The Ophthalmoscope And Its Use; The Normal Eye Ground

The Ophthalmoscope And Its Use; The Normal Eye Ground.
By B. ALEX. RANDALL, A. M., M. D.,
OF PHILADELPHIA.

OPHTHALMOSCOPY is the visual exploration of the eye, and is more strictly limited to the study by transmitted light. Its utilization has inaugurated a new era in ophthalmology, from which most of its scientific development dates; but general medicine has been and is greatly concerned in the information thus gained. The ophthalmoscope ought to be in daily use in the hands of every physician, and it will be when the erroneous impression has been removed that its use is difficult to learn. A half hour's good instruction can give any intelligent person command of its technique and a dozen illustrations of its various revelations; and moderate practice alone, with loyal adhesion to the cardinal rules, will then serve to widen almost ad maximum the field of its employment. Compared with medical microscopy, its technique is very simple, although reasonable persistence in the face of difficulties may be less easy when dealing with a patient than in the quiet conditions of laboratory work. The beginner must not expect to succeed at once under adverse conditions which would try or even baffle the expert: the study of a patient in bed is comparatively hard, even with an electric light ophthalmoscope, and when intractable or otherwise difficult his examination may prove beyond the power of any one; yet it is to such very practical utilization that the physician may at once unreasonably desire to put the new accomplishment. Restricted at first to easy conditions, the art may be practised with few failures and rapidly growing comprehension; the infinite variations which fall within the physiological limits will be gradually learned and cease to be frequent enigmas' and the physician, made duly self confident by his success, will not too easily accept defeat when difficulties have to be surmounted. Learning that real cause only need disturb him, he will seek the ground of his difficulties in the narrow group of requirements; and when these have all been met can feel assured that he has located, if Dot overcome, the obstacles, and learned as much, perhaps, as the circumstances would permit to any one.

The Ophthalmoscope. The ophthalmoscope, augen spiegel of the Germans, is a mirror for throwing light into the eye. Elaborate and costly forms have been devised in numberless variety, intended to meet almost every possible requirement in the way which the designer thinks best; but it must not be forgotten that any one can in a moment improvise an instrument better adapted sometimes to the needs of the case before him than any which be could find in the shops, and competent for a considerable group of cases. A bit of looking glass with a bole scratched in its silvering, two or three microscope slides held together in the fingers, or three or four cover glasses in the end of a split stick improvisations of the original Helmholtz mirrorcan reveal the commencing changes at the macula of renal disease which might easily escape the user of the most high priced ophthalmoscope. But this “weak light" instrument is an over refinement for the majority of cases: the condensed illumination of a perforated concave mirror is more generally usefill, and the brow mirror of the otologist and laryngologist may revert to its earlier use, when Ruete first employed it for ophthalmoscopy.

Yet an instrument designed for wide diversity of ophthalmoscopic work, and convenient in size and construction, is naturally to be preferred. The original ophthalmoscope of Helmboltz; is practically unknown to most modern oculists, and its surpassing value in some directions has been eclipsed by less cumbersome rivals (Fig. 117). The convex mirror of Zehender, on which the light is concentrated by a lens, has as completely passed away, and almost every ophthalmoscopist of to day utilizes, with scant or no recognition, the perforated mirror of Ruete. Behind this is generally placed the revolving disk of lenses added by the optician Rekoss single, double, or even treble and upon these fundamental elements have been rung changes more numerous than could be here recorded. Some of the best of these arrangements worthy of being credited to the designer we owe to the lamented Dr. Edward G. Loring. The modifications of his later instrument (Fig. 118) are all questionable gains at the cost of undoubted loss, and are almost as numerous as the individual users. That of the writer (Fig. 119) aims at unusual completeness of the series of lenses, cylindrical as well as spherical, brought seriatim to the sight bole without removing the instrument from the eye, and boasts a minimum deviation from the dimensions, weight, and balance of the best 11 Loring'' Dr. Edward Jackson's admirable use of slides of lenses (Fig. 120) forms the simplest of " refraction ophthalmoscopes," most warmly to be commended to the non expert; while Couper's chain of lenses (Fig. 121) or Morton's modification of it offers a most ingenious solution of the difficulty of bringing a wide series of uncombined glasses close behind the sight hole of the tilted mirror. For the practitioner who is willing to make but small outlay the simple Liebreich mirror, with its clip to hold its few lenses, will prove fairly satisfactory.

Optical Principles of the Instrument. These need concern its user little at first. Rule of thumb methods will suffice for the great majority of cases, and the minutiae of the dioptries of the eye, upon which depend such questions as the amplification of the erect image and the height or depth of objects, involve formulas from which most oculist shrink. We will consider only the manifest facts, easily observed and verified, which go to make up the possibilities and limitations of the instrument, and will consider the refraction and accommodation of the eye only so far as they force themselves upon the attention of the ophthalmoscopist.

