the part of the animal was not impaired by the removal of the so-called auditory regions of the cortex. Kalischer suggests that this reaction taught by training is not destroyed by the operation which so greatly impairs the common reaction to the master's call, because the former is a simpler process more allied to reflex action. In it the attention of the dog is already fastened upon the object, namely the food, and the stimulus given by the note excites a reaction which simply allows the act of seizing the food to take place, or on the other hand stops it. In the case of answering the call of the master the stimulus has to excite attention, to produce perception of the locality whence it comes, and to invoke a complicated series of movements of response. He finds that destruction of the posterior colliculi of the mid-brain, which have long been known to be in some way connected with hearing, likewise destroys the response to the call of the master, but did not destroy the trick taught to his dogs of taking meat offered at the sound of a note of one particular pitch but not at notes of other pitch given by the same instrument.

_Other Senses and Localization in the Cortex Cerebri._--Turning now to the connexion between the function of the cortex and the senses other than those of the great distance-receptors just dealt with, even less is known. Disturbance and impairment of skin sensations are observable both in experiments on the cerebrum of animals and in cases of cerebral disease in man. But the localization in the cortex of regions specially or mainly concerned with cutaneous sensation has not been made sufficiently clear to warrant statement here. Still less is there satisfactory knowledge regarding the existence of cortical areas concerned with sensations originated in the alimentary canal. The least equivocal of such evidence regards the sense of taste. There is some slight evidence of a connexion between this sense and a region of the hippocampal gyrus near to but behind that related to smell.

As to the sensations excited by the numerous receptors which lie not in any of the surface membranes of the body but embedded in the masses of the organs and between them, the _proprioceptors_, buried in muscles, tendons and joints, there is little doubt that these sensations may be disturbed or impaired by injury of the _cortex cerebri_. They may probably also be excited by cortical stimulation. But evidence of localization of their seat in, and their details of connexion with, the cortex, is at present uncertain. Many authorities consider it probable that sensations of touch and the sensations initiated by the proprioceptors of muscles and joints (the organs of the so-called muscular sense) are specially related to the post-central gyrus and perhaps to the pre-central gyrus also. The clearest items on this point are perhaps the following.

Besides the regions instanced above, in the limbic (olfactory), occipital (visual), and temporal (auditory) lobes, as exhibiting precocity of development, there is a region showing similar precocity in the fronto-parietal portion of the hemisphere. This is the region which in the Primates includes the large _central fissure_ (sometimes called the fissure of Rolando). To it fibres are traced which seem to continue a path of conduction that began with afferent tracts belonging to the spinal cord, and tracts which there is reason to think conduct impulses from the receptor-organs of skin and muscles. The part of the cortex immediately behind the _central fissure_ seems to be the main cortical goal for these upward-conducting paths. That _post-central_ strip of cortex would in this view bear to these paths a relation similar to that which the occipital and temporal regions bear to afferent tracts from the retina and the cochlea. There are observations which associate impaired tactual sense and impaired perception of posture and movement of a limb with injury of the _central region_ of the cortex. But there are a number also which show that the motor defect which is a well-ascertained result of injury of the _pre-central_ gyrus is sometimes unaccompanied by any obvious defect either of touch or of muscular sense. It seems then that the motor centres of this region are closely connected with the centres for cutaneous and muscular sense, yet are not so closely interwoven with them that mechanical damage inflicted on the one of necessity heavily damages the other as well. There is evidence that the sensory cortex in this region lies posterior to that which has been conveniently termed the "motor." These latter in the monkey and the man-like apes and man lie in front of the central fissure: the sensory lie probably behind it. A.W. Campbell has found changes in the cortex of the post-central convolution ensuing in the essentially sensory disease, _tabes dorsalis_, a disease in which degeneration of sensory nerve-fibres of the muscular sense and of the skin senses is prominent. He considers that in man and the man-like apes the part of the post-central gyrus which lies next to and enters into the _central fissure_ is concerned with simpler sensual recognitions, while the adjoining part of that convolution farther back is a "psychic region" concerned with more complex psychosis connected with the senses of skin and muscle. His subdivision of the post-central gyrus is based on histological differences which he discovers between its anterior and its posterior parts and on the above-described analogous differentiation of a "sensory" from a "psychic" part in the visual region of cortex.

