Chapter X1 Muscles Of The Eye | Operating Microscopes

Anatomy and Physiology. The eyeball is a spheroid, and the movements which it executes are only those of revolution around a centre whose position never varies.

In the normal condition the position of the eyeball itself does not vary.

The six muscles which are concerned in its movements form two groups viz., i, the four recti ; 2, the two obliques. The conjoined action of the two latter is to draw the eye forwards ; the recti, antagonistic to the obliques in this respect, draw it backwards, and thus the eye is in a state of equilibrium when these muscles are acting together normally. When they are not, the ball can be turned by them in any direction around its fixed centre, without changing its situation.

To determine the position of such a sphere, it is not sufficient to indicate the changes in position of any single point ; for should this point be the pole of any axis, the sphere may change its position by rotation around that axis, the point itself remaining fixed. It is, therefore, necessary to determine another point or line. On the eyeball we take as points of precision the centre of the cornea, and the vertical meridian passing through this centre. In this way we determine the rotation of the eye by indicating the direction in which the centre of the cornea is carried, and the inclination which is communicated to the vertical meridian.

The direction in which a muscle acts is given by a line which joins the middle points of its two insertions. The plane which unites this line to the centre of revolution of the eyeball, which is at the same time its spherical centre, is called the plane of the muscle. The axis of revolution of the muscle is the line perpendicular to its plane at the point of rotation.

Let us now consider the anatomical relations of the eyeball with the oculo motor muscles. The recti muscles take their origin from the fundus of the orbit in the fibrous ring which surrounds the Optic nerve ; their course to their terminal insertion is rectilinear till they reach the greatest circuniference of the ball. From their point Of contact, before they have pierced Tenon's capsule, to their terminal insertion, they describe 'I curve similar to that Of the struc ture which they cover. In the rest of their course, except where their tendons are inserted into the sclerotic, these muscles are free till they reach the equator of the eyeball ; their internal surface is lined with a smooth membrane, which is a prolongation of the fibrous envelope. The anterior insertions must be studied separately (Fig. 181). Before their insertion the muscles of the eye pierce the fibrous envelope of the eyeball, and wherever they pierce it, it forms a sheath around them, which is gradually lost in the perimysium. These prolongations are of great importance in the modern operation of strabotomy. They appear as two thin membranes (lateral sheaths) in the margins of the muscles, whose internal surface is covered by them with dense and close set fascia. We thus see that, by these prolongations, the muscles may act on the eyeball, even when their insertion is cut near the sclerotic.

The internal rectus (Fig. 18z, 5), the strongest of all the muscles of the eye, runs parallel to the internal wall of the orbit, and is inserted in front by an aponeurosis, 8 millimetres in breadth, the middle point of which is on a level with the centre of the cornea and about 5 millimetres from the corneal margin (Fig. 181, 6).

The external rectus (Fig. 182, 3), the longest of the recti muscles of the eyeball, for the greatest part of its course runs along the external wall of the orbit, and to a large extent covers the ball ; it is inserted in front by a tendon 6 millimetres broad, at the level of the centre of the cornea and at about 7 millimetres from its margin (Fig. 18i~ 7).

It follows, from what has been said, that the plane of these two muscles is horizontal, and is the same for both. Their axis of revolution is, therefore, vertical, and coincides with the vertical axis of the eye, which forms a right angle with the optical axis. Let us now suppose that the eye is in its initial position, that is to say, let the optical axis be horizontal, and the centre of the cornea be directed forwards ; the internal rectus will turn the eye horizontally inwards, whilst the vertical meridian preserves its direction ; the external rectus will turn the cornea horizontally outwards without changing the direction of the vertical meridian.

The superior rectus (Fig. 182, 4) runs parallel with the superior wall of the orbit, from behind forwards, and from without inwards, so that a straight line joining the middle point of its two insertions forms with the optical axis an angle of 2o degrees ; its tendon, 7 or 8 millimetres broad, expands into a slightly convex aponeurosis, which is inserted obliquely at 7 millimetres from the superior margin of the cornea (Fig. IS', 4), in such a manner that its internal extremity is 2 millimetres nearer the cornea than its external.

The inferior rectus is also directed from behind forwards, and from without inwards; the middle part of its aponeurotic insertion, 7 millimetres broad, is inserted at 5 millimetres from the inferior margin of the cornea, and i millimetre within the vertical meridian. This insertion is oblique (Fig. 181, 5) and so placed that its internal extremity is 2 millimetres nearer the cornea than the external.

For greater simplicity, let us suppose that these two muscles have the same plane; it will be vertical and run obliquely forwards and outwards, making an angle Of 2o degrees with the optical axis. The axis of revolution of these two muscles will then be horizontal, running from before backwards, and from within outwards, and will make an angle Of 7o degrees with the optical axis (Fig. 182, aa).

Let us suppose that the eye is now moved from its initial position by rotation round this axis.

The superior rectus turns the eye upwards and inwards, and inclines its vertical meridian slightly inwards.

The inferior rectus rotates the cornea downwards and inwards, and inclines its vertical meridian slightly outwards.

The displacement of the cornea by the action of these two muscles will be greater the nearer it is to the external angle ; and the alteration in the inclination of the meridian will be greater the nearer it is to the internal angle.

The superior oblique ( I ig. 182, 6) arises at the bottom of the orbit, and is directed at first forwards ; then it diminishes to it tendon which passes over a pulley ( P'Ig. 182, 9), after which it expands, and passing beneath the superior rectus from within outwards ( Fig. 10), it is inserted in the sclerotic oil the temporal side of the posterior circumference of the ball, by an aponeurosis 6 millimetres broad, the convexity of which is directed backwards and outwards, whilst its posterior extremity is about 7 millimetres, and its anterior 12 or 14 millimetres, from the optic nerve.

The inferior oblique arising from the internal and anterior aspect of the orbital floor, external to the lachrymal sac, first passes backwards and outwards beneath the inferior rectus. Soon, after running about 5 millimetres in this direction, it turns abruptly upwards and backwards, so as to pass between the external rectus and the eyeball ; then, becoming broader and thinner, it is inserted by a short tendon near the superior oblique (Fig. 182, 2). Its insertion, io millimetres broad, presents a convexity upwards and forwards, the superior extremity of which is 14 millimetres from the optic nerve, whilst the inferior is only 4.

