Looking and seeing

OF T H E IRISH J O U R N A L MEDICAL S C I E N C E THE OFFICIAL JOURNAL OF THE ROYAL ACADEMY OF MEDICINE IN IRELAND

SIXTH SERIES. NO. 1~9. S~PTE~SER, 1936.

LOOKING AND SEEING.

(MOVEMENTS AND FIXATION OF THE EYES.)

By GORDO~ HOL~ES.

I MUST first express my appreciation of the honour done me in inviting me to give the John Mallet Purser Lecture, for of the many eminent and popular teachers of past generations in this

School few have enjoyed the affection and respect of its students in a greater measure than he to whose memory this Lecture has been established. Many of us cherish remembrances of his sym- pathy and kindly advice when sought, and particularly of his painstaking and lucid instruction. I f he has not left behind him a large record of scientific research, of which few were more capable, it is only because during the many years in which he taught here the " Institutes (or Fundamentals) of Medicine," he devoted himself wholly to the i~struction and training of his students. To me personally it has always seemed that the essential feature of his teaching was insistence on basic principles, on getting a grasp of the simpler problems and developing fuller knowledge from them.

When old students of your School exchange reminiscences one of the most frequent questions is: " Do you remember Johnny Purser's exam ? He always started off by asking: ' What is urine? or what is bile? or some other embarassingly simple question." But Purser realised that such apparently simple problems are the only sound foundation of knowledge.

I have selected for this Lecture a subject which would, I fe~l sure, have appealed to Professor Purser. I can imagine him asking an unwary student: " How do you ~ook at and see that object?" But even this simple question entails the consideration of many problems, most of which are in their essence unsolved. The essential nature of visual perception, as of every other activity in which mind is concerned, is obscure, but some fifteen years ago I was able to put before you in the Montgomery Lectures the results of investigations which indicated the portions of the brain through the agency of which we see, and those by which we can localise in space objects seen.

I t is on only one aspect of this subject that I intend to speak

* Being the Sixbh John Mallet Purser Lecture delivered at Trinity College, Dublin, June 4th, 1936.

566 IRISH JOURNAL OF MEDICAL SCIENCE

to-day; it is: " How do our eyes move to obtain accurate vision of an object, and how do we keep the object in distinct vision?"; in other words , " How do we loak at an object and arrange to see it distinctly?" The answer might be given : " I move my eyes by an effort of will as I would move my arm, and I keep the object in central vision by appropriate muscular contractions as I would if i.t were grasped in my hand." But the matter is not quite so simple as this. I t is true that the eyes are moved by the contractions of muscles which obey the same laws as those which bring about purposive movements of the arm, but only a small proportion of the ocular movements are voluntary or purposive, and it is certainly not merely by an effort of will that%ur eyes can be kept fixed on an object that interests us.

The subject is so complicated that it is only by breaking it up into its simpler components that we can obtain an insight into it. Experimentally this can be done by removing one or more of the components and then observing how the simpler mechanism works; this is one of the methods of physiology. 'But in certain researches our fellow beings, who can co-operate with us in our investigations, are more suitable subjects than the experimental animal, and the observations we make can be interpreted more easily and more directly, particularly when we are dealing with the more highly evolved functions of the nervous system. Clinical investigations designed to throw light on normal functions, however, present more difficult problems than those of the laboratories, for the clinician must accept the conditions as they occur; he cannot arrange for them to happen as the experimentalist may do, nor can he at will repeat the experiment which disease has performed. The fact t ha t chance has thrown in my way the opportunity of making certain observations bearing on fixation and the reflex movements of the eyes is my excuse for venturing into the physiological field.

I must preface my remarks by a review of certain facts known to you all. Each eye is roughly a sphere, the posterior part of which is covered by a receptor organ sensitive to light, the retina. A ray of light falling on any portion of the retina can produce in consciousness a sensation which is referred to the corresponding point in the field of vision; but in each eye there is a small area of more acute vision, known as the macula, on which we arrange for the rays to fall if we wish to see most distinctly the object from which the rays are reflected. This we do by movements of the eyes so that the visual axes are directed to that object. But as we employ two eyes it is necessary that both should work together, that the images received by both should fall on corresponding points of the two retin~e and, further, that the peripheral apparatus for accommodating the eyes should be called into play in order that the images formed on the retin~e should be sharp and distinct. A nervous mechanism in the lower portions of the brain, actually in the midbrain and pons Varolii, is responsible for assuring the accurate co-operation and accommodation of the two eyes, whether they are employed reflexly or voluntarily. The organisation of this mechanism requires no further reference here.