It will be noted that although certain regions of the cortex are found connected closely with certain of the main sense organs, there are important receptive organs which do not appear to have any special region of cortex assigned to their sensual products. Thus, there is the "vestibular labyrinth" of the ear. This great receptive organ, so closely connected in function with the movements and adjustment of the postures of the head and eyes, and indeed of the whole body, is prominent in the co-ordination necessary for the equilibrium of the body, an essential part of the fundamental acts of progression, standing, &c. Yet neither structural nor functional connexion with any special region of the cortex has been traced as yet for the labyrinthine receptors. Perceptions of the position of the head and of the body are of course part of our habitual and everyday experience. It may perhaps be that these perceptions are almost entirely obtained through sense organs which are not labyrinthine, but visual, muscular, tactual, and so on. The labyrinth may, though it controls and adjusts the muscular activities which maintain the balance of the body, operate reflexly without in its operation exciting of itself sensations. The results of the unconscious reflexes it initiated and guided would be perceptible through other organs of sense. But against this purely unconscious functioning of the labyrinth and its nervous apparatus stands the fact that galvanic stimulation of the labyrinth is accompanied by well-known distinctive sensations--including giddiness, &c. Moreover, the prominent factor in sea-sickness, a disorder richly suffused with sensations, is probably the labyrinth. Yet there is marked absence of evidence of any special and direct connexion between the _cortex cerebri_ and the labyrinth organs.

Also there is curiously little evidence of connexion of the cortex with the nervous paths of conduction concerned with pain. As far as the present writer can find from reference to books and from the clinical experience of others, "pain" is unknown as an _aura_ in cortical epilepsy, or at most is of equivocal occurrence.

The preceding brief exposition of some of the main features of the localization of function in the _cortex cerebri_, gradually deciphered by patient inquiry, shows that the scheme of partition of function so far perceptible does not follow the quaint lines of analysis of the phrenologists with their supposed mental entities, so-called "faculties." On the contrary it is based, as some of those who early favoured a differential arrangement of function in the cerebrum had surmised, on the _separateness of the incoming channels from peripheral organs of sense_. These organs fall into groups separate one from another not only by reason of their spatial differentiation at the surface and in the thickness of the body, but also because each group generates sensations which introspection tells us are of a species unbridgeably separate from those generated by the other groups. Between sensations of hearing and sensations of sight there is a dissimilarity across which no intermediate series of sensual phenomena extend. The two species of sensations are wholly disparate. Similarly there is a total and impassable gap between sensations of touch and sensations of sight and sound. In other words the sensations fall into groups which are wholly disparate and are hence termed species. But within each species there exist multifold varieties of the specific sensation, e.g. sensations of red, of yellow, &c. We should expect, therefore, that the conducting paths from the receptive organs which in their function as sense-organs yield wholly disparate sensations would in so far as subserving sensation diverge and pass to separate neural mechanisms. That these sense-organs should in fact be found to possess in the cortex of the cerebrum separate fields for their sensual nervous apparatus is, therefore, in harmony with what would be the _a priori_ supposition.