The plane of the two oblique muscles is vertical, and is directed from behind forwards and from within outwards, forming with the optical axis an angle Of 55 degrees. It follows from this, that the axis of revolution for these two muscles is horizontal, and passes from before backwards, meeting the optical axis at an angle Of 35 degrees (Fig. 1182, bb).

The superior obliqae rotates the cornea downwards and outwards, and inclines the vertical meridian of the cornea inward The inferior oblique rotates the cornea upwards and outwards, and inclines the vertical meridian outwards.

The more the eye is turned towards the nose the greater will be the effect of these muscles on the displacement of the cornea, and the more the eye is turned towards the temporal side, the greater will be their effect on the inclination of the vertical meridian. Having thus studied the effect of each of the muscles of the eyeball separately, it remains for us to examine the part each plays in the various movements of the eye (laws of Donders).

In looking horizontally straight forwards, outwards or inwards, the vertical meridian of the cornea is not inclined; it remains vertical. In looking straight forwards all the muscles of the eye are in equilibrium; there is no deviation of the cornea or inclination of the meridian. The external rectus is sufficient to enable us to look horizontally outwards, for we have seen that its action is to turn the eye outwards without inclining the vertical meridian ; it therefore fulfills the necessary conditions of this movement. Similarly, to look horizontally inwards the internal muscle suffices.
To look vertically forwards, upwards or downwards, the vertical meridian is not inclined; it remains vertical. In looking vertically upwards, the superior rectus must come into play; but, as we have already seen, its contraction not only turns the eyeball upwards but also inwards, and it inclines the vertical meridian inwards. Therefore, in turning the eye vertically upwards, there must be a second force which counterbalances the subsidiary effects of the rectus superior ; the inferior oblique alone does so, directing the eye upwards and slightly outwards, whilst it inclines the vertical meridian slightly outwards, and thus counteracts the subsidiary movements of the superior rectus. Therefore, in looking vertically upwards, the movement is executed by the combined action of the superior rectus and inferior oblique, which acting together lift the eye straight upwards. Similarly, in looking vertically downwards, for reasons which need not be repeated, we combine the actions of the inferior rectus and superior oblique muscles.
In looking obliquely upwards and to the left, the vertical meridians of both eyes are inclined parallel to each other and to the left ; that of the left eye outwards ; that of the right inwards. For the execution of this upward and outward movement of the left eye, we must first take into account the superior and external recti muscles; but the combined action of the two cannot incline the vertical meridian outwards, for, whilst the external rectus does not influence the inclination of the meridian, the superior rectus, on the other hand, turns it inwards. III this movement, therefore, some other muscle must be brought into action, which not only counteracts the intrnal inclination of the superior rectus, but also inclines it outwards', and thus procure the parallelism of the meridians III both eyes. The inferior oblique alone can have this effect, since it turns the eye upwards, and so far is associated with the superior rectus. But, in addition, it inclines the vertical meridian outwards, which latter effect is all the more marked when the eye is turned outwards by the external rectus, for in that position the influence of the oblique muscles on the inclination of the vertical meridian is more pronounced. In looking obliquely outwards and upwards the eye is moved by the combined action of the external and superior recti and the inferior oblique.
In looking obliquely downwards and to the left, the vertical meridians are inclined in parallel lines to the right ; that of the left eye inwards, that of the right outwards. To execute this movement with the left eye, we have first of all the action of the external and inferior recti ; but, as the first of these muscles has no influence on the meridian, and as the other inclines it outwards, there must be, as in the preceding case, a third muscle which produces the necessary inclination of the meridian ; this muscle is the superior oblique, which alone has this effect. The movement necessary to look downwards and outwards is then executed by the combined action of the inferior and external recti and the superior oblique.
In looking obliquely upwards and to the right, the vertical meridians are inclined in parallel lines to the right ; that of the right eye outwards, that of the left inwards. To perform this upward and inward movement, we have first of all the superior and internal recti; but the inclination of the meridian inwards, produced by the action of the first, would be too great compared with the outward inclination of the other eye to allow the necessary parallelism to be preserved. A third muscle, therefore, must control the action of the superior rectus. It is the small oblique which exerts this influence on the meridian, and as the eye is in such a position (inwards) that the action of the oblique muscles on the vertical meridian is very slight, the effect of the inferior oblique cannot be too great. To look upwards and inwards we then require the combined action of three muscles viz., the superior rectus, the internal rectus and the inferior oblique.
To look obliquely to the right and downwards, the vertical meridians of the two eyes are inclined in parallel lines to the left ; that of the right eye inwards and that of the left outwards. To execute this movement of the left eye downwards and inwards we have the inferior and internal recti ; but the action of the first on the inclination of the meridian would be too great to allow the meridians to remain parallel to each other: this action is limited, however, to the superior oblique. To look obliquely downwards and inwards the combined action of three muscles is then required viz., the inferior and superior rec~i and the superior oblique.

ART. I. Paralysis of the Muscles of the Eye.

A. GENERAL CONSIDERATIONS.

Affections of the innervation show themselves by a diminution of the muscular contractility, which may be only diminished or totally destroyed : hence we may distinguish various degrees in this affection, varying from a simple insufficiency to a complete paralysis. The number of muscle affe It'd differs a( cording 14) the of nerves affected.

If the disease takes hold of the sixth pair, the symptoms are manifested in the external recti ; if the fourth pair be attacked, the superior oblique alone will be paralyzed ; if the third pair, either one, or several, or even all the muscles supplied by this pair will be affected. In some cases the paralysis supervenes simultaneously in several pairs of nerves.

Every paralysis of a muscle is at first manifested by a diminution of the mobilit of the eye in the direction in which that muscle acts in the normal condition. Yet we must not forget that there may be a loss of mobility without paralysis (as in cases of orbital tumor and symblepharon), and an incomplete paralysis without apparent loss of mobility. The absolute and normal mobility of the eye is subject to considerable physiological variation. As a general rule, we estimate that, in a healthy eye, the strongest adduction, (rotation of the eye towards the nose) will turn the eye so far in, that the internal margin of the pupil is bidden behind the caruncle, whilst in extreme abduction (rotation of the eye towards the temple) only the margin of the cornea reaches to the external commissure.