The eye is a camera obscura, provided with a complex lens system capable of changing focus and armed with a diaphragm the iris which varies the size of its central opening the pupil limiting the amount of light which enters and the optical imperfections of the image. This pupil generally appears black because the light entering it is reflected back, after partial a sorption, in exactly the direction from which it came. As the observer's head is not generally a source of light, but an obstacle, cutting off all illumination from that direction, his eye receives none of the returning rays. If the pupil be wide, however, and the retinal surface less than the focal distance behind it, as is common in children and in animals, it is not difficult to obtain a red reflex from within the eye. Ophthalmoscopy aims to secure uniformly this result, by so reflecting a beam of light that the observer's eye is always in position to receive the returning rays, and not only to obtain a diffused glare from the pupil, but to see numberless details within. For this a number of optical conditions have to be met, depending not only upon the refraction of the eye in general, but upon that of the observed eye in particular, and involving even the conditions of the observer's eye. To these we first must turn.

By the law of the conjugate foci of lenses, light from within the illuminated eye emerges in parallel rays if the eye be emmetropic, divergent if hyperopic, convergent if it be myopic. To make such rays furnish a clear image of the interior two methods are in vogue, and various optical apparatus is needful for each. The simpler method is known as the 11 direct," or that of the "upright image," in contrast to the "indirect," which gives an "inverted image."

Direct Method of Ophthalmoscopy. In this method the mirror is placed before the observer's eye, so as to throw light through the pupil of the observed eye, and the two are brought close together (Fig. 122). If the observed eye be emmetropic, parallel rays pass from it into the observer's, and if this be also emmetropic, a clear image is obtained without further aid. If the observed eye be hyperopic, myopia or accommodation in the observing eve may neutralize it and permit of seeing clearly; if not exactly thus adjusted, a convex lens must be introduced to render parallel the divergent rays. If, on the contrary, the eye be myopic, the observer must employ a concave glass to bring the convergent rays to parallelism, unless himself hyperopic enough to be focussed for such convergence. Thus it is requisite that there shall be a series of concave and convex lenses at command, which may be skillfully used as required in order to afford clear views in all conditions of refraction.

But this, while inconvenient in some respects, constitutes one of the great advantages of the direct method; for the lens thus required to give a sharp image of the retinal details becomes, under proper conditions, the measure of the ametropia. That this should be accurate assumes that the observer must be metropic or allow for his error of refraction, and make no accommodative effort that would change it from this basis. The lens thus used must be properly placed before the observed eve. It ought to be about 13 mm. from the cornea, at the anterior focus of the lens system, and it should be tilted little if at all, since this has a distorting effect. The ophthalmoscope should be so constructed as to give a considerable series of glasses coming seriatim to the sight hole, which should not be too small nor tunnel like from thickness of the instrument; and, as the light must be taken from the side of the patient's head, the mirror should incline in the needed direction, leaving the rest of the ophthalmoscope straight.

The field of view open to the direct method is never larger than the pupil, and grows steadily smaller as one draws farther away from the observed eye. So the advantage of a dilated pupil is evident: although an expert can approach so close, locate so well the image presented, and proceeding from it to each other desired part of the eye ground, can build up from this series of glimpses so satisfactory a mosaic, that he may explore with ease through a 3 min. pupil when a tyro might find difficulty even were the pupil dilated to 6 or 8 mm. The periphery of the lens and the extremes of the eye ground cannot be seen through a contracted pupil, however expert the ophthalmoscopes; and a case demanding such study must have a drop or two of a mydriatic, such as 1 per cent., solution of homatropin or 0.5 per cent. of atropin, instilled and given time to act.

When there is inequality of the refraction in the various meridians of the eye, constituting astigmatism, there is a distortion of the image of the eye ground, and all details are not equally well seen with the same lens. If, as is most common, this be due to excess of curvature of the cornea in its vertical meridian, fine vertical vessels in the retina will be sharply seen with a stronger convex or weaker concave lens than any others, especially the horizontal vessels adjacent; and thus a ready means is afforded of recognizing and measuring astigmatism (see also page 199).

Indirect Method of Ophthalmoscopy. The indirect method has certain decided advantages. The magnification obtained is less and the field proportionately larger; hence a better general view can be thus gained. Then its sharpness is largely independent of the refraction of the eye, unsteady movements are less disturbing, and it can supplement the direct method in many important relations. Differences of level count for less, although quite perceptible, and may reveal their true relief, previously misunderstood.' A simpler instrument is competent, since a concave mirror, a double convex lens of 2 3 inches focus (14 20 D.), and one or two lenses to clip behind the sight hole meet all requirements.

In this method the eye is illuminated from a distance of 25 30 cm., and the emerging rays, unless already strongly convergent, are intercepted with the convex lens held some 5 cm. in front, so that they are brought to a focus near by. Here a real inverted image is formed in the air (Fig. 123), and this, and not the eye ground itself, is studied by the observer, generally with the help of a convex lens to magnify it. The principle is the same as that of the compound microscope, while the direct method is like the use of a simple lens, the lens system of the observed eye serving to magnify all the details of its own interior. The myopic observer may often dispense with any magnifier back of his mirror, and if the observed eye be very myopic, it forms the requisite image near enough in front to obviate the need for an object glass. Here, then, the mere concave mirror may serve all needs, and in circumstances where the satisfactory use of the direct method is very difficult.

