The Brain and the Voice in Speech and Song Part 3

[Ill.u.s.tration: Fig. 15]

[Description: FIG. 15.--Diagrammatic representation of a motor neurone magnified 300 diameters. Whereas the nerve cell and its branching processes (the dendrons) form but a minute speck of protoplasm, the nerve fibre which arises from it, although microscopic in diameter, extends a very long distance; in some cases it is a yard long; consequently only a minute fraction of the nerve fibre is represented in the diagram.]

The great brain or cerebrum consists of two halves equal in weight, termed hemispheres, right and left; and the grey matter covering their surface is thrown into folds with fissures between, thus increasing enormously the superficial area of the grey matter and of the neurones of which it consists without increasing the size of the head. The pattern of the folds or convolutions shows a general similarity in all human beings, certain fissures being always present; and around these fissures which are constantly present are situated fibre systems and communities of neurones having particular functions (_vide_ fig. 16.) Thus there is a significance in the convolutional pattern of the brain. But just as there are no two faces alike, so there are no two brains alike in their pattern; and just as it is rare to find the two halves of the face quite symmetrical, so the two halves of the brain are seldom exactly alike in their pattern. Although each hemisphere is especially related to the opposite half of the body, the two are unified in function by a great bridge of nerve fibres, called the corpus callosum, which unites them. The cortical centres or structures with specialised functions localised in particular regions of one hemisphere are a.s.sociated by fibres pa.s.sing to the same region in the opposite hemisphere by this bridge.

[Ill.u.s.tration: Fig. 16]

[Description: FIG. 16.--Diagram of the left hemisphere of the brain showing localised centres, of which the functions are known. It will be observed that the centres for the special senses, tactile, muscular, hearing, and vision, are all situated behind the central fissure. The tactile-motor kinaesthetic sense occupies the whole of the post-central convolution; the centre for hearing (and in the left hemisphere memory of words) is shown at the end of the first temporal convolution, but the portion shaded by no means indicates the whole of the grey cortex which possesses this function; a large portion of this centre cannot be seen because it lies within the fissure forming the upper surface of the temporal lobe. Behind this is the angular gyrus which is connected with visual word memory. The half-vision centre, and by this is meant the portion of brain which receives impressions from each half of the field of vision, is situated for the most part on the inner (unseen) surface of the occipital lobe. In front of the central fissure is situated the motor area, or that region destruction of which causes paralysis of the muscles moving the structures of the opposite half of the body. If the situations indicated by black dots be excited by an interrupted electric current, movements of the limbs, trunk, and face occur in the precise order shown, from the great toe to the larynx. In front of this precentral convolution are the three frontal convolutions, and it would seem that the functions of these convolutions are higher movements and attention in fixation of the eyes; moreover, in the lowest frontal region, indicated by fine dots, we have Broca's convolution, which is a.s.sociated with motor speech; above at the base of the second middle frontal convolution is the portion of cortex in which is localised the function of writing. Taste and smell functions reside in brain cortex only a small portion of which can be seen, viz. that at the tip of the temporal lobe.]

Muscles and groups of muscles on the two sides of the body which invariably act together may thus be innervated from either hemisphere, e.g. the muscles of the larynx, the trunk, and upper part of the face.

Gall, the founder of the doctrine of Phrenology, wrecked his fame as a scientist by a.s.sociating mental faculties with conditions of the skull instead of conditions of the brain beneath; nevertheless, he deserves the highest credit for his discoveries and deductions, for he was the first to point out that that part of the brain with which psychic processes are connected must be the cerebral hemispheres. He said, if we compare man with animals we find that the sensory functions of animals are much finer and more highly developed than in man; in man, on the other hand, we find intelligence much more highly developed than in animals. Upon comparing the corresponding anatomical conditions, we see, he said, that in animals the deeper situated parts of the brain are relatively more developed and the hemispheres less developed than in man; in man, the hemispheres so surpa.s.s in development those of animals that we can find no a.n.a.logy. Gall therefore argued that we must consider the cerebral hemispheres to be the seat of the higher functions of the mind. We must moreover acknowledge that the following deductions of Gall are quite sound: "The convolutions ought to be recognised as the parts where the instincts, feelings, thoughts, talents, the affective qualities in general, and the moral and intellectual forces are exercised." The Paris Academy of Science appointed a commission of inquiry, May, 1808, which declared the doctrine of Gall to be erroneous.