We therefore begin by examining the absolute mobility of the eye which is supposed to be affected, verifying our observation by comparing it with the mobility of the other.

On making this examination, when one muscle is completely paralyzed we shall find that the eyeball can no longer be turned in the direction in which that muscle acts. If the paralysis is less complete, the eye may be more or less directed in this direction ; when it attains the limit of its movement, we shall find that its efforts are terminated by a spasmodic movement which rapidly exhausts the muscular power still available. We must distinguish the spasms which take place in the direction of the affected muscle from those by which the other muscles strive to replace the function of the I enfeebled one.

A second important sign is obtained by the examination of the associated movements of the two eyes. To move in a given direction, the same nervous impulse is given to the two eyes. If, then, there exists a muscular paralysis in one eye, the degree of innervation which suffices for the healthy side does not suffice for the diseased one. The optical axis of this eye is no longer directed towards the object of fixation ; it deviates towards the side opposite to the paralyzed muscle ; thus we have a _paralytic strabismus. This deviation of the eye will naturally be the more marked the farther the patient looks in the direction in which the muscular power is absent. If, then, we cover the healthy eye with the hand, the other will be obliged to fix itself by making a more or less extensive movement of rotation in the direction' of the paralyzed muscle ; this symptom is decisive in any case where it is difficult to determine which is the abnormal side.

In performing this experiment, we notice in the second place another characteristic phenomenon. After having hidden the healthy eve behind the hand, and brought the affected one to a point of fixation, if we examine the position of the healthy eye under the hand, we notice that it has also moved in the same direction as the paralyzed eye, but that the amount of its movement is twice, thrice, or four times greater (secondary deviation). In order to adjust the eye, the paralyzed muscle must make a certain effort.

This effort, which is accompanied in the healthy eve by the identical nervous impulse, must necessarily produce on it a much greater effect, and the more complete the paralysis of the affected eye the greater will be the effect on the healthy side.

A third symptom of every muscular paralysis of the eye is a marked deficiency in the projection of fixed objects, that is to say, an inability to determine their exact situation in space. Thus, if, shutting the healthy eye, we ask the patient to place the tip of his finger very rapidly on any object whatsoever (the surgeon's finger, for example) situated on the side of the paralyzed muscl a, without turning his head, the patient's finger will always pass on the side corresponding to the paralyzed eye; a circumstance which is easily understood. For we judge of the posi¬tion of objects in space by the amount of effort necessary to direct the optical axis on any object ; thus, if a muscle is paralyzed, the nervous impulse necessary to fix an object is greater than in the normal condi¬tion. Consequently, the patient estimates it at a greater distance on the paralyzed side than it is in reality. It is well to remember that these movements must be performed rapidly, for if not, the patient can correct his judgment as he advances his finger. On this false projec¬tion depends the vertigo, of which such patients complain when they close the healthy eye, which form of vertigo must be carefully distin¬guished from that other which is felt when both eyes are used, and which depends on the presence of diplopia. Monocular vertigo, if we may so express it, is much more marked. when several muscles are affected.

Diplopia that is, a disturbance of binocular vision is another important symptom of muscular paralysis. If the deviation is one of convergence , the two images are homonymous ; the image situated on the right hand is seen by the right eye, and the image on the left, by the eye of the same side. Thus, in excessive convergence of one eye (see Fig. tile image of tile object, it, is formed on the internal side of the right retina at it', and is projected, according to tile laws of reflection, at a". ]I* Hit deviation is divergent ( Fig. 185), tile images are crossed, that is to say, the right image is perceived by the left eye, and the left image by the right eye, for (Fig. 185) the image of the point, a, formed on the external part of the right retina at a', is projected to a". For the same reasons a deviation downwards will produce a more elevated image than is perceived by the eye at the normal level, as also a deviation upwards must produce an image at a lower level. When the deviation is in a diagonal direction for example, upwards and outwards the respective position of the double image is analogous.

The distance which separates the two images will be in direct proportion to the distance separating the macula, from the image in the deviating eye. Consequently, this distance depends on the degree of deviation, and will increase the farther the patient tries to look in the direction of the paralyzed muscle.

When the deviation is very small, as in cases of slight muscular insufficiency, the diplopia is often concealed, because the images are in part blended with each other; only objects appear larger than they are in reality, and their margins present diffusion circles, which destroys the definition of their outlines. As the deviation increases, the image of the affected eye becomes further separated from that of the other. and the separation may be so great that the patient entirely suppresses the impression in the deviating eye; which impression, moreover, becomes less intense as it approaches the periphery of the retina. When this takes place the patient no longer complains of diplopia.

To aid the patient in distinguishing the two objects and the distance which separates them, it is well to place a piece of colored glass before the healthy eye. We may also make use of prismatic glasses in detecting a concealed diplopia. Thus, if we place a prism so that it refracts vertically, one of the two images is superimposed, and the patient can then easily indicate their lateral distance from each other. Prisms are also of great use, in cases of muscular paralysis of the eye, in verifying a diagnosis which has been founded on the symptoms indicated. When we have detected binocular diplopia, a prismatic glass diminishes the distance between the two images, or unites them, by bringing the luminous rays into the neighborhood of the fovea centralis, or even on to it. It is easily seen that the exact position in which we must place the prism before the eye indicates the direction of the deviation which has followed the muscular paralysis.

A characteristic sign of diplopia, and consequently of muscular paralysis, is that the patient prefers to keep one eye closed, and is unable to move about with both eyes open.

Persons affected with muscular paralysis of the eye also hold their heads in a peculiar way ; they do not place objects at which they are looking directly before them. In fact, such patients instinctively place objects in the part of the visual field in which they do not see double, turning their heads to the side. We shall see that certain paralyses involve a very peculiar and almost pathognomonic position of the ]read.

In conclusion, we should mention that, according to a physiological law based on custom, our eyes converge if we look downwards, whilst they diverge when we look upwards. Consequently the symptoms of muscular paralysis will be modified according to the direction of the eyes. If the paralysis cause divergence, the divergence will be increased when the patient looks upwards, and, conversely, a pathological convergence will increase when he looks downwards.