In this method much depends upon the clearness of the object lens held near the observed eye; and one of ample size and of material, like pebble, not easily scratched, has distinct advantage. A protecting mounting is often useful. The reflection from the pole of the cornea is less troublesome than when contrasted with the weaker illumination of the direct method; but the reflections from the front and back surfaces of the objective lens compel a little tilting of it to throw them out of the way.

An element of astigmatic distortion is thus introduced which must be allowed for. A round optic disk may be made to appear oval, the longer diameter corresponding with the least inclined diameter of the lens. When the eye is astigmatic a similar distortion of the disk. appears, which may be modified by tilting the lens ; but irrespective of this, to and fro movement of the lens corrects and reverses the apparent lengthening of the nerve head, which reveals whether it is anatomically or only optically elongated.

Size of the Image and Mensuration of Fundus Details. The problems as to the amplifications afforded by the upright and by the inverted image and the mensuration of objects in the fundus are complex and variable. Even in the 11 reduced eye" many other factors must be determined in order to permit of precise statement of the result. Suffice it here to say that in the emmetropic eye the upright image, when projected to 10 inches, is about sixteen fold the linear size of the retinal surface seen ; and an optic disk 1.5 mm. in diameter ' will seem 24 mm. broad when projected to 25 cm. An easy test of this is to hold a quarter dollar or shilling before the one eye while the other views the disk, and find the point where the images seem of equal size : this distance will vary little from 10 inches. In hyperopia .the enlargement is less, in myopia more, the myopic eye having virtually an extra magnifying lens in it as contrasted with the emmetropic and still more the hyperopic. The indirect method affords about one third as much amplification as the direct, increasing as the object glass is weakened and the ocular strengthened. Hence myopia gives smaller and hyperopia larger images by this method.

Another interesting point, still more practical, is the mensuration of the axial lengthening or shortening as afforded by prominences or depressions of the eye ground. Having determined the refraction at the general retinal level, the ability (aside from astigmatic conditions) to see some object with stronger convex or weaker concave lenses marks its protrusion above that level, and the following table shows the amount of elevation calculated for the "reduced emmetropic eye:"

On the contrary, the need of stronger concave or weaker convex lenses to bring the object sharply to view demonstrates its depression below the general level, as also shown in the table. The prominence of a swollen optic nervebead or of a tumor mass may thus be measured, and comparison will show the variations of its advance or recession So, too, a glaucomatous or other cupping of the nerve or the staphylomatous bulging in a coloboma may be exactly determined, when at first glance it may have seemed doubtful whither .the ill focussed surface was raised or depressed. The same table holds approximately for general conditions of axial shortening or lengthening, with the proviso that emmetropia (or any other refraction) may exist with different axial length. , if only the power of the refractive media be adjusted to such lengths. The axis of 23.8 mm., which may be assumed for the average adult emmetropic eye, has grown from some 16 mm. in infancy; and while a diopter or so of congenital hyperopia may possibly have been outgrown, the eye may be said to have changed its length and its refraction exactly pari passu.' As the other diameters of the globe are generally approximately the same as the axis, and the corneal diameter is about one half as great, a correction can be thus gained, perhaps, when in an eye not showing typically myopic or hyperopic deformity we wish to estimate from the .refraction its exact length 'and the Position of objects not on the retinal level within, as may be desired in case of operation for the removal of a foreign body in the vitreous. (See also page 201.)

The mensuration of objects or distances on or near the retinal level can generally best be given in terms of the cardinal objects there presented for comparisons. e. g " broad as the retinal vein ... .. two disk diameters out," etc. The actual size can easily be then estimated with as close approximation as would be possible with the complicated apparatus devised for actual measurement.

Examination of the Media. Previous to the employment of either method of examination of the fundus it is generally advisable to investigate the media lying in front of it both by focussed incident light (oblique illumination, see page 146) and by transmitted light.

For the latter it suffices to illuminate the eye with the concave mirror from eight or ten inches away, when any opacity in cornea, lens, or vitreous will appear as a dark silhouette against the reddish background. Magnification of this by a convex lens behind the mirror enhances the delicacy of the test, and often brings to view minute details otherwise invisible. Beginning at some 25 cm. away with a + 4 D. lens, the surgeon can study each eye, both looking straightforward and in oblique positions ; and then, approaching closer and using stronger lenses, he can focus at will upon the cornea, lens layers, anterior or posterior capsule, or the various depths of the vitreous, until at the closest range the strongest available amplification may be utilized. Foreign bodies escaping every other effort at their detection are thus readily seen, and opacities or vascularities of the cornea form striking objects.

The preliminary observation from a distance has a great advantage also in the determination of refractive errors, for little or no eye ground detail comes sharply to view, except in hyperopia or marked myopia: in the latter, slight movement of the eve or head will show that the image is inverted. Irregularities of refraction also 'become thus readily manifest, flattened facets left by loss of substance appearing like blisters in a window pane to distort the details seen through them and give the image as in high hyperopia. The condition known as conicity of the cornea, or lens may thus appear to give a dark center or surrounding zone, although the tissues be perfectly transparent; and if the observer draw back a meter or more and use a long focus or plane mirror, every eye will give shadows in the pupil with slight rotations of the mirror, and the method becomes what is known as the shadow test or retinoscopy, our most delicate means of estimating the refraction (see page 202). Notable differences of eye ground level are conspicuous when studied from a distance of 20 or 30 cm., and this constitutes the best way of studying detachments of the retina, vitreous opacities, and intraocular tumors.