But, as emphasized at the beginning of this article, the receptive organs belonging to the surfaces and the depths of the body and forming the starting-points for the whole system of the afferent nerves, have two functions more or less separate. One of these functions is to excite sensations and the other is to excite movements, by reflex action, especially in glands and muscles. In this latter function, namely the reflexifacient, all that the receptive organs effect is effected by means of the efferent nerves. They all have to use the efferent, especially the motor, nerves of the body. So rich is the connexion of the receptive organs with the efferent nerves that it is not improbable that, through the central nervous organ, each receptive organ is connected with every motor nerve of the whole nervous system,--the facts of strychnine poisoning show that if this is not literally true it is at least approximately so. Hence one of the goals to which each afferent fibre from a receptive organ leads is a number of motor nerves. Their conducting paths must, therefore, converge in passing to the starting-points of the motor nerves; because these latter are instruments common to the use of a number of different receptive organs in so far as they excite reflex actions. On the other hand those of their conducting paths which are concerned in the genesis of sensation, instead of converging, diverge, at least as far as the _cortex cerebri_, or if not divergent, remain separate. These considerations would make it appear likely that the conducting path from each receptive organ divides in the central nervous system into two main lines, one of which goes off to its own particular region of the _cortex cerebri_ whither run conductors only of similar sensual species to itself, while the other main line passes with many others to a great motor station where, as at a telephone exchange, coordinate use of the outgoing lines is assured to them all. Now there is in fact a portion of the cortex in mammals the functions of which are so pre-eminently motor, as judged by our present methods, that it is commonly designated the _motor cortex_ (see fig. 24). This region of the cortex occupies in the Primates, including Man, the pre-central gyrus. Among the items of evidence which reveal its motor capabilities are the following.

_The Precentral or Motor Region of the Cortex._--The application to it of electric currents excites movements in the skeletal muscles. The movements occur in the half of the body of the side crossed from that of the hemisphere excited. The "motor representation," as it is termed, is in the cortex better described as a representation of definite actions than of particular muscles. The actions "represented" in the top part of the gyrus, namely next the great longitudinal fissure, move the leg; those in the lowest part of the gyrus belong to the tongue and mouth. The topical distribution along the length of the gyrus may be described in a general way as following a sequence resembling that of the motor representation in the spinal cord, the top of the gyrus being taken as corresponding with the caudal end of the spinal cord. The sequence as the gyrus is followed downwards runs: perineum, foot, knee, hip, abdomen, chest, shoulder, elbow, wrist, hand, eyelids and ear, nose, mouth and tongue. The nature of the movement is very fairly constant for separate points of this motor cortex as observed both in the same and in similar experiments. Thus flexion of the arm will be excitable from one set of points, and extension of the arm from another set of points; opening of the jaw from one set and closure from another, and so on. These various movements if excited strongly tend to have characters like those of the movements seen in an epileptic convulsion. Strong stimulation excites in fact a convulsion like that of epilepsy, beginning with the movement usual for the point stimulated and spreading so as to assume the proportions of a convulsion affecting the entire skeletal musculature of one half or even of the whole body. The resemblance to an epileptic seizure is the closer because the movement before it subsides becomes clonic (rhythmic) as in epilepsy. The determination of the exact spots of cortex in which are represented the various movements of the body has served a useful practical purpose in indicating the particular places in the cortex which are the seat of disease. These the physician can localize more exactly by reason of this knowledge. Hence the surgeon, if the nature of the disease is such as can be dealt with by surgical means, can without unnecessarily damaging the skull and brain, proceed directly to the point which is the seat of the mischief.

The motor representation of certain parts of the body is much more liberal than is that of others. There is little correspondence between the mere mass of musculature involved and the area of the cortex devoted to its representation. Variety of movement rather than force or energy of movement seems to demand extent of cortex. The cortical area for the thumb is larger than those for the whole abdomen and chest combined. The cortical area for the tongue is larger than that for the neck. Different movements of one and the same part are very unequally represented in the cortex. Thus, flexion of the leg is more extensively represented than is extension, opening of the jaw has a much larger cortical area than has closure of the jaws. It is interesting that certain agents, for instance strychnine, and the poison of the bacilli which cause the disease known as tetanus or lock-jaw, upset this normal topography, and replace in the cortex flexion of the limb by extension of the limb, and opening of the jaw by closure of the jaw. There is, however, no evidence that they do this by changing in any way the cortical mechanisms themselves. It is more likely that their action is confined to the lower centres, bulbar and spinal, upon which the discharge excited from the cortex plays. The change thus induced in the movement excited by the cortex does, however, show that the point of cortex which causes for instance opening of the mouth is connected with the motor nerves to the closing muscles as well as with those of the opening muscles. This is an item of evidence that the "centres" of the cortex are connected with the motor nerves of antagonistic muscles in such a way that when the "centre" excites one set of the muscles to contract, it simultaneously under normal circumstances causes inhibition of the motor neurones of the opposed set of muscles (reciprocal innervation). In the great majority of movements excited from the motor cortex of a single hemisphere of the cerebrum, the movement evoked is confined to one side of the body, namely to that opposite to the hemisphere stimulated. There are, however, important exceptions to this. Thus, adduction of both vocal cords is excited from the cortex of either hemisphere. The movement of closure of the eyelids is usually bilateral, unless the stimulation be very weak; then the movement is of the eyelids of the opposite side only. The same holds true for the movements of the jaw. It, therefore, seems clear that with many movements which are usually bilaterally performed in ordinary life, such as opening of the jaw, blinking, &c., the symmetrical areas of the motor regions of both hemispheres are simultaneously in action.