Gall moreover surmised that the faculty of language lay in the frontal lobes, and Bouillaud supported Gall's proposition by citing cases in which speech had been affected during life, and in which after death the frontal lobes were found to be damaged by disease. A great controversy ensued in France; popular imagination was stirred up especially in the republic by the doctrine of Gall, which was an attempt to materialise and localise psychic processes. Unfortunately Gall's imagination, encouraged by a widespread wave of popular sympathy, overstepped his judgment and launched him into speculative hypotheses unsupported by facts. His doctrine of Phrenology was shown to be absolutely illogical; consequently it was forgotten that he was the pioneer of cerebral localisation.

SPEECH AND RIGHT-HANDEDNESS

The next step in Cerebral Localisation was made by a French physician, Marc Dax, who first observed that disease of the left half of the cerebrum producing paralysis of the right half of the body (right hemiplegia) was a.s.sociated with loss of articulate speech. This observation led to the establishment of a most important fact in connection with speech, viz. that right-handed people use their left cerebral hemisphere as the executive portion of the brain in speech. Subsequently it was shown that when left-handed people were paralysed on the left side by disease of the right hemisphere, they lost their powers of speech. But the great majority of people are born right-handed, consequently the right hand being especially the instrument of the mind in the majority of people, the left hemisphere is the leading hemisphere; and since probably specialisation of function of the right hand (dexterity) has been so closely a.s.sociated with that other instrument of the mind, the vocal instrument of articulate speech, the two have now become inseparable; for are not graphic signs and verbal signs intimately interwoven in the development of language and human intelligence?

What has determined the predominance of the left hemisphere in speech? I can find no adequate anatomical explanation. There is no difference in weight of the two hemispheres in normal brains. Moreover, I am unable to subscribe to the opinion that there is any evidence to show that the left hemisphere receives a larger supply of blood than the right. Another theory advanced to explain localisation of speech and right-handedness in the left hemisphere is that the heavier organs, lung and liver, being on the right side have determined a mechanical advantage which has led to right-handedness in the great majority of people. This theory has, however, been disposed of by the fact that cases in which there has been a complete transposition of the viscera have not been left-handed in a larger proportion of cases. The great majority of people, modern and ancient, civilised and uncivilised, use the right hand by preference. Even graphic representations on the sun-baked clay records of a.s.syria, and the drawings on rocks, tusks, and horns of animals of the flint-weapon men of prehistoric times show that man was then right-handed. There is a difference of opinion whether anthropoid apes use the right hand in preference to the left. Professor Cunningham, who made a special study of this subject, a.s.serts that they use either hand indifferently; so also does the infant at first, and the idiot in a considerable number of cases. Then why should man, even primitive, have chosen the right hand as the instrument of the mind? Seeing that there is no apparent anatomical reason, we may ask ourselves the question: Is it the result of an acquired useful habit to which anatomical conditions may subsequently have contributed as a co-efficient? Primitive man depended largely upon gesture language, and the placing of the hand over the heart is universally understood to signify love and fidelity. Uneducated deaf mutes, whose only means of communicating with their fellow-men is by gestures, not only use this sign, but imply hatred also by holding the hand over the heart accompanied by the sign of negation. Moreover, pointing to the heart accompanied by a cry of pain or joy would indicate respectively death of an enemy or friend. Again, primitive man protected himself from the weapons of his enemies by holding the s.h.i.+eld in his left hand, thus covering the heart and leaving the right hand free to wield his spear. The question whether it would have been to his advantage to use either hand indifferently for spear and s.h.i.+eld has been, to my mind, solved by the fact that in the long procession of ages evolution has determined right-handed specialisation as being more advantageous to the progress of mankind than ambidexterity.

Right-handedness is an inherited character in the same sense as the potential power of speech.

LOCALISATION OF SPEECH CENTRES IN THE BRAIN

In 1863 Broca showed the importance in all right-handed people (that is in about ninety-five per cent of all human beings) of the third _left_ frontal convolution for speech (_vide_ figs. 16 and 17); when this is destroyed by disease, although the patient can understand what is said and can understand written and printed language, the power of articulate speech is lost. _Motor Aphasia_. This portion of the brain is concerned with the revival of the motor images, and has been termed by Dr. Bastian "the glosso-kinaesthetic centre," or the cortical grey matter, in which the images of the sense of movement of the lips and tongue are formed (_vide_ fig. 17). A destruction of a similar portion of the cortex in a right-handed person produces no loss of speech; but if the person is left-handed there is aphasia, because he, being left-handed, uses the third _right_ inferior frontal convolution for speech. These facts have for long been accepted by most neurologists, but recently doubts have been cast upon this fundamental principle of cerebral localisation by a most distinguished French neurologist, M. Marie; he has pointed out that a destructive lesion of the cortex may be accompanied by subcortical damage, which interrupts fibres coming from other parts of the brain connected with speech.