Admirable, too, is this method for learning the position of opacities, since the movements of the eve are about a fixed center of rotation back of the posterior pole of the lens; and every visible object anterior to this will seem to move in the direction of the gaze, and everything posterior in the opposite direction, the rapidity and extent of the excursion indicating by parallax its distance from that center.

Auto ophthalmoseopy. A word may also be said as to auto opbtbalmoscopy, although its value is limited. Several methods may be employed, but the simplest is that of Coccius, to hold the plane skiascopy mirror between the eye and the shaded light, so that the light falls into the pupil through the ample sight hole, while the emergent rays are caught by the margin of the opening and reflected back to the macula (Fig. 124). Upon the dark background behind the lamp the observer will then project the image of the small illuminated area, and with a little care the disk can be found and studied and the vessels followed out a long way in any direction except close to the macula. The picture is not a mere suggestion, like the Purkinje image, but can be drawn in good detail; and lie who is working up even ground sketches, and has no other model at band, can thus often freshen his impressions of form or color at the moment when he most needs them. The inability to see the macular vessels is compensated by the endoscopic methods of bringing them into view (Fig. 134), either by the convex mirror of Ayres or the pin hole of Mandelstanim.

Illumination is, first and last, the most important element of success in all these measures. A steady and ample source of light of fairly uniform color is therefore essential. Daylight is undoubtedly the truest illumination under which to study conditions where faint gradations of color are at times all important; yet even within the hours when it is obtainable it varies greatly in any consultation room. Its use may be ignored except as a matter of curiosity or in some leukemic conditions, when it may be noteworthy if the fundus looks as yellow under it as the normal eye does by lamplight.

An Argand gas burner, so mounted upon a hinged bracket or an adjustable stand that it may be shifted to any desirable position, is almost always obtainable or may be substituted by any good oil lamp. A second chimney, of glass or isinglass, will shut off much radiant heat from the observer, and still more from the patient nearer by ; while an opaque chimney of iron or asbestos with a vertically oval opening about 3 to 5 cm. will be found useful in restricting the light to the desired direction, leaving everything else in shadow. With this precaution the ophthalmoscopic room need not be very dark, although strong rays of daylight should be excluded by shutters or shades; and it is very well to have several blackish surfaces conveniently placed to form fixation points for the patient's gaze during examination. The eyes may be thus kept steady, while the dark surface affords nothing to call forth accommodative strain or pupillary contraction. Either of these may prove serious obstacles to some of our measures, and it is worthy of much care to avoid them.

The test and glare are trying even to well eyes, and must be mercifully and judiciously tempered for over sensitive cases if we would obtain full success and avoid actual injury. Here the use of the plane or weak light mirror may have decided value, or the reduction of the light by turning it down or narrowing the aperture through which it shines. If gas fixtures are used, it is very desirable to so arrange them that the light may be near the patient or the observer as desired, and with a range of 4 to 6 in. for skiascopy a need best met by having a bracket at each end of the room, one being also used for illumination of the test type. It is inadvisable to have the light too close to the patient, and much beat reflected by the mirror and directly radiated from the flame may be spared him by putting the burner a foot or more back of his head. It is important, too, that the light shall be as nearly as possible behind the head, so as to avoid needless rotation of the mirror; but it must Dot be cut off by the patient's bead when the macula or temporal retina is being studied. It will be found that if it is far enough to the side to illuminate the lid margins at the outer canthus, it will meet all conditions. Moderate tilting of the mirror will then suffice to throw the light into the eye, and the instrument can be brought so close that it touches the brow and eyelashes of the patient without having the light cut off.

Position of Surgeon and Patient. One of the cardinal errors of the beginner is in not getting close enough: the field of view is thus restricted, the corneal reflex more disturbing, and refractive errors unduly distorting or blurring to the details. In highly ametropic eyes great differences in the required lens depend upon its distance from the anterior focal point of the eye some 13 mm. from the corneal pole; and in high myopia a satisfactory view can sometimes be obtained only when the observer's brow is actually touching that of the patient. This presupposes the condition, essential in most cases, that the observer use his right eye for the patient's right and hold the ophthalmoscope in his right hand, and vice versa

The convenience, or even the possibility, of doing this depends in part upon the seating of patient and observer, and the face to face position usual abroad is not at all the best. It is better that the observer's chair should be close beside the patient's, with the seats fully overlapping; and then, unless very discrepant in height, each may sit erect and at ease. A child is often of better height standing by the ophthalmoscopist's seat, and, on the other band, satisfactory studies can be most hastily made when the observer stands by the sitting or standing patient. If the light be on a swinging bracket, it can be instantly swung from one side to the other, while the ophthalmoscopist transfers himself and his seat to that side for the study of that eye. Each will learn the position most satisfactory to himself, and habitually adopt it, but a constrained pose is to be deprecated as imperilling accuracy and thoroughness. Children often tend to nod forward if quiet, or, on the contrary, to wriggle and turn, so as to need some steadying: the free hand may do good service, therefore, in lightly grasping the occiput, while the thumb rests in the concha, controlling any rotation (Figs. 125, 126).