In regard to all these movements elicitable by artificial stimuli from the motor cortex it is obvious that were there clearer evidence that the pallial region from which they are elicitable is fairly directly connected with corticopetal paths subserving cutaneous sensation or "muscular sense," the movements might be regarded as falling into the category of higher reflexes connected with the organs of touch, muscular sense, &c., just as the movements of the eyeball excitable from the visual cortex may be regarded as higher reflexes connected with vision. The evidence of the connexion of the reactions of the motor cortex with cutaneous and muscular senses appears, however, scarcely sufficient to countenance at present this otherwise plausible view, which has on general grounds much to commend it.

It is remarkable that movements of the eyeball itself, i.e. apart from movement of the lids, are not in the category of movements elicitable from the precentral gyrus, the "motor" cortex. They are found represented in a region farther forward, namely in front of the precentral gyrus altogether, and occupying a scattered set of points in the direction frontal from the areas for movements of arm and face. This frontal area yields on excitation conjugate movements of both eyeballs extremely like if not exactly similar to those yielded by excitation of the occipital (visual) region of the cortex. It is supposed by some that this frontal area yielding eye-movements has its function in this respect based upon afferent conductors from other parts of the eyeball than the retina, for instance upon kinaesthetic (Bastian) impressions or upon sensual impressions derived from the cornea and the coats of the eyeball including the ciliary and iris muscles. The ocular muscles are certainly a source of centripetal impulses, but their connexion with the cortex is not clear as to either their nature or their seat. The question seems for the present to allow no clearer answer. It is certain, however, that the frontal area of eye movements has corticofugal paths descending from it to the lower motor centres of the eyeballs quite independent of those descending from the occipital (visual) area of eye-movements. Further, it seems clear that in many animals there is another cortical region, a third region, the region which we saw above might be considered auditory, where movements of the eyeball similar to those elicitable in the occipital and frontal cortex can be provoked. A. Tschermak is inclined to give the eyeball movements of the frontal region the significance of reflex movements which carry the visual field in various directions in answer to demands made by sensory data derived from touch, &c., as for instance from the hand. The movements of the eyeballs elicitable from the occipital region of the cortex he regards as probably concerned with directing the gaze toward something seen, for instance, in the peripheral field of vision. The occipital movement would, therefore, be excited through the retina, and would result in bringing the yellow spot region of the retinae of both eyes to bear upon the object. This view has much to justify it. The movements of the eyeballs excited from the cortex of the auditory region would in a similar way be explicable as bringing the gaze to bear upon a direction in which a sound had been located, auditory initiation replacing the visual and tactual of the occipital and the frontal regions respectively.