LOOKING AND SEEING 567

Movements of the eyes can be obtained by excitation of many parts of the central nervous system, including the forebrain. In the second frontal convolution of each side of the brain there is a centre from which such movements are easily excited, and according to an almost general consensus o f opinion it is by activation of this centre that so-called voluntary or purposive movements are produced. But when this centre is destroyed, even when the frontal eentres of both hemispheres are injured, ocular movements are still possible, which is not surprising i f the assumption already made is true, and each of you may verify it by person.ai examination, tha t voluntary effort is responsible for a small proportion only of our ocular movements. A flash of light or the unexpected movement of an object to one side at once excites a deviation of the eyes towards it, and a sound may attract our gaze in the direction in which we localise it.

Since the early days of experimental physiology it has been known that stimulation of the visual and auditory areas of the cortex of the brain also cause movements of the eyes. These movements are the experimental counterpart of those which are normally excited by a sudden and unexpected visual or auditory stimulus. We shall see later why I emphasise " suddenness " and " unexpectedness " as essential features of an adequate stimulus. Other nervous mechanisms situated subcortically, or deeper i.n the brain, may also cause ocular deviations unassociated with changes in consciousness, especially those connected with the vestibular apparatus of our ears, on which we depend largely in orientating and maintaining the positions of our bodies in space.

In studying the movements of the eyes we have consequently to consider several physiological mechanisms through which ocular movements can be excited reflexly, that is in response to a stimulus ei ther with or without the intervention of ,consciousness, as well as that by which voluntary or intentional deviations are brought about. The problem would be simplified if it were possible to study each of these mechanisms separately by removing or suppressing the activity of the others. This is rarely possible in man, but occasionally disease abolishes one or more functions and leaves a simpler state for ~nvestigation. But in pursuing research on these lines it is essential to realise that the condition produced by such experiment or disease is not merely due tO loss of one or more components of normal function, for the activity of structures which remain intact are disordered as a result of their isolation from the rest of the ~unctioning organ.

A typical ease in which as a result of disease the power of voluntary movement of the eyes was lost may be described to illustrate the functions of certain reflexes which were preserved, more particularly as it will focus attention on the main subject of this lecture, fixation of the eyes, or the power of maintaining our gaze on an object whether it is at rest or in movement.

A man who had had several strokes as a result of occlusion of cerebral vessels complained of disturbances of vision, but other symptoms which he presented must be referred to as they throw


light on the nature of his ocular troubles. As far as the functions subserved by the bulbar mechanisms are concerned he was almost a pure reflex machine. There was complete paralysis of the voluntary movements of his face, but his face moved well, even excessively, when he was made to smile or laugh. He could not protrude his tongue when asked to, but he licked away readily lemon juice from his lips. He was unable to open his mouth or chew to order, but he masticated vigorously bread placed between his teeth. He failed to close his eyes to order, but he blinked at once when a threatening object approached his face. The voluntary movements of his eyes were similarly affected ; he was unable to look to the left, upwards or downwards, or to converge on a near object, and deviation to the right was seri~ously restricted. When, however, his eyes were fixed on a point he could follow this point when it was moved slowly in any direction, and if his head was rotated passively his eyes remained in their original direction, in other words they deviated though a corresponding angle in the opposite direction. Frequently too his eyes turned in the direction of an unexpected sound.

When in a normal person one labyrinth is sthnulated by a galvanic current, by rapid rotation, or by irrigating one ear with cold water, the eyes deviate towards the opposite side in a series of rapid jerks with ~slower recessions towards the side stimulated; this we call nystagmus. The slow phase of such nystagmus is generally accepted to be the result of labyrinthine stimulation. In the patient whose condition we are considering no nystagmus occurred when his labyrinthine apparatus was sthnulated, but his eyes deviated strongly, and remained deviated for several seconds after the stimulation ceased, in the direction of the slow phase, even though this movement was impossible on voluntary effort.