Limited by the pupil into which it is thrown, the beam of light utilized in the direct method is that from a portion of the mirror close around the sight hole, and but little larger than the pupil. This must be quite accurately centered with the pupil, as is sometimes best done by throwing the light from a little distance upon the cheek, when the dark center of the illuminated area marking the sight hole can be seen, and this then centered in the pupil. At the bedside a light with a lens giving a parallel beam is useful. If a candle only be available, inclination of this gives a broader flame and a less limited area of light on the retina.

Three principal obstacles are met in the study of the interior of the eye reflections, opacities, and refractive errors.

Reflections. To the beginner these are very annoying. He hardly ever approaches sufficiently close to the eye, his fundus illumination is rarely the best, and the brilliant corneal reflex seems to occupy most of the pupillary space, and frequently is regarded as the whitish optic disk for which he is instructed first to look. In a narrow pupil this reflection from the cornea (and to an extent generally unperceived those also from the front and back surfaces of the lens) is ever all obstacle which the expert cannot wholly ignore, and may at times find insurmountable. Generally lie can look to the inner side of it or through its margin, and approach so near that its perception is slight. A small sight hole also reduces its annoyance by increasing the fundus-illumination and cutting off some of the rays reflected front the cornea.

Reflections are present at all the boundaries between the media, but only those upon the retina are apt to be noticed when not specially sought. In childhood, particularly, the whole retina is often covered, especially along the larger vessels, with shimmering, " watered silk" reflections, which shift with each motion of the mirror, and by the reversed direction of their movement show that they are formed by concave surfaces where the prominence over the vessels passes into the general retinal level. Of the same nature is the more definite reflex streak parallel to the nasal side of the disk, to which Weiss has called attention as being prodromal of myopia (see page 187, Fig. 132), and the bright streak (so called light reflex) always to be seen along the retinal vessels, especially the arteries, has been thus explained.

In the macular region a haIo can often be seen by the indirect method, generally horizontally oval, and having a diameter two or more times that of the disk. This is less easily seen in the upright image, unless a strongly concave mirror be used; and unless the ring of reflecting, mirror just around the sight hole be centered exactly with the pupil, only a portion of it will be visible. So, too, as to the little reflection from the fovea centralis, which is apt to be crescentic or comma shaped unless the mirror is exactly centered. Then the tiny concavity reflects the entire ring of brightness surrounding the sight hole, while the center of its floor gives back a central point of light. Like most retinal reflections, these are best seen when the surface is a little beyond the focus, and are more apt to aid than disturb, since they serve to locate the points deserving minute scrutiny, and are lost as the retinal structures are precisely focussed.

Opacities of the Media. These are at times prohibitory of study of what lies beyond them, and unless their presence and character be perceived they may prove very harassing or misleading by suggesting partial obscuration of the fundus details, retinal lesions, or refractive errors. But due employment of focal illumination and the lighting of the fundus from a little distance will rarely fail to reveal the real Difficulty and serve to locate it exactly. Against the red field of the illuminated pupil every such opacity will show dark in proportion to its lack of transparency; with a magnifying lens behind the mirror most minute and faint objects may be discerned readily. Not only real opacities, but also irregularities of surface, such as conicity of the cornea or lens, flattened facets, or plications as of the capsule, can be thus revealed, and the resultant impairment of vision correctly interpreted. Most difficult of all are the cases of turbidity of the media, since there are often no formed elements to give definition to the opacity, which merely obscures the view. Where the aqueous humor is at fault the altered appearance of the iris often furnishes the clue; but a discolored lens or a turbid vitreous can at times puzzle the most expert and permit of diagnosis only by exclusion.

Location of Opacities. This is of frequent importance. When far back near the retina the anterior position of opacities can generally be appreciated, if not estimated, by parallax, as compared with the movement of the retinal vessels; but the expert easily measures in the erect image by the interposition of convex lenses how much forward an object lies. Near emmetropia each diopter gives a difference of 0.3 mm. theoretically increasing to the myopic side, decreasing in hyperopia (e. g. + 6 D. = 1.77 mm. ; 6 D. = 2.13 mm., Na 1). Anterior opacities, on the other hand; are generally referred to the pupillary margin, and by their motion relative to it in movements of the eye their distance back or front is determined. The center of corneal curvature, which is near the posterior pole of the lens, may also be used, as pointed out by Jackson: the image of the mirror can always be seen in the line of this point, and any motion in reference to it determined. As previously stated, the rotation center of the globe is the cardinal point of reference (p. 179).

Refractive Errors. These can markedly complicate the diagnosis if the observer be not well posted. It is often surprising how much can be discerned in an unfocussed eye ground, not only when hyperopia allows a clear view of details from a distance or to an observer who does not relax his accommodation, but even when considerable myopia or astigmatism precludes sharpness of definition. To the indirect method these cases offer small difficulty: moving the objective lens a little to or from the eye compensates for large axial variations, while a little tilting of it makes or corrects astigmatism as great as is often Piet. Yet even to the direct method more is revealed than might be expected, and careful focussing is called for to decide whether all the distortion or blur present is really due to the refractive error. Much anatomical anomaly or pathological lesion can be concealed by the imperfection of the view; and minor changes in nerve, choroid, and retina are thus habitually passed over unseen or ignored by ophthalmoscopists of long experience. The habit of sketching the findings in the examinations has here one of its prime functions; and the use of stereotyped forms on which to fill in details is to be condemned, at least for the beginner. Each drawing, however rude and imperfect, should portray with all possible precision the apparent form of the disk, the trend of its vessels, and the conditions of its margins; since the minute observation here called for may prove unexpectedly valuable in these very cases, and begets an exactness of perception essential and invaluable both in refraction measurement and in the clinical observation of diseased conditions.