In the study of speech defects it is useful to employ a diagram; a certain part of the brain corresponds to the _Speech Zone_ there indicated, and lesions injuring any part of this area in the left hemisphere cause speech defects (_vide_ fig. 17). All neurologists, M. Marie included, admit this, and the whole question therefore is: Is a destruction of certain limited regions of the superficial grey matter the cause of different forms of speech defects, or are they not due more to the destruction of subcortical systems of fibres, which lie beneath this cortical speech zone?

There is a certain portion of the speech zone which is a.s.sumed to be connected with the revival of written or printed language, and is called the _visual word-centre_. There is another region connected with the memory of spoken words--the _auditory word-centre_; you will observe that it is situated in the posterior third of the first temporal convolution, but this does not comprise nearly the whole of it, for there is an extensive surface of grey matter lying unseen within the fissure, called the transverse convolutions, or gyri. Lesions of either of these regions give rise to _Sensory Aphasia_, which means a loss of speech due to inability to revive in memory the articulate sounds which serve as verbal symbols, or the graphic signs which serve as visual symbols for language.

[Ill.u.s.tration: FIG. 17]

[Description: FIG. 17.--Diagram to ill.u.s.trate the Speech Zone of the left hemisphere (Bastian). This scheme is used to explain the mechanism of speech, but probably the centres are not precisely limited, as shown in the diagram; it serves, however, to explain disorders of speech. Destruction of the brain substance in front of the central fissure gives rise to what is termed Motor Aphasia and Motor Agraphia, because the patient no longer recalls the images of the movements necessary for expressing himself in articulate speech or by writing. Destructive lesions behind the central fissure may damage the portion of the brain connected with the mental perception of the sounds of articulate language, or the portion of the brain connected with the mental perception of language in the form of printed or written words--Sensory Aphasia; the former entails inability to speak, the latter inability to read.

This speech zone acts as a whole, and many disorders of speech may arise from destructive lesions within its limits. It has a special arterial supply, viz. the middle cerebral, which divides into two main branches--an anterior, which supplies the motor portion, and a posterior, which supplies the posterior sensory portion. The anterior divides into two branches and the posterior into three branches, consequently various limited portions of the speech zone may be deprived of blood supply by blocking of one of these branches. The speech zone of the left hemisphere directly controls the centres in the medulla oblongata that preside over articulation and phonation; innervation currents are represented by the arrows coming from the higher to the lower centres.]

These several cortical regions are connected by systems of subcortical fibres to two regions in front of the ascending frontal convolution (_vide_ fig. 17), called respectively the "glosso-kinaesthetic" (sense of movement of tongue) and the "cheiro-kinaesthetic" (sense of movement of hand) centres.

Now a person may become hemiplegic and lose his speech owing either to the blood clotting in a diseased vessel, or to detachment of a small clot from the heart, which, swept into the circulation, may plug one of the arteries of the brain. The arteries branch and supply different regions, consequently a limited portion of the great brain may undergo destruction, giving rise to certain localising symptoms, according to the situation of the area which has been deprived of its blood supply. Upon the death of the patient, a correlation of the symptoms observed during life and the loss of brain substance found at the _post-mortem_ examination has enabled neurologists to a.s.sociate certain parts of the brain surface with certain functions; but M. Marie very rightly says: None of the older observations by Broca and others can be accepted because they were not examined by methods which would reveal the extent of the damage; the only cases which should be considered as scientifically reliable are those in which a careful examination by sections and microscopic investigation have determined how far subcortical structures and systems of fibres uniting various parts of the cortex in the speech zone have been damaged. Marie maintains that the speech zone cannot be separated into these several centres, and that destruction of Broca's convolution does not cause loss of speech (_vide_ figs. 16, 17). There are at present two camps--those who maintain the older views of precise cortical centres, and those who follow Marie and insist upon a revision.

