Part 59

4th. =Determine the vascularity of the conjunctiva.= When free from pigment as it habitually is in pigs and birds this is easily done, while in animals like the horse, in which the bulbar portion, which covers the sclerotic, is largely pigmented, we can scrutinize only the pigment free parts. =In health= there should be only a few, fine, pink vessels which move with the mucosa when pressed aside on the bulb. =In congestion= the surface may appear brick red, and the vessels are irregular, large, tortuous and are seen to anastomose at frequent intervals. These move on the bulb when pressed. The congestion is usually deepest on the palpebral mucosa and in the cul de sac, and may be whitened for an instant by pressure through the eyelid. To expose the conjunctiva the right fore finger and thumb may be pressed on the upper and lower lids respectively of the left eye, and the left finger and thumb for the right, allowing them to slide backward above and below the eyeball. Another method is to seize the cilia and edge of the upper eyelid between the finger and thumb, and draw it downward and outward from the bulb, and then deftly invert it over the tip of the finger. In the old the unpigmented conjunctiva may appear yellow from the presence of subconjunctival fat, or this may appear at any age from hepatic disease (distomatosis) or icterus. It is swollen, or dropsical in anæmia, distomatosis, etc.

5th. =Examine the ciliary vessels whether they are congested or not.= These are distinguished from the conjunctival vessels in that they radiate in straight lines outward from the margin of the transparent cornea and do not move on the sclerotic under pressure. They are enlarged and very red in congestion of the ciliary circle and in iritis. In eyes devoid of pigment over the sclerotic, there is usually a circular, narrow, white zone between the congested area and the margin of the transparent cornea.

6th. =Examine the Membrana Nictitans.= See that its free margin is uniformly smooth, even, and thin and that there is no swelling, congestion nor morbid growth on any part of the structure.

7th. =See if the transparent cornea is perfectly and uniformly smooth, transparent and glistening and if it reflects clear, erect images of all objects in front of it.= The image of a round object which shows any irregularity in the curvature of its margin implies a deviation from an uniform curvature of the cornea: the image narrows in the direction of the smaller arc and broadens in the direction of the larger one (see keratoscopy, and corneal astigmatism).

8th. =A foreign body on or in the cornea= may be recognized in a good light, but better and more certainly under focal oblique illumination (see this heading).

9th. =A corneal ulcer= may be similarly recognized. It is made more strikingly manifest by instilling into the lower cul de sac a drop of a solution of fluorescin and rubbing it over the eye by moving the eyelids with the finger. This will stain the whole cornea. If now the excess of stain is washed away by a few drops of boric acid, the healthy part of the cornea is cleared up and the ulcer retains a bright yellowish green tint.

10th. =Opacity or Floating objects in the aqueous humor= (flocculi of lymph, pus, pigment, blood, worms) are always to be looked for. They may be detected by placing the eye in a favorable light. They may be still more clearly shown under focal illumination (see below).

11th. =Changes in the iris and pupil= may also be noticed in a good light. The surface should be dark in the horse, and of the various lighter shades in the smaller animals, but in all alike clear, smooth and polished, without variation of shade in spots or patches and without bulging or irregularity at intervals. Apart from the congenital absence of pigment in whole or in part, which may be found in certain sound eyes, a total or partial change of the dark iris of the horse to a lighter red, brown or yellow shade implies congestion, inflammation, or exudation. The =corpora nigra= in the larger quadrupeds should be unbroken, smooth, rounded, projecting masses outside the free border of the upper portion of the iris. It should show a clear, polished surface like the rest of the iris. =The pupil= should be evenly oval with its long diameter transversely (horse, ruminant), circular (pig, dog, bird), or round with an elliptical outline on contracting and the long diameter vertical (cat). It should contract promptly in light and dilate as quickly in darkness. Place the patient before a window, cover one eye so as to exclude light, then cover the other eye with the hand and quickly withdraw. The pupil should be widely dilated when the hand is withdrawn and should promptly contract, and it should actively widen and narrow alternately until the proper accommodation has been secured. Any failure to show these movements implies a lesion in the brain, optic nerve, or eye which impairs or paralyzes vision, interferes with accommodation or imprisons the iris. In locomotor ataxia the pupil contracts in accommodation to distance, but not in response to light.