Here is an instance of loss of voluntary or willed movement of the eyes with retention of reflex movements which can be excited by retinal, auditory or labyrinthine stimulation.

During these tests an interesting fact was observed: when the eyes moved towards one side in response to a sound or on passive displacement of his head, they swung back quickly to their position of rest; on labyrinthine stimulation this occurred after a short delay which probably represented persisting stimulation of the semicircular canals. On the other hand, when his eyes followed an object in any direction they remained in their new position as long as they were fixed on the object despite the patient 's efforts to bring them back; under these circumstances he was quite as power- less to move them from the left to the central position of rest as in the opposite direction. If, however, fixation was broken by placing a screen in front of his face, or by blinking, his eyes at once slid back. On further testing it was found that if his head was passively rotated while his eyes were not interested in any object they receded quickly to their position of rest when the movement ceased; ~f, on the other hand, his eyes were fixed on a point when his head was moved they remained in compensatory deviation on


the point as long as fixation was uninterrupted. This has been also observed by Bielschowsky, Behr and others.

I t therefore seemed probable that fixation played a part in the symptoms presented by the patient. Further observations confirmed this. I t has been already mentioned that his eyes followed moving point provided its movement was slow and uniform in rate, but if the movement was too rapid or irregular, with the result that the images of the point were momentarily displaced from the maculm, his eyes ceased to follow it. But the crucial experiment was obtained by labyrinthine stimulation; this produced, when his eyes were closed or shaded, or if he were in the dark, an ocular deviation so strong and persistent as to give the patient cons;.der- able discomfort, but i t failed to produce any movement if, when stimulation was commenced, his eyes were fixed on a point.

We have, therefore, in addition to loss of voluntary movement and preservation of reflex movements, a tendency to pathologically exaggerated fixation, an inability to disengage the eyes from any object on which they are fixed. I have referred elsewhere to this phenomenon as " spasm of fixation," a suitable term provided it is understood that it is merely an exaggeration of a normal reflex. I t was owing to this reflex that though voluntary movement was lost the patient was able to follow with his eyes a moving object on wh~h they were fixed, and that by compensatory deviation the eyes remained directed to a stationary object when his head was moved either actively or passively. That the essential factor was a fixation reflex excited by visual impressions was shown by the fact that the reflex was broken when fixation was interrupted, as by a screen suddenly placed i~ front of the eyes, or by blinking. As fixation means the maintenance of the images of the object fixated on the retinal areas of most distinct vision, we must regard it as essentially a macular reflex.

The obvious explanation of the pathological exaggeration of the fixation reflex i~ this and in similar cases is that it is a " release phenomenon," that the physiological mechanism of the normal reflex has become over-active owing to absence of inhibitory impulses from more highly evolved centres which normally control it and subordinate it to the well-bei~g of the body as a whole. I t is, therefore, comparable to the increase of the knee-jerks and other reflexes found in association with disease of the cortical motor centres or of their projection fibres.

Another curious, and at first puzzling, phenomenon which has been repeatedly observed in these patients who have lost the power of voluntary ocular movements is that when they are at rest and disinterested in their surroundings their eyes may be seen to wander about as though exploring or searching the room in which they happen to be. These movements may resemble those of normal eyes, but they are usually less systematic and objectJ~ve, more sporadic and aimless. Usually they cannot be attributed to any immediate cause, but occasionally they seem to be excited by an unexpected sound, by a flash of light or the sudden movement of an object, and sometimes they appear to be part of an emotional


reaction, The explanation of their occurrence seems to be that when the patient is disinterested and attention does not fix the eyes on any point, when therefore the fixation reflex is not in action, the eyes which are not under voluntary control are at the merey of subordinate reflexes which may produce movement.

I t is obvious that we are here dealing with reactions which di~er from those which we usually describe as reflex as they require the intervention of consciousness, for fixation does not imply merely the maintenance on the macula of each eye of an image of the object which is fixed, but also the perception of the object. Further. we must invoke interest or attention to initiate the reflex. We mustl therefore, regard the reflex as a cortical function if we believe, as we have every reason to believe, that the cortex is the organ of consciousness. There is, in fact, no evidence that any retinal reflex, except the reaction of the pupil to light, persists in man or in the higher mammals af ter destruction of the visual cortex.