B. SYMPTOMS OF THE VARIOUS FORMS OF MUSCULAR PARALY. SIS OF THE EYE.

1. Paralysis of the Sixth Pair.
(EXTERNAL RECTUS.)
(In the succeeding sections we shall speak of the left eye ; the deductions which we are about to make may easily be applied to the right eye.]

(a) Complete Paralysis. Beginning with the median line, the eye remains completely immobile if we attempt to make it move towards the temple. Yet, oil careful observation, we generally find a slight movement of abduction only, this movement does not take place directly outwards, Nit either outwards and upwards, or outwards and downwards. This movement is the product of the contractions of the oblique muscles, which thus seek to supply the place of the external rectus. If now we cause both eyes to fix on an object held to the left, and to the outside of the median line, the right eye will follow it perfectly, but the left will remain behind ; we, therefore, have a convergent strabismus, which increases the farther the object is carried to the left ; and along with this there is homonymous diplopia.

In studying the phenomena arising from diplopia, we generally use with great advantage a deep violet glass, which is held before the healthy eye ; the patient looking with both eyes at a lighted candle, placed three or four feet in front of him. This glass serves a triple purpose : ist, It clearly differentiates the two images, so that it is always easy for the surgeon to know which is the image seen by the left eye and which by the right, and thus he makes sure of their respective positions ; 2d, the image of the paralyzed eye, being formed on an eccentric part of the retina, is not so easily seen as that of the normal eye, so that the colored glass, by diminishing the intensity of the image of the healthy eye, renders the perception of the two images more easy; A again, the glass considerably obscures the visual field, and thus allows the eye to concentrate its attention on the candle flame.

On placing a violet colored glass before the right eye, the patient will see a violet flame to the right and a white one to the left. The distance between the two flarnes is increased as we carry the candle to the left, but both remain exactly in the same plane) at the same height.

We may cause the distance between the two objects to disappear, by using a prism with its base turned outwards ; for, as we have seen, the retinal image of the affected eye is formed internally to the yellow spot ; consequently a prism, with its base outwards, turning luminous rays outwards brings them on to the macula. In making this examination, we may also place before the other eye a prism having its base upwards or downwards, so as to make a difference in the height of the two images, for by so doing we prevent the patient from making any effort of fusion ; the prism, which brings the one image exactly on the top of the other, gives the correct measurement of the degree of deviation.

By reason of the physiological convergence and divergence, which, as we have said, take place when the patient looks upwards and downwards, the line which separates that portion of the visual field in which the vision is simple from that in which it is double, will be inclined from above downwards, and from within outwards. In cases of partial paralysis, moreover, the line of demarcation of which we speak, will vary with the point at which we begin our examination. If we begin with a point at which vision is simple, the tendency to fusion will be more felt, and the paralyzed eye will make every effort, until a point is reached at which it is obliged to yield. If, on the other hand, we begin at a point where the diplopia is already manifest, the muscle in question does not experience any special stimulus to contraction, and the diplopia will be present at a line somewhat nearer the median line than in the former case.

Sometimes we find a contraction of the muscle antagonistic to the paralyzed one. That contraction of the internal rectus which accompanies the paralysis of the external is sometimes developed in the early stages of the disease, sometimes later ; in other cases it is never present. Its presence is detected by the abnormal increase of the convergence, as well as by the extension of diplopia into the other part of the field of vision.

The special symptoms of secondary deviation and false projection manifest themselves in the following way: If we cover the eye which is supposed to be healthy with the hand or a piece of ground glass, whilst the left eye is directed to a point situated on the left, we notice great convergence of the right eye, much more considerable than that of the left one when the healthy eye fixes the same point. This very pronounced strabismus is at once observed in cases where the non paralyzed eye was previously affected with amblyopia, and in which the paralyzed eye was used for purposes of fixation.

The false projection always takes place to the outside of the fixed object (on the same side as the paralyzed muscle). Again, the patient will rotate his head outwards (to the left) round its vertical axis, and will hold objects at which he is looking to the right. He will thus replace the action of the paralyzed muscle by the rotation of his head, and will at the same time bring in front of him the part, of the visual field in which he sees objects single. (b) Incomplete Paralysis. In this, the symptoms are fundamentally the same as in complete paralysis, but not so well marked. The mobility of the eye is only reduced, and towards the external limit of its movement we shall find the jerking contractions of which we have spoken.

We may more particularly notice the characteristic movements of readjustment which take place when, causing both eyes to fix on some object, we cover the healthy one the diseased eye, in order to keep the object fixed, makes a small excursion outwards, and the other eye undergoes a secondary deviation to a much greater extent.

Diplopia occurs only when the point of fixation is at the cxtrenic left. It is important to measure the amount of deviation in this class of cases with the prism so that we may ascertain my improvement or aggravation of the paralysis. (Muscles affected: Internal rectus, superior rectus, inferior rectus, inferior oblique, levator of the upper eyelid, sphincter of the iris and ciliary muscle.]

(a) Paralysis of the Internal Rectus. When this muscle is paralyzed, we find an inability to move the eye inwards, divergent strabismus and crossed diplopia affecting the entire internal (right) portion of the field of vision. When we cover the healthy eye, the diseased one makes an inward movement of readjustment, and at the same time we may notice the secondary deviation of the right eye outwards. The false projection takes place inwards (to the right) of the object. To avoid the diplopia, the patient rotates his head to the right, and also holds objects at which he is looking to his left. The distance between the two images diminishes when a prism is placed before the eye with its base inwards, and if the prism is of the proper strength, the diplopia entirely disappears.

(b) Paralysis of the Superior Rectus. When this muscle is paralyzed, we have, conformably with its physiological action, a deficiency in the movements upwards and inwards, and a loss of power in the rotation of the vertical meridian inwards. Consequently, when the patient looks upwards, the eye will deviate downwards and outwards, and the vertical meridian will turn outwards.

On placing an object in the superior half of the field of vision, we shall find a cross diplopia, and the image belonging to the diseased eye will be at a higher level than that of the other one. The superior extremities of the two images diverge. When the healthy eye is hidden, the secondary deviation takes place upwards and outwards, since the affected eye is forced to make a considerable effort upwards and inwards to gain the fixation point.

The patient rotates his head on the horizontal axis backwards, and holds objects in the inferior half of the field of vision. A prism, with its base upwards and slightly inwards, will bring the two images closer together, and, if of sufficient power, will cause their union.