12th. =Other causes of pupillary immobility= include: (a) Permanence of a pupillary membrane, which has remained from the fœtal condition and may be recognized by oblique focal illumination and invariability of the pupil: (b) Adhesion of the iris to the capsule of the lens—complete or partial—in the latter case the adherent portion only remains fixed, while the remainder expands and contracts, giving rise to distortions and variations from the smoothly curved outline: (c) Adhesion of the iris to the back of the cornea—complete or partial—and leading to similar distortions: (d) Glaucoma in which intraocular pressure determines a permanent dilatation of the pupil and depression of the optic disc: (e) The pupil is narrowed in iritis, and is less responsive to atropia or other mydriatic: (f) Lesions of the oculo-motor nerve may paralyze the iris and fix the pupil. The first three and the fifth of these conditions may be recognized by the naked eye, alone, or with the aid of focal illumination, the fourth may require the aid of the ophthalmoscope and the sixth which cannot be reached by such methods, might in exceptional cases be betrayed by other disorders of the oculo-motor nerve (dropping of the upper eyelid, protrusion of the eyeball, squinting outward).

13th. =Coloboma= (fenestrated iris), and =lacerated iris= are recognizable by the naked eye in a good light, or by the aid of focal illumination.

14th. =Tension of the eyeball= (Tonometry). Elaborate instruments constructed for ascertaining ocular tension are of very little use in the lower animals. The simplest and most practicable method is with the two index fingers placed on the upper lid to press the eyeball downward upon the wall of the orbit using the one finger alternately with the other as if in search of fluctuation. The other fingers rest on the margin of the orbit. All normal eyes have about the same measure of tension and one can use his own eye as a means of comparison. The educated touch is essential. In increased tension, the sense of hardness and resistance, and the indisposition to become indented on pressure is present in the early stages of internal ophthalmias (iritis, choroiditis, retinitis), phlegmon of the eyeball, glaucoma, hydrophthalmos, and tumors of the bulb.

Oblique Focal Illumination.

This is so essential to clear and definite conclusions and is so easily practiced on the domestic animals that every veterinarian should make himself familiar with the method. The method is based on the fact that when two perfectly transparent media touch each other a reflection of luminous rays takes place only at the surface. But in case any opacity exists in any part of the thickness of one of these media, it reflects the rays from its surface no matter what may be its position in the medium. Thus corneal opacities appear as gray blotches and under careful focal illumination it may be determined whether these are on the conjunctival surface, in the superficial or deeper layers of the cornea or in the membrane of Descemet. Similarly cloudiness or floating objects in the aqueous, reflect the luminous rays, and so with opacities in the lens or its capsule, or in the vitreous. In the same way the surface of the iris and corpora nigra may be carefully scrutinized. For satisfactory examination of the media, back of the iris, the pupil should be first dilated, by instillation under the lid of a drop or two of a 3 per cent. solution of atropia, and the examination proceeded with twenty minutes later. Homatropin is preferable to atropin as being less persistent in its action, and less liable to produce conjunctivitis. If it fails to produce the requisite dilatation, it may be followed by a drop of a 4 per cent. solution of hydrochloride of cocaine, which will secure a free dilatation, lasting only for one day in place of seven days as with atropin. The cocaine further removes pain and favors the full eversion of the eyelids.

The instruments required for focal illumination are a biconvex lens of 15 to 20 diopters, and a good oil lamp or movable gas jet. The light of the sun is not satisfactory. The examination ought to be conducted in a dark room, or less satisfactorily in semi-darkness. The lamp is held by an assistant at the level of the eye to be examined, either in front or behind, or first one and then the other, so that the rays of light may fall upon the eye obliquely. If the lids are kept closed it may be necessary to expose the cornea by pressing on the lids with the finger and thumb. The light is held 8 or 10 inches from the eye and the lens is interposed between it and the eye and moved nearer and more distant until the clearest illumination has been obtained of the point to be examined. In this way every accessible part of the eye may be examined in turn. The examiner may make his results more satisfactory by observing the illuminated surface through a lens magnifying three or four diameters. It is important to observe that the eye of the operator must be in the direct line of reflection of the pencil of light.