Differences of Level in the Eye ground, These are always to be expected. The normal disk has a prominence which justifies its name of papilla, although its center is often excavated nearly to the level of the cribriform lamina.

The excentric location of the disk commonly exaggerates the greater protrusion of the nasal side, and its major vessels are decidedly prominent. This promi¬nence may be much increased by edema and inflammatory infiltration, while the lower level of the outer margin and adjacent posterior pole grows deeper with the atrophic changes and stretching of 11 posterior staphyloma." This phrase, like that of “conus," is often employed as to conditions not strictly fulfilling its primary meaning; but the opposite view, that bulging at this point due to inflammatory softening does not take place, meets daily refuta¬tion. These points must always be taken into consideration, Dot only in relation to the present refraction of the eye, but also as to its past and future.

In the direct examination, then, we measure the direction of the rays of light, which, emerging from the observed eye, form a sharp image on the observer's retina. But this relation is affected by the observer's refraction as well as the patient's. Only upon an emmetropic eye will parallel rays be exactly focussed; and any interposed lens Deeded to make sharp the image measures the momentary ametropia of the patient = that of the observer. But it is only the refraction at the moment which is measured, and this may be very far from the static refraction which we desire. The ophthalmoscopists must learn what is his true static refraction, and as far as possible relax always to this condition. The author believes every one can learn so to do, although fatigue, headache, or improper conditions will at times preclude utilization of the faculty. If the examiner does not, any fixed allowance for his unrelaxed accommodation is so utterly vague as to be of little value. Those who habitually Use mydriatics to the total paralysis of accommodation, and accept in their measurements nothing as " near enough " to right which can possibly be improved upon, learn that total relaxation and total paralysis are identical in almost all cases, and that the " tone" of accommodation of which Donders wrote decreases under scrutiny to the vanishing point.

The Normal Fundus. The prime feature and landmark of the eyeground is the nerve head, with it& branching central artery and vein. This lies some 15' to the nasal side, and a little higher than the posterior pole of the globe, and appears as a whitish disk from which the vessels ramify in the fundus (Fig. 127). It is surrounded by the red choroid, which usually defines sharply its margin; and the frequent massing of choroid pigment here may give a gray or black edge, which is occasionally half as broad as the disk. The opening through the choroid is normally smaller than that of the sclera, and hides all trace of this ; but at times, without recognizable absorption of the choroid or its pigment, a ring of white scleral tissue (scleral ring) can be seen, partial or complete, within the choroidal ring. (See Plate 1.)

Consisting Of the nerve fibers which enter to the retinal level and then disperse, the disk often presents a slight prominence or papilla, in the center of which the diverging tissues form a porus opticus. This may be inconspicuous, especially in early life; but is at times both wide and deep, one edge or perhaps all steep or overhanging, while part of it is usually shelving. The most conspicuous feature is the group of branching vessels. Both artery and vein may come to the summit of the papilla before dividing, but commonly both branch in the bottom of the porus, while occasionally only the subdivisions in bewildering number emerge from the nerve head. Little difficulty should be experienced in distinguishing the broader, darker veins with their crimson tint from the scarlet arteries, which are near the color of the background; but the smaller branches differ less until they cease to be differentiable. On the larger veins and on all the arteries distinguishable as such, a bright streak of reflection (" light reflex ") marks the central convexity and shifts slightly with variations of the light.

The branching is usually dichotomous, giving an upper and a lower artery, which again divides into a temporal and a nasal branch, while the veins present fair parallelism. Small vessels, not always visibly arising from the central, generally pass outward toward the macula; and at this margin especially, independent cilio retinal vessels, not always of small size, are frequently met. The branches pass from the disk with sinuous curving sweep, as a rule, and with slowly diminishing caliber extend toward the periphery. On the disk, especially as they curve down into the excavation, the veins often present visible pulsation, and in rarer cases of disproportionate pressure the arteries also empty and fill, particularly in glaucoma; crossing and entwining of vein and artery are common (Figs. 128, 129), but it is extremely rare for vein to cross vein, or artery artery. Anastomosis of the vessels, almost always on the disk, is also of the rarest occurrence (Fig. 130).

The rear limit of the nerve head is the cribriform lamina, at which the optic nerve fibers lose their sheaths and enter the eye as naked axis cylinders.

This varies in depth, but can generally be distinguished, especially at the porus; and a deep excavation generally has as its bottom this mottled sieve tissue.

