Text book of veterinary medicine, Volume 3 (of 5)

by James Law

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.