The influence of the paralysis on the height and on the inclination of the meridian varies with the position of the eye ; if the eye is directed outwards, the axis of rotation corresponds with the transverse diameter of the eye, and therefore the influence of the paralysis on the height of objects will be at a maximum but on the inclination of the meridian will be nil ; the opposite is the case if the eye is directed inwards. In looking upwards and inwards, the distance between the two images is much diminished, but the inclination of tile image belonging to the left eye is very marked. This point is of great importance in distinguishing paralysis of the superior rectus from that f the inferior oblique.

(c) Paralysis of the Inferior Rectus. We find a deficiency of movement downwards and inwards, and an inability to rotate the meridian outwards. If the healthy eye be hidden, the other makes a movement of readjustment upwards and inwards; and the secondary deviation takes place upwards and outwards. The diplopia is crossed, the image of the affected eye is the lower of the two, and the extremities of the images converge towards each other. The image belonging to the affected eye seems to be nearer the patient than the image of the healthy side. The lateral distance between the two images increases as the fixed object is carried directly from above downwards ; the difference in height increases when the object is carried towards the side of the paralyzed eye ; lastly, the obliquity of the images increases when the object is carried towards the side of the healthy eye.

This form of paralysis is very troublesome to the patient, as he has to keep his head down and hold objects above him. A prism, with its base downwards and slightly inwards, will bring the double images nearer each other, and, if of proper strength, will unite them.

(d) Paralysis of the Inferior Oblique. The affected eye deviates downwards and inwards. When the healthy eye Is covered, the other readjusts itself by a movement upwards and outwards. Consequently, the secondary deviation of the healthy eye takes place upwards and inwards. The double images are situated in the superior part of the visual field, their superior extremities diverge and the diplopia is homonymous. The divergence of the images increases if the patient looks upwards and outwards, the difference in their height is specially marked when he looks upwards and inwards. A prism, with its base upwards and outwards, will bring the two images together, and unite them into one.

Paralysis of this muscle alone is moreover exceedingly rare.

When the paralysis of the third pair is complete, we have, in addition to the signs already mentioned, drooping of the superior eyelid, due to paralysis of the levator muscle. If we raise up the lid, the pupil is found to be partially dilated (see article Mydriasis, p. 185), and immobile from paralysis of the sphincter of the iris. Lastly, the accommodation of the eye is reduced, being sometimes altogether absent (paralysis of the ciliary muscle, p. 407). Sometimes we find a slight exophthalmos Which is caused by the diminished tension of tile muscles which draw the eyeball backwards, of which three are supplied by tile motor oculi nerve.

In the early stages of the paralysis the eyeball does not seem to be deviated when the patient looks directly before him, but soon the predominating action of the external rectus draws it towards the temple. The eye can be directed inwards only very imperfectly, and scarcely beyond the middle point of the palpebral fissure. The eye cannot be turned upwards at all, and can only be turned downwards by the superior oblique; consequently the movement is very imperfect, and is accompanied by a well marked inclination of the vertical meridian inwards.

The objective symptoms of complete paralysis of the third pair are so characteristic that it is almost superfluous to examine the diplopia. The images are crossed, and the distance between them increases the farther the fixed object is held to the side of the healthy eye. When the patient looks upwards, the image of the diseased eye is above the other, but is below it when be looks downwards.

If the patient attempt to walk with only his affected eye open, be will suffer from vertigo (caused by false projection of his field of vision) to such an extent that be will stumble and be obliged to stop; generally, however, the drooping of the upper eyelid will protect him from the inconveniences of this vertigo, as well as from diplopia supervening when both eyes are open.

3. Paralysis of the Fourth Pair.
(SUPERIOR OBLIQUE.)

The deviation of the affected eye is only felt when the fixed object is situated in the inferior half of the field of vision. The eye is turned upwards and inwards. When the healthy eye is covered, the other is rotated from above downwards and from within outwards; and this will increase in proportion as the fixed object is carried farther to the side of the healthy eye. The secondary deviation takes place downwards and inwards.

Diplopia is only manifest in the inferior portion of the field of vision, and is, consequently, very annoying when the patient ascends or descends a staircase, or walks on a narrow footpath*. The double images are homonymous; the image belonging to the healthy eye is found above the other, and their superior extremities converge. This obliquity increases when the fixed object is carried towards the side of the diseased eye, whilst the lateral distance between the two images and the difference of their level diminish. When the fixed object is held on the same side as the diseased eye, the lateral distance between the two images decreases, whilst the difference in height. is greater.

The image coming from the affected eye always seems nearer the patient than that coming from the healthy eye.

A prism with its base downwards and outwards will bring the double images nearer each other. The patient keeps his head down and inclined towards the side of the healthy eye, in order to avoid, as far as possible, the inconveniences of the diplopia.

When, at a later period, paralysis of the superior oblique is complicated with retraction of its antagonist, the inferior oblique, the diplopia, extends to the superior half of the field of vision. The images in this situation, however, are crossed, which circumstance is due to the excessive action of the inferior oblique increasing the outward deviation of the healthy eye. The difference in height of the two images increases when the fixed object is carried towards the healthy side ; their obliquity increases in the opposite direction.

C. PROGRESS AND TERMINATION OF PARALYSIS OF THE OCULAR MUSCLES.

The progress of the different forms of paralysis varies with the degree of the paralysis and with its cause. The forms which are of central origin are, as a rule, slower in passing off and more difficult to treat successfully than those which are due to some peripheral influence.

Paralysis may end in various conditions, in the enumeration of which we shall begin with the most favorable

Complete restoration of mobility.
Incomplete restoration of the muscular power.