But in presenting a hypothesis to explain any phenomenon it is advisable to look at it from every side, to adduce negative as well as positive evidence; in this case to ask what will happen if the fixation reflex is enfeebled or abolished. Clinical observations can luckily furnish some evidence on the point.

There is normally a position of rest of the eyes, known as the position of primary fixation, in which the visual axes are directed straight in front and slightly downwards. This position depends on the tone of the extrinsic ocular muscles, not on their active contraction, but any deviation, whether voluntary, automatic or reflex, from this position requires active muscular contraction. I have been able to collect a considerable number of observations in which the most striking feature was the inability or the difficulty of the patient to keep his eyes fixed on any object outside their position of rest although all the ocular movements were intact, or, to put the matter more accurately, to maintain fixation by active muscular contraction.

This disturbance of fixation is most commonly seen on deviation of the eyes to one side, as the cerebral lesions to which it is due are usually unilateral, but I have observed it in attempts to fixate objects in all d~rections in space. When a patient afflicted with this condition is asked to look at an object, to one side for instance, he moves his eye promptly and accurately to it, but he cannot keep !hem directed on it ; almost at once they recede towards their position of rest and are brought back by a series of efforts which are obviously purposive or voluntary. The same disturbance is seen when the eyes attempt to follow a moving object, especially i t the movement is not uniform in rate, and when the head is moved passively during an attempt on the part of the patient to maintain ocular fixation. These patients often recognise thei,r difficulty and may describe it as an inability to see the object distinctly, or to focus it. I t may be, but of this I am not certain, associated with a disturbance of fusion, that is a failure to unify the separate impres~ sions from the retin~e of the two eyes into a single percept, which,


of course, depends among other factors on the accurate incidence of the visual axes of the two eyes on the object.

Such an inability to ma'mtain fixation may, of course, be due to many causes, for instance, to paresis of the muscles which move the eyes; to nystagmus whatever its origin may be; to defective eentral vision, especially that associated with central scotomata, to failure to orientate accurately in space the position of the object which should be fixed, and to inattention of the patient, but all these possible causes were carefully excluded in the observations to which I refer.

Another feature of this disturbance is that the failure to fixate accurately is always more pronounced the further the object to be fixed is from the line of the visual axes when the eyes are at rest, in other words, it is roughly proportional to the muscular effort demanded of the patient. I t seems, therefore, that voluntary effort alone does not sufliee to keep the eyes directed on an object when active muscular contractions are necessary for it, and that the greater the effort the less accurate are the attempts to do so.

We have~ :consequently, to deal with a condition eharacterised by failure of fixation though the muscular apparatus of the eyes is intact, and we can now enquire into the nature of this pathological defect and see what light it throws on the physiological processes by which fixation is normally maintained.

I t is commonly assumed that when we look at an object we do so by turning our eyes to it by a purposive contraction of the appropriate muscles and the reciprocal relaxation of their antagonists, and that we keep our eyes ia the new posi,tion by a steady main- r of the muscular contractions which were initiated by the voluntary effort, but these observations show that even though the contractions of the muscles are unaffected prolonged fixation may fail.

Further consideration indeed makes it improbable that voluntary effort alone can serve to keep an object in central vision, for even under the normal conditions of life the relative positions of the point at which we look and of our eyes axe always changing. When I walk, talk or even breathe the position of my head in space, and of my eyes within my head, is constantly altering, usually irregu- laxily and abruptly, while the point at which we gaze may move in any direction and at any rate. I t is u pr/ar/improbable that the fixation of the eyes on the point should, under these conditions, be wholly dependent on a purposive act, and that the necessary rapid and accurate adjustments of the visual axes by a conscious effort alone should be adequate to the task. Such a claim on conscious volition would be an intolerable distraction of attention.