Herbert Spencer says that "our intellectual operations are indeed mostly confined to the auditory feelings as integrated into words and the visual feelings as integrated into ideas of objects, their relations and their motions."

Stricker by introspection and concentration of attention upon his own speech-production came to the conclusion that the primary revival of words was by the feeling of movements of the muscles of articulation; but there is a fallacy here, for the more the attention is concentrated upon any mental process the more is the expressive side brought into prominence in consciousness. This can be explained by the fact that there is in consequence of attention an increased outflow of innervation currents to special lower executive centres, thence to the muscles, but every change of tension in the speech muscles is followed by reciprocal incoming impressions appertaining to the sense and feeling of the movement. The more intense the sense of movement, the greater will be the effect upon consciousness. In fact, a person who reads and thinks by articulating the words, does so because experience has taught him that he can concentrate his attention more perfectly; therefore his memory or understanding of the subject read or thought of will be increased. Very many people think and commit to memory by this method of concentrating attention; they probably do not belong to the quick, perceptive, imaginative cla.s.s, but rather to those who have power of application and who have educated their minds by close voluntary attention. Galton found a large proportion of the Fellows of the Royal Society were of this motor type. But the fact that certain individuals make use of this faculty more than others does not destroy the arguments in favour of the primary revival of words in the great majority of persons by a subconscious process in the auditory centre, which is followed immediately by correlated revival of sensori-motor images.

Although the sensori-motor images of speech can be revived, it is almost impossible without moving the hand to revive kinaesthetic impressions concerned in writing a word. Both Ballet and Stricker admit this fact, and it tends to prove that the sense of hearing is the primary incitation to speech.

Charcot in reference to the interpretation of speech defects divided persons into four cla.s.ses--auditives, visuals, motors, and indifferents.

There are really no separate cla.s.ses, but only different kinds of word-memory in different degrees of excellence as regards the first three; and as regards the fourth there is no one kind of memory developed to a preponderating degree. Bastian doubts the second cla.s.s, but does not deny that the visual type may exist; for Galton has undoubtedly shown that visual memory and power of recall of visual word images varies immensely in different individuals, and it is unquestionable that certain individuals possess the visualising faculty to an extraordinary degree; some few, moreover, can see mentally every word that is uttered; they give their attention to the visual symbolic equivalent and not to the auditory. Such persons may, as Ribot supposes, habitually think and represent objects by visual typographic images. Lord Macaulay and Sir James Paget were notable possessors of this visualising faculty. The former is said to have been able to read a column of "The Times" and repeat it _verbatim_; the latter could deliver his lectures _verbatim_ as he had written them. Both saw mentally the print or MS. in front of them.

Nevertheless it is a question of degree how much motor images enter into silent thought and into the primary revival of words in different individuals. Mach in "a.n.a.lysis of Sensations" says: "It is true that in my own case words (of which I think) reverberate loudly in my ear. Moreover, I have no doubt that thoughts may be directly excited by the ringing of a house-bell, by the whistle of a locomotive, etc., that small children and even dogs understand words which they cannot repeat. Nevertheless I have been convinced by Stricker that the ordinary and most familiar, though not the only possible way, by which speech is comprehended is really _motor_ and that we should be badly off if we were without it. I can cite corroborations of this view from my own experience. I frequently see strangers who are endeavouring to follow my remarks slightly moving their lips."

THE PRIMARY SITE OF REVIVAL OF WORDS IN SILENT THOUGHT

Since destructive lesions of the speech zone of the left hemisphere in right-handed persons leads to inability to revive the memory pictures of the sounds of words as heard in ordinary speech, the revival of visual impressions as seen in printed or written characters, and of the kinaesthetic (sense of movement) impressions concerned with the alterations of the minute tensions of the muscle structures employed in the articulation of words, it must be presumed that the left hemisphere in right-handed persons is dominant in speech and silent thought; it may even dominate the use of the left hand for many movements. But does not the right hemisphere take a part? Yes; and I will give my reasons later for supposing that the whole brain is in action. During the voluntary recall of words in speech and thought by virtue of the intimate a.s.sociation tracts connecting the grey matter of the whole speech zone, it is not a single part of this zone which is in action, but the whole of it; and when we a.s.sign to definite parts of the speech zone different functions in connection with language, we really refer to areas in which the process is most active or is primarily initiated, for the whole brain is in action just as it is in the recognition of an object which we see, hear, feel, or move. What really comes before us is contributed more by the mind itself than by the present object.