=Cornea.= By focusing the light in succession over the different parts of the surface of the cornea, all inflammations, vascularities, opacities, ulcers, and cicatrices will be shown and their outlines clearly defined. By illuminating the deeper layers of the cornea proper, the lesions of keratitis, opacities, ulcers and cicatrices will be shown. To complete the examination of the cornea the light should be focused upon the iris so that it may be reflected back through the cornea. This will reveal the most minute blood-vessels, any cell concretions on Descemet’s membrane, or any foreign body in the cornea which may have been overlooked.

=Aqueous Humor.= Unless the cornea is densely opaque, the anterior chamber can be satisfactorily explored by the oblique focal illumination. The cloudiness or milkiness of iritis or choroiditis furnishes a strong reflection from its free particles of floating matter, its blood and pus globules, and its flocculi of fibrine. The latter have usually a whitish reflection, the blood elements a red (hypohæma), and the pus a yellow (hypopion). The writhing movements of a filaria scarcely need this mode of diagnosis. Sometimes, and especially in the horse, detached flocculi of black pigment are found floating free in the aqueous and are highly characteristic.

By this illumination one can easily determine the distance of the cornea from the iris and lens (depth of anterior chamber) which is lessened by the forward displacement of iris and lens in undue tension in the vitreous (glaucoma, retinitis, tumors, bladderworms), or of the iris alone, in irido-choroiditis with accumulation of exudate in the posterior chamber of the aqueous. The depth of the anterior chamber may increase in cases of luxation or absence of the lens or softening and atrophy of the vitreous.

The adhesion of the iris to the back of the cornea may be satisfactorily demonstrated by focal illumination.

=Iris.= The lesions of the iris are exceedingly common in connection with recurring ophthalmia in the horse, and examinations in the intervals between attacks are of the greatest importance. The eye should be examined as already stated, at a window or door, and if available by the aid of a mirror. Any changes in form or color, or luster should be carefully noted, any tension of the eyeball, or angularity of the upper lid, and any slight blue opacity round the margin of the cornea. Then the prompt or tardy response of iris and pupil to light and darkness must be made out. To complete the test the eye should be treated with homatropin for three-quarters of an hour and with cocaine for ten or fifteen minutes, and then subjected to oblique focal illumination.

With partial posterior synechia the rest of the pupil is found dilated while the attached portion extends inward remaining fixed to the capsule of the lens. If the synechia is complete no dilatation whatever has occurred. The edges of the adherent iris extend inward as adherent projections, and any exposed portion of the lens is likely to show black points, the seat of previous adhesions that have been broken up. In such cases the periphery of the iris bulges forward from the accumulation behind it of aqueous humor or inflammatory exudate which cannot escape. The discoloration of the iris as the result of inflammation, stands out more definitely under the fuller illumination.

=Crystalline lens.= In exploring the crystalline lens or its capsule for opacities (cataracts) oblique focal illumination can be employed to the very best advantage, if the pupil has first been widely dilated by homatropine and cocaine. The light is concentrated on all parts of the anterior capsule in turn, then in succession on the different layers of the lens at all points and finally on the posterior capsule. The striking reflection from any points of opacity whether pigmentary, gray or pearly white is diagnostic, not only of cataract, but of its exact position—anterior or posterior, capsular or lenticular.

=Purkinje-Sanson images.= If the flame of a candle is passed in front of the eye, at a suitable distance, in a darkened room, and the observer looks into the eye obliquely from the opposite side, he observes three images of the flame, reflected respectively from the front of the cornea, from the anterior surface of the lens and from the back of the lens. The image from the cornea is erect, bright and clearly defined: that from the front of the lens is still erect, but larger and dimmer, because the difference between the index of refraction of the aqueous and lens is very slight: the third image, which is smaller and clearer than the last, is inverted, because the surface of reflection on the back of the lens acts as a concave mirror. The beginner may at first find it difficult to make out the image from the front of the lens but with a little care he can do so, and then by moving the light he should cause each image to pass over all parts of the reflecting surface in turn. Any unevenness or opacity at any point of the reflecting surface, will cause the image reflected from it to become blurred or diffused as it passes over it and thus, not the existence only, but the exact seat of such opacity is easily demonstrated. Opacities on the cornea cause blurring of the bright, erect image of the flame as it passes over that part: opacities on the anterior capsule of the lens blur the dim, erect image when passed over them: finally, opacities in the body of the lens or on its posterior capsule, blur the small inverted image as it passes over them.