In these two sets of cases, the disease, during its entire course, may be restricted to the paralyzed muscle. But it may occur also that its antagonist, freed from a portion of the resistance which it generally encounters, tends to retract. If this muscular tension lasts for a certain time, it may produce a permanent contraction, with all the symptoms of concomitant strabismus. We may then have to deal with one of the following conditions:

The paralysis is cured ; but while it lasted, the antagonist of the paralyzed muscle has become shorter, which causes a slight deviation in the direction of that muscle. This deviation, being very slight, may be overcome by muscular exercise (dynamical strabismus).
Tile shortening of the antagonistic muscle which has supervened during the period of the paralysis may be so great that the patient, even after the paralysis has quite passed off, is no longer able to overcome the contraction by muscular effort We have, therefore, a. permanent deviation with all the Symptoms of concomitant strabismus.
The contraction of the antagonist may take place although the paralysis is not completely cured ; so that we have at the same time the symptoms of paralysis of one muscle and those of contraction of its antagonist.
The last condition consists in 'complete paralysis of one muscle and exceedingly firm contraction of the other. The eye then follows the latter, and remains immovable in the angle of the same side. This condition is called _paralytic contraction.

D. PROGNOSIS OF PARALYSIS OF THE OCULAR MUSCLES.

The prognosis is much more favorable when the paralysis is of peripheral origin than when of central. In the former case it often completely disappears, except in cases where there is a lesion of the nerve or other cause which cannot be expected to be cured or pass off e. g., a tumor invading or pressing on the nerve.

The prognosis of paralysis of the third pair is less serious if the disease is of recent origin, and if it does not involve many muscles.

Paralysis of the sixth pair, although as a rule easily cured, often produces a contraction of the internal rectus, which leaves a permanent convergent strabismus.

When the paralysis is not due to some peripheral condition, we must be much more guarded in our prognosis; for it must not be forgotten that such ocular affections are often prodromatic, or appear in the course of serious diseases of. the nervous system.

E. ETIOLOGY OF PARALYSIS OF THE OCULAR MUSCLES.

The causes of paralysis of the muscles of the eyeball are either peripheral or central. The first are of two kinds, affecting the motor nerves either directly, or secondarily to neuralgia of the orbit and its neighborhood, in which case the motor nerves are influenced in a reflex manner. The want of exercise in a paralyzed muscle may then give rise to some disturbance of its nutrition, to atrophy with or without fatty degeneration, which may so modify the structure of the muscle that it. is unable to perform its functions, even after the primary cause has disappeared. It has not yet been proved that inflammation or over exertion has ever produced symptoms of paralysis of an ocular muscle.

If we leave aside influences which only exceptionally give rise to muscular paralysis, such as alcoholism, diphtheria, hysteria, facial neuralgia, etc., we find that the others may be grouped into three great classes:¬

I. Rheumatic affections.

2. Syphilis.

3. Affections of the nervous centres.

I. The rheumatic origin of the paralysis is easily recognized, when there are also present other affections produced by the same diathesis, or there is a history of a sudden and prolonged chill.

2. We must suspect a syphilitic origin when we detect specific symptoms in the previous history of the case. Paralysis of the ocular muscles is generally a tertiary symptom, rarely a secondary. A common cause of the paralysis is periostitis, exostosis or gummatous tumors, and still more frequently granular tubercles in the course of the nerve. Paralysis of syphilitic origin is relatively more frequent in the third pair.

3 In paralysis of cerebral origin the diagnosis is rendered easy by the presence of other concomitant symptoms. We often find an affection involving several muscles of the eye supplied by different nerves; while, at the same time, there is hemiplegia, characteristic headaches, vertigo, or a diminution of the intelligence. Besides, experience shows that the diplopia of paralysis of central origin persists, notwithstanding the means used to produce fusion of the images. If, after a very careful selection of the proper prism, we succeed in uniting the images, the least displacement or the slightest difference in the angle of the Prism brings back the diplopia.

Paralysis of the muscles of the eye is often met with in locomotor ataxy, and it is not unfrequently one of the first symptoms of cerebrospinal disease. The amount of paralysis is sometimes so slight that it can only be detected by the diplopia which accompanies it. A proper application of the physiological laws, relative to the action of the muscles and to the projection of the images, allows us to determine. easily, from the position of the double images, the presence of incipient affections ; and their clinical significance, in certain cases, is of great assistance in the diagnosis of the general affection.

As to the localization of the cerebral disease, it cannot always be indisputably settled by the special form of muscular paralysis present ; still there are certain rules, deduced from the observation of a great number of cases, which are of great use. Paralysis of the external rectus and the superior oblique, of central origin, generally depends on an affection of the opposite hemisphere; whilst paralysis of the third pair is connected with the hemisphere of the same side. Paralysis of the muscles of both eyes is due to affections of the corpora quadrigemina and of the pons.

Complete paralysis of it nerve indicates that the lesion is situated at it point where the nerve fibres are already united into one trunk, that is to say, near the base of the cranium ; if the cause is situated in the central parts, or near the origin of the nerve, the disease must be very extensive to involve all the fibres of the nerve. Again, it must not be forgotten that cerebral hyperemia, either active or passive, may of itself be sufficient to produce paralysis of the ocular muscles. It is also often due to basilar tubercular meningitis and pachymeningitis.

Paralysis of the muscles of the eye is frequently a symptom of disease of the spinal cord, often appearing long before the other symptoms. It is then characterized by a passing disturbance, sometimes in one muscle, sometimes in another ; sometimes even the two eyes are affected alternately.

Lastly, we should mention congenital paralysis and the isolated cases of complete paralysis of all the ocular muscles (nuclear paralysis).

F. TREATMENT OF PARALYSIS OF THE OCULAR MUSCLES.

The treatment employed must be a natural sequence of the cause of the disease. It consists of: i, The administration of suitable remedies; 2, the use of prismatic glasses and orthopedic treatment ; 3, surgical interference. If the disease be of rheumatic origin, this primarily involves that the patient should avoid everything which may cause a chill, and should protect the affected side of the head. In the early stages, the treatment must be antiphlogistic and derivative (antimony in nauseating doses, salicylate of soda, iodide of potassium, sudorifics, fly blisters in the neighborhood of the affected eye). After the inflammatory symptoms have subsided, electricity may be of great service. In syphilitic paralysis, all the anti syphilitic remedies, beginning with mercurials and iodide of potassium, are of use. The treatment of cases originating in cerebral disease includes the administration of such remedies as are suitable in that affection.

To prevent the diplopia which is so annoying to the patient, and which may occasion severe headache and vertigo, spectacles may be worn in which the diseased eye is protected by a piece of ground glass.