There is, however, a direct functional a.s.sociation between the auditory and glosso-kinaesthetic (sense of movement of the tongue) centres on the one hand and the visual and cheiro-kinaesthetic (sense of movement of the hand) on the other. No less intimate must be the connection between the auditory word-centre and the visual word-centre; they must necessarily be called into a.s.sociation actively in successive units of time, as in reading aloud or writing from dictation. Educated deaf mutes think with revived visual symbols either of lips or fingers. Words are to a great extent symbols whereby we carry on thought, and thinking becomes more elaborate and complex as we rise in the scale of civilisation, because more and more are verbal symbols inst.i.tuted for concrete visual images.

In which portion of the brain are words primarily and princ.i.p.ally revived during the process of thinking? I have already alluded to the views of Stricker and those who follow him, viz. that words are the revived images of the feelings of the sense of movement, caused by the alteration in the tension of the muscles of articulation occurring during speech, with or without phonation. There is another which I think the correct view, that words are revived in thought primarily as auditory images, so that the sense of hearing is essential for articulation as well as phonation; the two operations of the vocal organ as an instrument of the mind being inseparable. The arguments in favour of this are:--

1. The part of the brain concerned with the sense of hearing develops earlier and the nerve fibres found in this situation are myelinated[1] at an earlier period of development of the brain than the portion connected with the sense of movement of the muscles of articulation.

[Footnote 1: The covering of the fibres by a sheath of phosph.o.r.etted fat serving to insulate the conductile portion of the nerve is an indication that the fibre has commenced to function as a conductor of nervous impulses.]

2. As a rule, the child's first ideas of language come through the sense of hearing; articulate speech is next evolved, in fact the child speaks only that which it has heard; it learns first to repeat the names of persons and objects with which it comes into relation, a.s.sociating visual images with auditory symbols.

An example of this was communicated by Darwin to Romanes. One of his children who was just beginning to speak, called a duck a "quack." By an appreciation of the resemblance of qualities it next extended the term "quack" to denote all birds and insects on the one hand, and all fluid objects on the other. Lastly, by a still more delicate appreciation of resemblance the child called all coins "quack" because on the back of a French sou it had seen the representation of an eagle (Romanes' "Mental Evolution in Man," p. 183). Later on, children who have been educated acquire a knowledge of the application of visual symbols, and how to represent them by drawing and writing, and a.s.sociate them with persons and objects.

3. There is more definiteness of impression and readiness of recall for auditory than for articulatory motor sense feelings.

4. After the acquirement of speech by the child, auditory feelings are still necessary for articulate speech processes; for if it were not so, how could we explain the fact that a child up to the fifth or sixth year in full possession of speech will become dumb if it loses the sense of hearing from middle-ear disease, unless it be educated later by lip language.

5. Cases have been recorded of bilateral lesion of the auditory centre of the brain producing loss of hearing and loss of speech, the motor centres being unaffected. This is called Wernicke's sensory aphasia. The following case occurring in my own practice is probably the most complete instance recorded.

CASE OF DEAFNESS ARISING FROM DESTRUCTION OF THE AUDITORY CENTRES IN THE BRAIN CAUSING LOSS OF SPEECH

A woman at the age of twenty suddenly became unconscious and remained so for three hours; on recovery of consciousness it was found she could not speak; this condition remained for a fortnight; speech gradually returned, although it was impaired for a month or more. She married, but soon after marriage she suddenly lost her hearing completely, remaining permanently stone deaf; and although she could understand anything of a simple character when written, and was able imperfectly to copy sentences, she was unable to speak. Once, however, under great emotional excitement, while I was examining her by written questions, she uttered, "Is that." But she was never heard to speak again during the subsequent five years that she lived.

The utterance of those two words, however, showed that the loss of speech was not due to a defect of the physiological mechanism of the vocal instrument of speech, nor to the motor centres in the brain that preside over its movements in the production of articulate speech. She recognised pictures and expressed satisfaction or dissatisfaction when correct or incorrect names were written beneath the pictures; moreover, in many ways, by gestures, facial expression, and curious noises of a high-pitched, musical, whining character, showed that she was not markedly deficient in intelligence. Although in an asylum and partially paralysed, she was not really insane in the proper sense, but incapable of taking care of herself.