The physiological cup or excavation is usually present, and similar on the two sides, and, however deep and sharp cut, always leaves a marginal ring of the disk undepressed. The vessels can generally be seen to emerge through this tissue, which seldom overhangs he cup at all sides; and while the veins often present pulsation, this is rare y seen in the arteries unless the ocular tension is increased or aortic regurgitation is present (Fig. 131). An examination of the diagrams will make clear the differences between physiological and pathological excavations (see also p. 382).

Often there is a curvilinear reflex a little outside of the nasal nerve margin, due to the concavity where the prominent disk sinks into the adjacent retinal level (Fig. 132). Weiss, who called attention to this, regards it as prodromal of myopic stretching. In like manner a double ridged crescentic area to the nasal side was proven by Jaeger to be due to supra traction of the choroid; and Nagel and Weiss hold it to be a feature in many myopic changes. While none of these things are pathognomonic, they deserve to be seen and weighed.

The macula or center of most distinct vision near the posterior pole of the eye is the most important, but generally least conspicuous, region of the retina. The pupil is apt to be at its smallest when this is illuminated, the eye least steady, the corneal reflex most annoying, and the accommodation most variable. Under these conditions some of the older authorities used to be skeptical as to the visibility of the macula lutea. "Yellow spot" it is not normally in life, but only a region of deeper coloration, generally maroon in tint, with a little shifting reflex at its Center (foveal reflex). This, which is an inverted image of the ophthalmoscopic mirror given back from the nit like fovea as a concave mirror, has the form of the illuminated area of the ophthalmoscope annular if the sight bole is exactly centered before the pupil, but generally crescentic or comet shaped if excentric. A tinier central point from the center of the fovea is sometimes seen.

Outside of the macula, where the change in retinal thickness begins, a large ring or halo (macular reflex) may be seen, complete only when the mirror is exactly central, generally partial and faint in the upright image. As in the better definition of the indirect method, it constitutes a horizontally oval ring decidedly larger than the disk, and from 1 to 2 disk diameters out from its lower border (Fig. 133). Like all retinal reflexes, these phenomena are best seen with a strongly concave mirror, and seem to shift somewhat above the retina, fading as we focus down to the exact level at which they arise an additional proof that they are real images formed by concave reflecting surfaces. With advancing life all such reflexes are dim or lacking.

The center of the macula is devoid of blood vessels, as may be best seen by the entoptic study (Fig. 134) ; and the ophthalmoscope, failing to reveal the capillaries which surround it, can best place it by the way in which vessels approach it from all sides without reaching it (with rare exceptions). Its most important blood supply, like its nerve fibers, comes from the temporal margin of the disk, and the occasional presence of an independent cilio retinal artery has saved central vision in some cases of embolism of the central artery. More than in thicker parts of the retina, the stipple of the pigment-layer should be recognizable in all this region, and furnishes the most delicate focussing object in measuring the refraction in the optic axis. Senile changes are frequent in this region; albuminuric and other lesions are here most characteristic, and sometimes almost prodromal ; and hemorrhagic lesions are not very rare; so its scrupulous study should be the rule (see pp. 416, 420).

The periphery of the retina offers no special peculiarities, and is difficult to see only in proportion to the narrowness of the pupil. It is the seat of the earliest changes in retin it is pigmentosa ; its underlying choroid may show equatorial myopic stretching or splotches of disseminated choroiditis and other syphilitic affections lesions that are often most marked up and in ; while down and in, where skylight falls unobstructed by the brow, we commonly find any changes due to its irritation.

The color of the eye ground is a composite blending of factors varying in value in every case. In blondes the sheen of the almost invisible retina is backed by the orange red of the chorio capillaris, veiled by little retinal pigment: back of this are the broader bands of choroidal vessels, through as well as between which light is reflected from the sclera.' Only in the albino does this outer coat appear in its full whiteness, while in most eyes little light even reaches it through the pigmented tissues. The amount of pigmentation affects the tone and conceals the deeper layers in varying degree, until in the Negro the retinal pigment gives a slaty tapetum, almost as reflecting as that in the lower animals. Every gradation of pigmentation can be seen, not only in different eyes, but almost in the same eye, since the periphery is generally less dark, and the choroidal structure may show every; where except in the macula, where the pigment is richest. These peculiarities, especially at the nerve margin, are worthy of note, verbal or graphic as well as mental, in a large proportion of cases, since they mark minor but often important changes there in progress. So too as to the porus opticus, which is rarely marked in the infantile disk, but soon becomes definite, and at times increases greatly through atrophy or mechanical pressure.

Physiological Variations and Congenital Anomalies. Among the countless deviations from an ideal relation of the eye ground picture, variation in the vessels is most common. Often the division of the vessels is within the nerve, and only the branches, perplexingly subdivided, appear on the disk. The distribution may be accomplished by most roundabout curves, the whole group of vessels passing inward, or in some other direction, before separating toward the different quadrants of the retina. The main blood supply of the lower nasal retina may come from the upper nasal vessels (Fig. 130) or any similar irreg¬ularity; and large areas, even in two quadrants, may be supplied by no branch of the central artery, but by a cilio retinal vessel arising at the edge of the disk from the short ciliary vessels or communicating with the choroidal system (Fig.135). Tortuosity of 'vessels may be mere exaggeration of their normal sinuosities ; but at times, especially in strained hyperopic eyes, they may have the marked curves, vertical as well as lateral, usual in neuro retinitis. Single loops may lie across the disk or adjacent retina (Fig. 136) or protrude into the vitreous, or the single strand of the persistent hyaloid artery, generally devoid of blood, extends forward, in rare instances reaching or branching upon the posterior capsule of the lens. Small cystic outgrowths, especially to the nasal side, may mark a more atrophic stage of its condition (Figs. 137, 138).