Fixation of the eyes is comparable to the maintenance of the postures of other parts of our bodies, which numerous experiments have shown to be a purely reflex,process. I t is well known that an animal can stand erect and hold its head and ta~,l in normal attitudes even though it has been made a purely reflex automaton by removal of its forebrain, and in many diseased conditions in man postures are adopted by various parts of the body and main-


tained against external forces wholly indepently of volition. Introspection can also convince us that when we stand or sit erect we do not do so by a sustained voluntary effort. The postures of our eyes when they are not in a position of rest may a F r / a r /be assumed to depend on analogous reflexes: In the case of our limbs these postural reflexes are activated mainly by impulses coming from the muscles concerned in the posture, from their tendons and probably from the joints, but the labyrinths of our internal ears also play an important part in this function. There can be little doubt that proprioceptive impulses from the ocular muscles, and probably from other tissues in the orbits, excite reflexes or augment those already in action to maintain the postures of the eyes, but to simplify our problem this influence will be disregarded here.

Reflexes of labyrinthine origin certainly play a more important rSle in ocular fixation. As you know, each labyrinth is a dual mechanism; the otoliths of the utricle and saccule rest on sensitive hairs connected with nerve fibres, and every change in the position of the head in relation to gravity necessarily modifies the pressure they exert on the hairs, and this starts a new series of impulses. The i r function is, therefore, essentially static. The other organ is the semicircular canals in which nervous impulses are initiated by displacement of endolymph during movement of the head. Their nature is, therefore, kinetic.

These reflexes, which have been exhaustively studied in animals by Magnus and his colleagues, have an important function in the adjustments of the eyes. They are excited by every movement of the head, and particularly those which originate in the utricle (for the function of the saccule is in doubt), can bring about compensatory deviations of the eyes as a result of which the gaze is not deflected from the object in central vision. Their influence in man is indicated by the fact that when the labyrinths are destroyed, or when their afferent nerve fibres are divided, the object looked at seems to oscillate or jerk about whenever the head is moved, but this does not happen when the head is stationary. The labyrinths are, therefore, one of the physiological mechanisms by which ocular fixation is assured when the position, of the head alters in relation to the object in central vision. But these disorders of fixation are temporary, and usually they do not appear when the loss of labyrinthine function develops slowly. The labyrinthine reflexes are, therefore, not essential for fixation. This is also obvious from the fact that compensatory deviations from the otolithie organs occur chiefly when the eyes are closed or the animal is in the dark, and not so readily if its eyes are fixed on an object. I t seems, therefore, that reflexes of retinal origin must supplement and may even dominate those excited by labyrinthine impulses.

And as the disturbances of fixation described here are present even when the head is stationary, obviously they cannot be attributed to disorders of reflexes which are normally excited by movements or by new postures of the head. Moreover, in some of the patients in whom fixation was defective normal deviations of the eyes could be produced by labyrinthine stimulation. Finally, when


the eyes are observed directly unmistakable differences can be seen between the disturbances of fixation with which we are concerned and those which result from ablation of the labyrinths or interruption of impulses from them by section of the vestibular nerves; in the latter state the eyes when at rest or in movement are displaced by rapid jerks or by a more regular nystagmus, while in the former the eyes are steady at rest and their movements are regular and uniform, and the only abnormality in their motor functions is a failure to hold a fixation brought about by active muscular contractions.

Magnus has shown that reflexes from the muscles and other structures of the neck may also influence the postures of the eyes, but these reflexes are obviously not concerned in the phenomena which we are discussing.

Some other explanation must be sought for the fact that though the labyrinthine reflexes are intact fixation may be defective, especially when active muscular contractions are necessary to maintain it. The only other factors we can invoke are impulses of retinal origin which act through either cortical or subcort~cal mechanisms in controlling and maintaining ocular fixation.

We can now correlate two series of observations; in the one the eyes tend to remain fixed spasmodically on any point m central vision and cannot be moved from it by will, or even by the intervention of reflexes which normally lead to their deviation, unless the retinal impressions are interrupted; in the other series the patient is unable to maintain fixation accurately though the ocular movements are intact. These two series of observations are com- plementary; in the one the function of fixation is exaggerated, in the other it is lost or diminished. As retinal impressions, and more particularly those subserving central vision, are necessary for fixation, and as fixation may be independent of voluntary effort, we must assume that it depends in part at least on a central reflex mechanism activated by impulses from the macul~e by which the eyes are held directed on the object in central vision.