Prismatic glasses may be used for a twofold purpose : i, To relieve the patient temporarily of the diplopia ; 2, to exercise, and thus to strengthen, the weakened muscles. In dealing with diplopia, the strength of the prism and its direction must naturally depend on the amount of deviation and on the muscle which is paralyzed. Speaking generally, the prism should be placed before the eye so that its apex is in the same direction as the deviation, pointing outwards in divergence, inwards in convergence, upwards when the eye deviate,; upwards, and vice versa If the double images show both a vertical and horizontal deviation, it may be corrected by putting a prism horizontally before one eye, and another prism vertically before the other.

In the same way, when we are dealing with a lateral deviation which requires a prism of 10 for its correction, we may divide the effect of this prism by placing before each eye one Of 5'. In no case should we use a prism of more than 6' or 7' for each eye. This difficulty, and the variation in the two images according to the direction in which the patient looks, explain why, in most cases, it is impossible permanently to give prismatic glasses for the purpose of correcting the diplopia.

To strengthen the paralyzed muscle by means of prisms, we must first ascertain the prism required to completely neutralize the diplopia. This being found, we must try the effect of placing a somewhat weaker prism before the eye. The two images are then brought very near each other, and the patient may be able to obtain single vision by uniting them with an effort of the affected muscle. This is the very effort which we wish to excite in order to give exercise to the muscle in question. If the fusion of the two images, when placed near each other, does not take place, we must give up the idea of orthopedic treatment. If, however, fusion is produced, we leave the patient to exercise his eye with this prism, which will soon of itself be sufficient to counteract the diplopia without any muscular effort. We then exchange it for a weaker prism, and so on till the deviation is cured. The selection of prisms requires great care; a prism which is too weak fatigues the muscle instead of strengthening it, and one which is too strong will increase the contractions of the antagonist, and, consequently, the deviation of the eye.

Michel recommends orthopedic treatment consisting of passive movements of the paralyzed muscle. A fold of the conjunctiva, near the cornea and scleral intersection, is held with the fixation forceps, and the eye drawn as far as possible, several times in succession for about two minutes, in the direction of the muscle. These manipulations are intended to counteract the contraction of the opposing muscle, and to prevent atrophy of the diseased one by imitating its natural contractions. We frequently make use of it after anesthetizing the conjunctiva with cocaine. Electricity may be used in connection with this treatment.

When the treatment has given all the relief which can be expected from it, and there remains a certain amount of deviation and diplopia, surgical interference may be required. In the most simple cases, in which the loss of mobility does not exceed 3 or 4 millimetres, we may rectify it by advancing the affected muscle ; when tile loss of mobility is about 5 or 0 millimetres, muscular advancement, combined with tenotomy of the antagonist, and, if necessary, followed by tenotomy of the internal or external rectus of the other eye, brings the two eyes parallel, and restores simple binocular vision. Vertical deviations should be rectified by advancement of the muscle affected or by tenotomy of those muscles of the healthy eye which contract simultaneously with the affected muscles. If the deviation be very great, this operation must be followed by tenotomy of the antagonistic muscle. In no case should the oblique muscles be divided. The details of the operation will be explained when we speak of the operations for strabismus.

Any surgical interference of this kind should only be had recourse to when the paralysis is already of old standing, and when the deviation and diplopia have been stationary for a length of time. An operation performed too soon or inadvisedly, although at first apparently satisfactory, will soon be found to be insufficient, or still worse, if the paralyzed eye recover, it will give rise to a deviation in the opposite direction.

ART.II. Spasm of the Ocular Muscles. Nystagmus.

Tonic spasm of the ocular muscles is one of the rarest affections of the eye. It almost never occurs idiopathically, but only as a symptom of certain cerebral diseases. The same is true of conjugated spasmodic deviations of both eyes (and of the head). The deviation is towards the same side as the disease in affections of the hemispheres, and to the opposite side in affections of the pons and cerebellar peduncles (Prevost).

Nystagmus consists of constant oscillatory movements of the eyeball, most frequently in the direction of the lines of action of the external and internal recti muscles. In a few cases the movement is rotatory, and still more rarely is it vertical. It has sometimes been found that the movement is slower, or even stopped, when the patient looks in a given direction. Nystagmus disappears during sleep, but the movement increases or, when periodic, rapidly supervenes under the stimulus of emotion.

Vision is almost always very weak in cases of nystagmus. Still, cases are met with where it is sufficiently good to allow the patient to follow his ordinary work, or even to read. Sometimes the patient tries to compensate the effect of the ocular movement by movements of the head in the opposite direction.

The Etiology of this affection is not perfectly understood.' It TREATMENT OF NYSTAGMUS. most frequently comes on in the first years of life with congenita a amblyopia, microphthalmia, coloboma of the choroid, albinism, opaci ities of the cornea, cataract, etc. In all such cases, the predisposing cause seems to be a loss of then I power of fixation. Yet the number of observed cases of congenita a amblyopia without nystagmus evidently shows that it is not of itsel sufficient to produce the disease. It is more than probable that then r( must also be some disturbance of the muscular equilibrium, most likrly I of the internal and external recti. Thus nystagmus has been see;to attack colliers, whose work in a defective light often necessitates very inclined position of the head, so that they only use one eye at the same time this eye is kept at extreme abduction, which rapidl, 11 fatigues the external rectus, and obliges it to perform rhythmical move e ments, in order to bring the eye as often as possible into the latera a position which the collier's work requires. These rhythmical move e ments are accompanied by similar movements of the other eye, an( if this condition lasts for some length of time the nystagmus is found to be perfectly established.

Whilst in cases of congenital nystagmus, or in nystagmus super vening in infancy, the displacement of the retinal images is not per ceived by the patient, this phenomenon forms a prominent feature in I collier's nystagmus, especially in the early stages. Objects seem to ,( dance before the patient, causing a disagreeable vertigo and a sensation of nausea akin to sea sickness. Another cause of acquired nystagmus has been shown to be cerebra sclerosis (Cheroot), and this symptom may be compared to the trem bling movements of the hands which are seen when the patient attempt: to perform any given movement. These symptoms are not presen when the patient is at rest.

Congenital nystagmus, or nystagmus supervening in the early year of life, is often accompanied by strabismus.