Supernumerary depressions of the disk with emerging vessels are occasionally seen; more often there is a colobomatous gap, due to incomplete closure of the fetal cleft. This, which is normally open but for the sixth or seventh week, may be held open, probably by intra uterine inflammation, and give rise to most various and extreme malformations. The disk mav be alone colobomatous and show a depression, oftenest downward, of dark aspect and apparently immeasurable depth (Fig. 140), or the white sheath may be plainly seen beneath the gap. Sometimes the sheath alone is involved, and the disk, superficially normal, shows a peculiar greenish coloration near one margin that can be traced into its depths. Oftener the choroid shows a defect, usually downward, at times involving nerve and sheath, and perhaps extending broadly as far forward as can he seen (Fig. 141), while coloboma of iris or lens, or both, marks the greater extension (in time as well as area). Difficult of explanation are those rarer cases in which the defect is outward, inward, or even upward, where the fetal cleft can hardly be supposed to have had influence. Gap of the retina alone, true persistence of the fetal cleft itself, has hardly ever been described: some representative of retinal structure is usually present, when perhaps not even a vessel marks choroidal tissue, and the lack or stretching of scleral tissue forms a considerable staphylomatous concavity. Areas of defect at or near the macula (Fig. 142) are probably not related to the fetal cleft, but mark mere atrophy and non development resulting from fetal inflammation a process that may leave strands, knobs, or falciform folds of membrane protruding into the vitreous chamber, and is doubtless responsible for the persistence or perversion of most of that for which the faulty prenatal development is held accountable.

Conus was the name early given to the atrophic choroidal changes at the nerve margin, which sometimes present a form suggestive of a cone. Oftener it is a crescent embracing the outer half of the disk at times the nasal or other margin in some cases annular, though generally broadest out. With this is generally associated an ectasia or staphyloma posticum, due to coincident atrophy or yielding of the sclera. Noted at first exclusively with myopia, many writers have denied the kinship of the crescents seen in other refractive conditions ; and there is little doubt that several groups of conditions ought to be differentiated, just as there are high myopias in the illiterate who do no close work, unrelated to the eye strain myopia (Fig. 143). Any close and experienced observer has seen at times one of these forms (usually distinct) pass into another, generally with elongation of the visual axis; and he recognizes the relation, although he may feel unable to define or explain it. Whether Hasner's theory of drag by the too short optic nerve upon its scleral insertion has general or only occasional truth, the crescent most commonly begins at the outer margin as a region of altered color, doubtless inflammatory. Pigment is absorbed, to be deposited in most cases at the outer margin of the crescent, and as the atrophy advances the area increases in size, usually by the demarkation of another crescent beyond.

Three or four crescents at once may be thus shown in one eve in different stages of atrophic change. Rarely the process retrogrades and a crescent of altered color returns to the normal. Actual development of a large myopic crescent may never have been fully observed, for in most cases it and the advance of the myopic stretching can be stopped by atropine and alterative tonics; and some of us feel that our full duty has not been done in a case that does progress. Yet clinical study has been long and extended, and definite enough to bridge any gaps and show the usual identity of the processes; and strong anatomical evidence to the contrary could alone disprove it.

Probably another matter is presented by the condition called “congenital conus," ' conus downward," or” underlying conus." It has the form of a crescent of whitish color, apparently extending in under the margin of the nerve, generally below, although also noted in or out or at times even above. It is probably akin to coloboma of the nerve sheath, although not merging into this condition, seeming to underlie the upper layers of the nerve head, and to extend in at times as far as the central vessels. Most like the “scleral ring," normally or morbidly revealed, it yet presents recognizable differences, which seems to mark dissimilarity of nature. Where it is marked, full acuteness of vision can rarely be attained; and the usual presence of notable astigmatism and the frequency of aberrant vessels passing through it point to it as a congenital defect (Fig. 144).

An interesting anomaly, sometimes most striking in appearance, is furnished by marrow sheaths on the retinal fibers. Instead of being lost outside of the lamina, these elements are met in patches at or near the disk, of white fringed aspect, partly burying the retinal vessels under their opacity. The rule in the rabbit and other animals, this is an exception in man, and may constitute a huge broadening of the blind spot (Fig. 145). If extensive, they are apt to extend outward in the line of the major upper and lower temporal vessels, forming a crescentic white patch, within which the macula is seen decentered out. At the nerve they are apt to overlie the margin and to cast a greenish shadow inward; which is, of course, more marked if there be any atrophy of the nerve. They may easily be mistaken for snowy patches of infiltration, such as the “snow banks “of albuminuric or other retinitis, although generally far more fibrillar in their snowy whiteness; but the differentiation is not easy when they form small isolated patches unconnected with the disk (Fig. 146). Vision, except in the broadened blind spot, may be absolutely unaffected (see also p. 472).

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