The facts at our disposal go to show that this central reflex mechanism is situated in the cortex of the brain within, or in the i,mmediate proximity of, the area concerned in visual perception, that is the cortex in the lips and on the walls of the ealcarine fissures. In man and the higher animals no reflex movements of retinal origin can be obtained after destruction of the visual cortex. The afferent segment of the reflex are is almost certainly contained within the central visual system, that is in the optic radiations, for there is no evidence to suggest the existence of separate eortico- petal fibres for ocular reflexes.

The anatomists have provided us with a corticofugal path which probably serves as the efferent limb of the reflex arc. In 1889 von Monakow described fibres which take origin in the larger cells of the occipital lobe and pass to the anteri, or quadrigeminal bodies. Flechsig confirmed this observation, and a little later Beevor and Horsley and Probst independently demonstrated their course more


accurately by experiments on animals. More recently they have been studied by Bouman, Biemond, Poljak and Barris. In the hemispheres they lie medial to the optic radhtions, enter the retrolenticular segment of the internal capsule, and pass through the pulvinar thalami and through the anterior brachium to reach the superficial grey matter of the anterior quadrigeminal body. Barris, who has recently traced them, states that they end mainly in the " nucleus of the opth~ t ract ," a small collection of grey matter in the anterior end of the midbrain. According to Barris, Bouman and others a few fibres of similar origin and course can be traced to the pons Varolii; these may be concerned in the movements of the head which are usually associated with deviations of the eyes.

The exact origin of these fibres, which are known as the eortico- mesencephalic or palli~tectal tract, is still in dispute; some workers believe they come mainly from the primary visual centre in the area striata, others that they arise in the cortex around it. This is, however, a matter of littlo significance, for while the visual perception centre is strictly localised, the cortex around it is certainly actively engaged in the physical and somatic reactions excited by visual impulses. The important fact is that by these fibres the visual cortex in the occipital lobe is connected i~ the centrifugal sense with the dorsal part of the midbrain in the neighbourhcod of which there is a mechanism for the execution of ocular movements.

I t will be remembered that working with physiological methods Munk showed that ocular movements could be obtained by electrical stimulation of the occipital lobe, and by demonstrating that these movements were unaffected by isolating the occipital lobe from the rest of the brain, he proved that this region of the cortex must be connected directly with the subcortical eentres which effect ocular movements. Bechterew found that these movements were abolished by lesions of the anterior quadrigeminal bodies, but those caused by stimulation of the oculomotor centres in the frontal lobes were uninfluenced by this operation, and thus brought the proof that in this region there is an anatomical separation of the cerebral paths through which ocular movements can be effected.

These fibres of the occipito-mes~neephalic tract are undoubtedly engaged in reflexes which determine.movements and postures of the eyes in which volithm is not concerned, for all known facts go to show that voluntary movements are initiated from the frontal lobe, not from the region of the visual cortex. One of these reflexes serves to bring objects into central vision, for stimuli falling on the peripheral parts of the retin~ from an object outskle the line of central vision may excite movements which place images of the object on the macule. Another is concerned in the blinking reflex to threats, and other ocular reflexes of cortical origin are active in accommodation and fusion.

But my thesis is that some of these fibres connecting the occipital lobes of the brain with the upper part of the brainstem, chiefly with the teetum of the mesencephalon, subserve reflex fixation of the eyes. Their function is to fix reflexly the visual axes on any


object in central vision until the reflex is broken by the dominance of another reflex, as that excited by the image of another object, or by volition or attention. We have seen that " suddenness" and " unexpectedness " are important factors in visual and auditory stimuli which may cause deviation of the eyes; this shows that the conscious proceas we call " attention," which is easily disturbed by the unexpected, plays an important part in reflex fixation.

There are, therefore, two distinct and opposed cortical ocular reflexes, the one which is excited by extramacular stimuli effects movements of the eyes; the other, of macular origin, holds an object in central vision either by maintaining the posture of the eyes, or by moving them appropriately when either the object or the sub- jeet's head moves in space.