Treatment. Various attempts have been made, with little suc cess, to treat nystagmus by regular exercise of the ocular movements beginning with the direction in which the nystagmus disappears, or i considerably reduced. These exercises should be preceded by the us of such glasses as are adapted to the anomaly of refraction with whicl the eyes are affected (frequently astigmatism). If there are specific of the cornea or of the lens, we must make a passage sufficient for the transmission of luminous rays. In cases complicated with strabismus and even in certain cases where there is no strabismus, tenotomy o the muscles which are affected with chronic contrac.tions has been proposed (Bochin). The constant current, as also injections of strych nia, have likewise been recommended. The wearing of' blue tinted glasses often seems beneficial. But, in estimating the efficacy of such remedies, we must not forget that nystagmus often becomes much less in adult life, and may even completely subside.

ART. III. Strabismus.

A. GENERAL SYMPTOMS AND DIFFERENTIAL DIAGNOSIS.

In normal circumstances, the two eyes are so directed that the optical axes meet on the object at which we may be looking; but if a person affected with strabismus looks at an object, one of his eyes will be directed towards it, but the prolongation of the optical axis of the other will pass to its side.

In many cases the false direction is so obvious that it can be detected at a glance ; but there are other cases in which we can detect the want of 'symmetry without being able to say which is the deviating eye. We must then ask the patient to fix a point for instance, the tip of the finger and close one eye after the other. The eye which retains its previous position was normally directed on the fixed object ;, the other, compelled to fix the object, readjusts itself that is to say, brings its optical axis into the necessary position for distinct vision of the object placed before it. The direction in which it moves also indicates in what direction the eye was deviated ; if it moves inwards, the eye deviates outwards; if upwards, then the eye is displaced downwards, and vice versa.

We have just seen that, a point of fixation being given, on covering the healthy eye with the hand the squinting eye readjusts itself. If we watch the healthy eye behind the hand, we find that it changes its direction, and moves in conjunction with the other eye. Thus, if we are dealing with a convergent strabismus of the left eye, and cover the right one with our hand, the left readjusts itself by moving outwards, and we can at the same time see an associated inward movement of the right. It will also then present a convergent strabismus, which has received the name of secondary deviation. The degree of the secondary deviation is exactly the same as that of the primary strabismus.

In paralytic strabismus, the secondary deviation is much greater than the strabismus, for reasons explained in speaking of that affection.

If we now continue to keep the healthy eye closed, and cause the other to rotate in all directions, we shall find that it moves perfectly freely ; its mobility is only slightly increased towards the deviation, and a little diminished in the opposite direction, but is in amount exactly the same as that of the healthy eye.

Let us suppose, for example, a case of convergent strabismus of the left eye i Fig. 186), and let us measure the mobility of each eye separately. We find that the healthy eye can be directed inwards till the external margin of the cornea reaches the point a, and that it may be turned outwards till this margin reaches the point be The left eye, which is squinting, in extreme inward rotation goes a little farther than the point a', but in the opposite direction it stops a little sooner than the other, its corneal margin falling short of the point P. We may therefore say that the mobility of the squinting eye is slightly displaced in the direction of the strabismus, but its extent is the same as in the normal eye.

In paralytic strabismus, the mobility is diminished on account of the deficient action of the paralyzed muscle.

When the patient, keeping both eyes open, looks in various directions, the squinting eye moves with perfect freedom in conjunction with the healthy eye.

On studying this symptom, which has caused this variety of strabismus to be called concomitant, we also find that the degree of deviation is the same for all movements.

In the normal condition, the two eyes, having their optical axes at first perfectly parallel, preserve this parallelism for all lateral movements, because the same amount of nerve influence acts on the muscles which set the eyes in motion. Hence, there is a perfect harmony of all associated movements. Thus, for example, if (Fig. 187) tile left eye be turned so that the anterior pole of its optical axis (a, o) is directed towards the distant point, b, the other eye will accompany this rotation by a similar movement which preserves the parallelism of the two eyes.

III concomitant strabismus, the squinting eye accompanies the movements of its fellow, and to a like extent (the innervation being intact). Bat since the optical axes are not parallel before the movement begins, they are no more so when it is completed. For example, in the variety of strabismus represented in Fig. 188, the vertical axis of the right eye is deviated inwards, whilst that of the' left eye is directed straight forwards. If now the healthy left eye be turned towards the right, so that its Optical axis looks at the point b, the other eye will be turned as much towards the right. The extent of movement will be the same, and consequently the eyes will squint as previously.

This symptom may also be represented as in Fig. 18q, in which the left eye deviates. When the person looks to the left, the right eye will be turned towards the nose, so that the centre of the cornea, which is at first situated above the point a, will now be above the point b, the left eye, which is the squinting one, will perform an associated movement to a similar extent ; the centre of its cornea being displaced from a' to b, and the strabismus will remain the same.

In paralytic strabismus, the affected eye can no longer move conjointly with the other in the direction of the affected muscle, and the degree of deviation becomes greater the farther the person looks in that direction.

To take the linear measurement of the deviation, we must determine the distance between the centre of the cornea and the internal angle in convergent strabismus, and between the centre of the cornea and the external angle in divergent strabismus. Let us suppose that in the healthy eye this distance is 15 millimetres, in the other 7 millimetres, the deviation is then 8 millimetres. We may also, according to von Graefe's advice, measure with a pair of compasses the distance on the free margin of the lid between the point situated just below the cornea and the point above which the centre of the cornea should be if normally directed.

Amongst the various instruments adapted for linear measurement of strabismus we have figured Mr. Laurcrice's strabometer and the one which has been constructed to our own order.

The subjoined figures (igo and 191) are sufficiently plain, and require no detailed description.

The angle of strabismus, that is to say, the angle formed by the two optic axes, may be ascertained by means of the perimeter. The patient, whose head should be fixed in such a way that the centre of rotation of the deviated eye is situated at the centre of the perimetric arc, should be made to look steadily at a distant object, and in the direction of the middle of the perimeter. A candle flame is then moved along the perimeter until its image appears in the centre of the deviated cornea. The position of the flame will then indicate the angle sought for.

Strabismus may take place in any direction, most frequently, however, it is either convergent or divergent ; most rarely it is diagonal. We often find, along with a pronounced lateral deviation, a slight deviation upwards or downwards. This deviation is due to the phys