When other patients were getting into mischief this patient would give a warning to the attendants by the utterance of inarticulate sounds, showing that she was able to comprehend what was taking place around and reason thereon, indicating thereby that although stone deaf and dumb, it was probable that she possessed the power of silent thought. I observed that during emotional excitement the pitch of the sounds she uttered increased markedly with the increase of excitement. After having been discharged from Claybury Asylum she was sent to Colney Hatch Asylum. Upon one of my visits to that inst.i.tution I learnt that she had been admitted, and upon my entering the ward, although more than a year had elapsed since I last saw her, she immediately and from afar recognised me; and by facial expression, gesture, and the utterance of inarticulate sounds showed her great pleasure and satisfaction in seeing one who had taken a great interest in her case.

This poor woman must have felt some satisfaction in knowing that someone had interpreted her mental condition, for of course, her husband and friends did not understand why she could not speak. I may mention that the first attack of loss of speech was attributed to hysteria.

This woman died of tuberculosis seven years after the second attack, and examination of the brain _post-mortem_ revealed the cause of the deafness.

There was destruction of the centre of hearing in both hemispheres (_vide_ fig. 17), caused by blocking of an artery supplying in each hemisphere that particular region with blood. The cause of the blocking of the two arteries was discovered, for little warty vegetations were found on the mitral valve of the left side of the heart. I interpreted the two attacks thus: one of these warty vegetations had become detached, and escaping into the arterial circulation, entered the left carotid artery and eventually stuck in the posterior branch of the middle cerebral artery, causing a temporary loss of word memory, consequently a disturbance of the whole speech zone of the left hemisphere. This would account for the deafness to spoken language and loss of speech for a fortnight, with impairment for more than a month, following the first attack. But both ears are represented in each half of the brain; that is to say, sound vibrations entering either ear, although they produce vibrations only in one auditory nerve, nevertheless proceed subsequently to both auditory centres. The path most open, however, for transmission is to the opposite hemisphere; thus the right hemisphere receives most vibrations from the left ear and _vice versa_. Consequently the auditory centre in the right hemisphere was able very soon to take on the function of a.s.sociating verbal sounds with the sense of movement of articulate speech and recovery took place. _But_, when by a second attack the corresponding vessel of the opposite half of the brain was blocked the terminal avenues, and the central stations for the reception of the particular modes of motion a.s.sociated with sound vibration of all kinds were destroyed _in toto_; and the patient became stone deaf. It would have been extremely interesting to have seen whether, having lost that portion of the brain which const.i.tutes the primary incitation of speech, this patient could have been taught lip language.

There is no doubt that persons who become deaf from destruction of the peripheral sense organ late in life do not lose the power of speech, and children who are stone deaf from ear disease and dumb in consequence can be trained to learn to speak by watching and imitating the movements of articulation. Helen Keller indeed, although blind, was able to learn to speak by the education of the tactile motor sense. By placing the hand on the vocal instrument she appreciated by the tactile motor sense the movements a.s.sociated with phonation and articulation. The tactile motor sense by education replaced in her the auditory and visual senses. The following physiological experiment throws light on this subject. A dog that had been deprived of sight by removal of the eyes when it was a puppy found its way about as well as a normal dog; but an animal made blind by removal of the occipital lobes of the brain was quite stupid and had great difficulty in finding its way about. Helen Keller's brain, as shown by her accomplishments in later life, was a remarkable one; not long after birth she became deaf and blind, consequently there was practically only one avenue of intelligence left open for the education of that brain, viz. the tactile kinaesthetic. But the tactile motor sense is the active sense that waits upon and contributes to every other sense. The hand is the instrument of the mind and the agent of the will; consequently the tactile motor sense is intimately a.s.sociated in its structural representation in the brain with every other sense. This avenue being open in Helen Keller, was used by her teacher to the greatest possible advantage, and all the innate potentialities of a brain naturally endowed with remarkable intellectual powers were fully developed, and those cortical structures which normally serve as the terminal stations (_vide_ fig. 16) for the reception and a.n.a.lysis of light and sound vibrations were utilised to the full by Helen Keller by means of a.s.sociation tracts connecting them with the tactile motor central stations. The brain acts as a whole in even the simplest mental processes by virtue of the fact that the so-called functional centres in the brain are not isolated fields of consciousness, but are inextricably a.s.sociated one with another by a.s.sociation fibres.