There is no clear clinical evidence that in man disturbances of fixation may result from interruption of the afferent side of the fixation reflex arc. This is probably due to the fact that the centri- petal fibres are identical with those which subserve vision, and consequently when the afferent reflex path is broken vision also is lost, and where there is no vision there can be no fixation. The difficulty which many patients width homonymous hemianopia have, especially when the blindness comes up to the fixation point, in fixing objects on the blind side may be, however, an expression of disturbance of this reflex. In the greater part of their course the corticofugal fibres run in close anatomical relation to the optic radiations, but after they leave the posterior segment of the internal capsule they take a more dorsal and medial direction through the pulvinar and the anterior brachium to the anterior quadrigeminal body and are here separate from the visual p'athway. Such evidence as I have been able to collect from clinical observations and from a few postmortem examinations indicates that they are injured in this position in those cases in which defect of fixation exists as an isol.ated ocular symptom.

On the other hand, spasm of fixation, in the sense of difficulty in disengaging the eyes from the object in central vision, is due to the uninhibited activity of this cortical reflex which in normal conditions is under the control of the cortical eentres through which volitional movements of the eyes are effected. In the early days of cerebral physiology when functions were first localised in " centres " in the cortex of the brain it was assumed that the only duty of these " centres" was to bring about various reactions of the body in response to its needs or to its environment when acting under the influence of consciousness, or " will ", whatever it may be, but modern work has emphasised that they are largely concerned in keeping subcortical activities under proper control. The higher centres can be, in fact, compared to the second chamber of a legislature which is capable of action on its own, but its chief function is to prevent an assembly representative of many and diverse interests, but less well informed of all the bearings and of the facts of the situation and, therefore, less discriminative, from reacting to every impulse of the moment.

In normal conditions there is an adjusted co-operation between

576 I R I S H J O U R N A L OF M E D I C A L S C I E N C E

the frontal (or volitional) and occipital (or reflex) cortical oculomotor mechanisms. Through t he former we can move our eyes about and explore space in response to the call of volition; while occipital reflexes excite movements which turn our eyes, and usually our heads too, towards an object in the field of vision, but the occipital centres possess the equally important function of keeping the eyes fixed steadily on any point which claims attention or excites interest.

The importance of this reflex fixation of the eyes in the ordinary activities of life can scarcely be exaggerated; by its means objects of interest are held in distinct vision and our eyes are prevented, without conscious effort, f rom wandering from the work on which we are engaged. Skill in many occupations and games depends largely on its efficiency, it keeps our eye on the ball, but in both work and play it is essential tha t it is immediately responsive to, and modifiable by, volition, that there is an intimate co-ordination between voluntary and reflex activities.

The subject of this lecture is, therefore, of great significance in the ordinary routine of life, and an enquiry into its nature and ~nto the factors which control it, will I hope jus t i fy my incursion into physiology, even though I have been able to approach it only from the restricted field of clinical pathology.

POPULATION OF THE IRISH FREE STATE.

The returns of the recent census show the population of the Irish Free State to be 2,965,854. This represent~ a decrease of 6,138 persons compared with the last census t~ken, in 1928.

There are 1,518,807 males this year, compared with 1,506,889 in 1926, an increase of 11,918; but there are only 1,447,047 females, against 1,465,103 in 1928, a decrease of 18,056. There are thus 71,760 more ma es than females in the country to-day, the ratio of females to males being aS 953 to 1,000. The low ratio of females to males is the most striking f ea tu re of the Preliminary Report of the Census of Population taken on the night of April 26th-27th, 1936.

Since 1911 there has been an excess of males in the population, but to-day the Saorst~t ratio is lower than any of twenty-two other countries in Europe. At the 1926 census the Saorst~t ratio placed this country sixth lowest in the list of twenty-eight countries; this year's census places it second lowe6t in the same list of countries.

The published figures bring out the decrease in the rural population and ghe drift towards the towns. The four principal cities---Dublin, Cork, Limerick, Waterford--all show an increased popula$ion.

The net emigration during the past ten years was 169,316, an average of less than 17,000 per annum, compared with the 27,000 per anmml for the ten years 1916-1928.

Documents

Looking and seeing