Add to this method the oblique focal illumination and the images of the flame reflected from the three mirror surfaces (cornea, anterior and posterior lens surfaces) are made much clearer and more distinct than in any other way. To do this effectively the convex lens should be held so as to focus the flame in the air nearly in front of the cornea. The Purkinje-Sanson images are made very definite and clear. If the lens is approached nearer to the eye so as to throw the image of the flame within or behind the lens, a gray phosphorescent streak of light is seen in the depth of the pupil. This is due to the laminated structure of the lens as well as to the fact that the lens itself is not perfectly transparent even in its normal condition. The absence of the lens or its dislocation and displacement downward, below the line of vision may be inferred from the absence of this gray luminous reflection under this test.

OPHTHALMOSCOPE.

Principle of ophthalmoscope: Angle of incidence and angle of reflection in same line, light close to one side of the eye, reflected into it by a mirror, having a hole in the centre for eye of observer. Opacities show a dense white in transparent media: if in front of lens move with rolling of eye: if behind in opposite direction. To see fundus must use biconvex lens. Emmetropic eye: myopic: hypermetropic. Static refraction. Mydriatics: Atropine, homatropine, daturine, duboisine, hyoscyamine.

In the healthy eye, the pupil and iris, and in cataract, even the opaque anterior capsule of the lens, can be clearly seen. The reflection of the pupil, however, is dark and no object back of the iris can be observed. The reason of the difference is that the rays of light, entering through the whole cornea, are reflected at the same angle at which they strike the surface of the iris. The angle of incidence is the same as the angle of reflection. In the hollow fundus of the eye, however, the light entering through the narrow pupil, strikes the fundus at a point which is hidden from the observer, behind the iris, and being reflected by the concave fundus, in exactly the same line along which it entered, it remains invisible. To illuminate the fundus of the eye, for the observer, his line of vision must be made exactly the same as that in which the pencil of light enters the fundus. This is best effected by reflecting the light into the eye by the aid of a small plane or concave mirror having a hole in the center through which the observer looks into the pupil. The concave mirror gives the stronger illumination, but the plane article is more easily manipulated and tends to cause less active contractions of the pupil. This is the simplest form of ophthalmoscope. For careful examination of the fundus of the eye, it is best to have the subject in a dark chamber, with a single large flame of an oil lamp or gas (electric light with an obscure globe may answer). The light is held behind and on the same side as the eye to be examined, at the level of the eye and the perforated mirror and the eye of the observer are kept from 10 to 20 inches in front of the eye and also at the same level. For the horse or ox under favorable conditions in a stall, the light of day coming from a fansash over the door may serve the purpose. Nicholas assures us that it may be accomplished even under the shadow of a shed or a tree. In such a case it is better not to have too much glare of light as the reflection from cornea and lens may prevent accurate observation. A somewhat cloudy day may therefore prove advantageous.

In focusing the reflected light on the cornea, and then on the pupil and lens, any opacities in these will be shown as a grayish nebular reflection or a denser white according to their degree of opacity. The opacities in the cornea or aqueous, in front of the axis of vision in the lens move in the same direction and to the same degree as the eye rolls, while opacities on the posterior capsule or in the vitreous, move in a direction opposite to the motions of the eye, and to a degree corresponding to their distance back of the lens. Thus if the eye looks downward such opacities move upward; if it looks upward they move downward; if it looks inward they move outward; and if it looks outward they move inward.

To secure an image of the fundus of the eye, including the entrance of the optic nerve (optic papilla), the tapetum, the pigmentary surface and retina and vessels, accommodation must be made for the normal refraction of the eye of the patient, and even for that of the observer.