Turning from these still speculative matters to others less suggestive but of actual ascertainment, we find that the motor nature of the precentral cortex as ascertained by electric stimuli is further certified by the occurrence of disturbance and impairment of motor power and adjustment following destruction of that region of the cortex. The movements which such a part as a limb executes are of course manifold in purpose. The hind limb of a dog is used for standing, for stepping, for scratching, for squatting, and, where a dog, for instance, has been trained to stand or walk on its hind legs alone, for skilled acts requiring a special training for their acquisition. It is found that when the motor area of the brain has been destroyed, the limb is at first paralysed for all these movements, but after a time the limb recovers the ability to execute some of them, though not all. The scratching movement suffers little, and rapid improvement after cerebral injury soon effaces the impairment, at first somewhat pronounced, in the use of the limb for walking, running, &c., and ordinary movements of progression. Even when both hemispheres have been destroyed the dog can still stand and walk and run. Destruction of the motor region of the cortex renders the fore limbs of the dog unable to execute such skilled movements as the steadying of a bone for gnawing or the trained act of offering the paw in answer to the command of the master. Skilled acts of the limb, apart from conjoined movements in which it, together with all the other limbs, takes part, assume of course a larger share of the office of the limb in the Primates than in the dog; and this is especially true for the fore limb. It is when the fore-foot becomes a hand that opportunity is given for its more skilled individual use and for its training in movements as a tool, or for the handling of tools and weapons. It is these movements which suffer most heavily and for the longest period after injury of the motor region of the cortex. Hence the disablement ensuing upon injury to the cortex would be expected to be most apparent in the Primates; and it is so, and most of all in Man. Further, in Man there ensues a condition called "contracture," which is not so apparent or frequent a result in other animals,--indeed, does not occur at all in other animals except the monkey. In contracture the muscles of the paretic limb are not flaccid, as they are usually in paralysis, but they are tense and the limb is more or less rigidly fixed by them in a certain position, usually one of flexion at elbow and wrist. This condition does not occur at first, but gradually supervenes in the course of a number of weeks. In Man the destruction of the motor area of the cortex cripples the limb even for the part it should play in the combined limb movements of walking, &c., and cripples it to an extent markedly contrasting with the slight disturbances seen in the lower mammals, e.g. the dog.

As regards the recovery of motor power after lesions of the motor cortex, two processes seem at work which are termed respectively _restitution_ and _compensation_. By the former is understood the recovery obtained when a part of a "centre" is destroyed, and the rest of the centre, although thrown out of function at first, recovers and supplements the deficiency later. An example of restitution would be the recovery from temporary hemianopia caused by a small injury in one occipital lobe. By compensation is understood the improvement of an impaired nervous function, traceable to other centres different from those destroyed supplying means to compass the reaction originally dependent on the centres subsequently destroyed. Instances of such compensation are the recovery of taxis for equilibrium subsequent to destruction of the labyrinth of the ear, where the recovery is traceable to assistance obtained through the eye. It will be noted that these instances of recovery by restitution and by compensation respectively are taken, from cases of injury inflicted on receptive rather than on motor centres. It is doubtful how far they really apply to the undoubted improvement that does within certain limits progress and succeed in partially effacing the paresis immediately consequent on lesions of the motor area. It has to be remembered that in all cases of traumatic injury to the nervous system, especially where the trauma implicates the central nervous organ, the first effects and impairment of function resulting are due to a mixed cause, namely on the one hand the mechanical rupture of conducting paths actually broken by solution of their continuity, and on the other hand the temporary interruption of conducting paths by "shock." Shock effects are not permanent: they pass off. They are supposed to be due to a change at the synapses connecting neurone with neurone in the grey matter. They amount in effect to a long-lasting and gradually subsiding inhibition.

For diseases of the brain see NEUROPATHOLOGY, INSANITY, SKULL
(_Surgery_), &c. (C. S. S.)

FOOTNOTE:

[1] The literature of the pineal region is enormous. Studnicka (in
_Oppels Vergleichende mikrosk. Anat._ Teile 4-5, 1904, 1905) gives
285 references. The present conception of the generalized arrangement
is: ([alpha]) A single glandular median organ from the fore-brain
called the paraphysis. ([beta]) A pouch of the ependymal roof of the
ventricle called the dorsal sac. ([gamma]) A right and left
epiphysis, one of which may be wholly or partially suppressed. These
may change their position to anterior and posterior in some animals.

BRAINERD, DAVID (1718-1747), American missionary among the Indians, was born at Haddam, Connecticut, on the 20th of April 1718. He was orphaned at fourteen, and studied for nearly three years (1739-1742) at Yale. He then prepared for the ministry, being licensed to preach in 1742, and early in 1743 decided to devote himself to missionary work among the Indians. Supported by the Scottish "Society for Promoting Christian Knowledge," he worked first at Kaunaumeek, an Indian settlement about 20 m. from Stockbridge, Massachusetts, and subsequently, until his death, among the Delaware Indians in Pennsylvania (near Easton) and New Jersey (near Cranbury). His heroic and self-denying labours, both for the spiritual and for the temporal welfare of the Indians, wore out a naturally feeble constitution, and on the 19th of October 1747 he died at the house of his friend, Jonathan Edwards, in Northampton, Massachusetts.

His _Journal_ was published in two parts in 1746 by the Scottish
Society for Promoting Christian Knowledge; and in 1749, at Boston,
Jonathan Edwards published _An Account of the Life of the Late Rev.
David Brainerd, chiefly taken from his own Diary and other Private
Writings_, which has become a missionary classic. A new edition, with
the _Journal_ and Brainerd's letters embodied, was published by Sereno
E. Dwight at New Haven in 1822; and in 1884 was published what is
substantially another edition, _The Memoirs of David Brainerd_, edited
by James M. Sherwood.

BRAINERD, a city and the county-seat of Crow Wing county, Minnesota, U.S.A., on the E. bank of the Mississippi river, about 127 m. N.W. of Minneapolis. Pop. (1890) 5703; (1900) 7524, of whom 2193 were foreign-born; (1905) 8133; (1910) 8526. It is served by the Minnesota & International and the Northern Pacific railways. The latter maintains here large car and repair shops, and a sanatorium for its employees. There are also the Sisters of St Joseph hospital, a county court house, a public library and a Y.M.C.A. building. A dam across the Mississippi provides water power (about 60,000 H.P.) which is utilized extensively for manufacturing purposes. Lumbering is an important industry, and there are saw mills and planing mills, and an extensive creosote plant for treating railway ties and timber. There are also flour mills, paper and pulp mills, cigar factories, a brewery, a large foundry and a grain elevator. In 1906 large quantities of iron ore were discovered in the vicinity, the new range, the Cuyuna, running through the city from north-east to south-west. Brainerd, named in honour of David Brainerd, was settled in 1870, and chartered as a city in 1883.

BRAINTREE, a market town in the Maldon parliamentary division of Essex, England; 45 m. N.E. of London by a branch line from Witham of the Great Eastern railway. Pop. of urban district, 5330. The parish church of St Michael is a fine edifice of Early English work with later additions. A corn exchange, mechanics' institute and public hall may also be mentioned. The bishops of London had formerly a palace in the town, but there are no remains of the building. The manufactures of silk and crape have superseded that of woollen cloth, which was introduced by the Flemings who fled to England to escape the persecution of the duke of Alva. Matting and brushes are also made. On the north lies the large village of BOCKING, with the Perpendicular parish church of St Mary, similar industries, and a population of 3347.

BRAINTREE, a township of Norfolk county, Massachusetts, U.S.A., on the Monatiquot river about 10 m. S. of Boston. Pop. (1890) 4848; (1900) 598l, including 1250 foreign-born; (1905, state census) 6879; (1910) 8066. The New York, New Haven & Hartford railway crosses the town and has stations at its villages of Braintree, South Braintree and East Braintree, which are also served by suburban electric railways. In South Braintree are the Thayer Academy (co-educational; opened 1877) and the Thayer public library, both founded by and named in honour of General Sylvanus Thayer (1785-1872), a well-known military engineer born in Braintree, who was superintendent of the United States Military Academy in 1817-1833 and has been called the "father of West Point." There are large shoe factories and other manufactories. Bog iron was early found in Braintree, and iron-works, among the first in America, were established here in 1644. Braintree was first incorporated in 1640 from land belonging to Boston and called Mount Wollaston, and was named from the town in England. At Merry Mount, in that part of Braintree which is now Quincy, a settlement was established by Thomas Morton in 1625, but the gay life of the settlers and their selling rum and firearms to the Indians greatly offended the Pilgrims of Plymouth, who in 1627 arrested Morton; soon afterward Governor John Endecott of Massachusetts Bay visited Merry Mount, rebuked the inhabitants and cut down their Maypole. Later the place was abandoned, and in 1634 a Puritan settlement was made here. In 1708 the town was divided into the North Precinct and the South Precinct, and it was in the former, now Quincy, that John Adams, John Hancock and John Quincy Adams were born. Quincy was separated from Braintree in 1792 (there were further additions to Quincy from Braintree in 1856), and Randolph in 1793.

See D.M. Wilson, _Quincy, Old Braintree and Merry Mount_ (Boston,
1906); C.F. Adams, Jr., _Three Episodes of Massachusetts History_
(Boston, 1892 and 1896); W.S. Pattee, _History of Old Braintree and
Quincy_ (Quincy, 1878).

BRAKE, a town of Germany, in the grand duchy of Oldenburg, on the left bank of the Weser, about halfway between Bremen and the mouth of the river. Pop. 5000. It was for centuries the port of Bremen; and though, since the founding of Bremerhaven, it no longer possesses a monopoly of the river traffic as before, it still continues to flourish. Large docks have been constructed, and the place has a considerable import trade in English coal. Shipbuilding and weaving are carried on to some extent.

Brake in Oldenburg must be distinguished from the village of the same name in the principality of Lippe, known as Brake bei Limgo, which gave its name to the cadet line of the counts of Lippe-Brake (1621-1709).

BRAKE. (1) A term for rough-tangled undergrowth, connected, according to the _New English Dictionary_, with "break," to separate. The "brake-fern" (_Pteris aquilina_) is the common "bracken," and is a shortened form of that northern Eng. word, derived from a Scand. word for "fern" (cf. Swed. _braken_), though often confused with "brake," undergrowth. (2) A term applied to many implements and mechanical and other appliances, often spelled "break." Here there are probably several words, difficult to separate in origin, connected either with "break," to separate, and its derived meanings, or with the Fr. _braquer_ (appearing in such expressions as _braquer un canon_, to turn or point a gun), from O. Fr. _brac_, modern _bras_, an arm, Lat. _bracchium_. The word is thus used of a toothed instrument for separating the fibre of flax and hemp; of the "break-rolls" employed in flour manufacture; of a heavy wheeled vehicle used for "breaking in" horses, and hence of a large carriage of the wagonette type; of an arm or lever, and so of the winch of a crossbow and of a pump handle, cf. "brake-pump"; of a curb or bridle for a horse; and of a mechanical appliance for checking the speed of moving vehicles, &c. It is noteworthy that the two last meanings are also possessed by the Fr. _frein_ and the Ger. _Bremse_.

Brakes, in engineering, are instruments by means of which mechanical energy may be expended in overcoming friction. They are used for two main classes of purpose: (1) to limit or decrease the velocity of a moving body, or to bring it completely to rest; and (2) to measure directly the amount of frictional resistance between two bodies, or indirectly the amount of energy given out by a body or bodies in motion. Machines in which brakes are employed for purposes of the second class are commonly known as dynamometers (q.v.). The other class is exemplified in the brakes used on wheeled vehicles and on cranes, lifts, &c. Here a body, or system of bodies, originally at rest, has been set in motion and has received acceleration up to a certain velocity, the work which has been done in that acceleration being stored up as "actual energy" in the body itself. Before the body can be brought to rest it must part with this energy, expending it in overcoming some external resistance. If the energy be great in proportion to the usual resistance tending to stop the body, the motion will continue for a long time, or through a long distance, before the energy has been completely expended and the body brought to rest. But in certain cases considerations of safety or convenience require that this time or distance be greatly shortened, and this is done by artificially increasing the external resistance for the time being, by means of a brake.