Complete Text (Part 14)

a corresponding degree. Fig. 36. Correction of Hypermetropia. How is hypermetropia corrected? By placing a convex glass before the eye of sufficient power to bring the far-point to infinity (Fig. 36). 108 THE EYE. What is astigmatism? It is a condition of the refraction of the eye in which rays of light passing through one of the principal meridians are refracted differently from those passing through the other meridian. Astigmatism is either regular or irregular. "What are the principal meridians in an astigmatic eye ? The meridians of greatest and least refraction : they are usually at right angles to each other. What is regular astigmatism? A condition in which the dioptric surfaces are regularly curved, but in which the refraction is different in the different meridians. It may be corneal or lenticular, or both. If corneal, the curvature of one meridian of the cornea is more acute than the curvature of the meridian at right angles to it. If lenticular, the curvatures of the lens vary in the same manner, or the lens is set at an angle with the visual line, which pro- duces the same result. What are the varieties of regular astigmatism ? One meridian may be hypermetropic or myopic, while the other meridian is emmetropic. This is termed simple hypermetropic or sim- ple myopic astigmatism. Both meridians may be hypermetropic or myopic, one to a greater extent than the other. It is then known as compound hypermetropic and compound myopic astigmatism, respect- ively. One meridian may be hypermetropic and the other myopic. This condition is known as mixed astigmatism. What is the cause of astigmatism ? It depends on the shape of the eye, and is not necessarily the result of diseased processes. How is astigmatism corrected? By the use of cylindrical glasses, so adjusted as to correct the differ- ence between the refraction of the principal meridians. To this may be added a plus or minus spherical glass, which may be employed when there is hypermetropia or myopia common to both meridians. What is irregular astigmatism ? It consists in the presence of facets or irregular surfaces over the cornea. These are usually due to ulcerative processes that have pre- ceded. How is this detected? Examination with the oblique illumination will serve to disclose a few smaller opacities in the majority of cases. Where this is not easy of demonstration the peculiar broken appearance of the image of the fun- THE EYE AS AN OPTICAL INSTRUMENT. 109 dus will suffice to make the diagnosis. The use of a Placido's disk will also aid in determining the nature of the condition. "What are the methods employed for the detection of errors of refraction ? They are subjective and objective. To the subjective method belong trial-lenses, Schemer's test, Thompson's ametrometer, and others. The objective tests are Placido's disk, retinoscopy or skioscopy, the ophthal- moscope, and others. In testing the refraction of an eye, especially in younger people, it is always well to employ a mydriatic for the purpose of putting the accommodation at rest. If this is not done, tonic spasm may hide the real condition, and a proper correction of the error of re- fraction cannot be obtained. Atropine, homatropine, or duboisine may be used. How is the refraction determined by the use of the trial-lenses ? The patient is placed at a distance of 20 feet from the trial-cards, which are sufficiently illuminated. Glasses are placed before the eye (each eye should be tested separately), beginning with the plus lenses. If vision is not blurred when a weak plus lens is placed before the eye, the far-point must be beyond infinity, and the condition must be one of hypermetropia. The plus glass is increased until the test-letters begin to be blurred. For simple hypermetropia the strongest glass that gives distinct vision represents the amount of hypermetropia, and the strong- est glass that gives distinct vision immediately after the eye has recov- ered from the mydriatic should be prescribed. If the plus glass blurs the vision, a weak minus should be substituted, and a stronger minus glass placed successively before the eye until a clear image is formed on the retina. The weakest glass that will correct the condition is the one that should be given. In order to correct astigmatism a minus or plus cylinder may be tried in the various meridians, and the glass selected which will give the most distinct vision. Scheiner's test is based on the fact that rays of light entering two small apertures, passing into the eye, fall on the same point on the retina in the emmetropic eye, but in the hypermetropic or myopic eye the points are duplicated, since the rays of light impinge upon the retina before they come together in the hyper- metropic eye and after they come together in the myopic eye. Thomp- son's ametrometer consists in the adjustment of two small flames so that they can be changed to correspond to the different meridians of the eye. The circles of diffusion produced by these lights in the various forms of ametropia indicate the refractive error. Scheiner's and Thompson's tests are but little used, because of their inaccuracies. What are the subjective methods? Placido's disk consists of a circular plane surface 23 cm. in diameter. In its centre is an opening of about ^ inch in diameter, which is sur- rounded by concentric white and black rings, each of about J inch in 110 THE EYE. width. Extending backward from this central opening is a short tube of the same diameter. This instrument is employed by placing the pa- tient's back to the light and directing him to look into the opening in the centre of the disk, the disk being placed at about a foot from the patient's face and parallel to the plane of the face. On looking through the tube the observer sees a reflection of the disk from the cornea of the patient. If the corneal surface is regular in its curvature, the image of the circles on the disk will appear regular. If astigmatism is present, the image will become oval, its shortest diameter corresponding to the meridian of most acute curvature. If irregular astigmatism is present, an irregular image of the disk will be produced. This test is serviceable only to indicate the condition of the cornea regarding its curvature. THE OPHTHALMOSCOPE. "What is the ophthalmoscope ? It is an instrument devised for the purpose of examining the interior of the eye. It was discovered by Helmholtz in 1851. As now con- structed it consists of a mirror perforated in its centre, and so arranged that lenses may be rotated into place behind the central perforation. The mirror acts as a source of light with which to throw light into the interior of the eye. A portion of the light that enters the eye is re- flected from the retina and choroid, passes out through the pupil, and passes through the small aperture in the centre of the mirror, entering the eye of the observer. The mirror of the modern ophthalmoscope is usually concave, with a principal focus of from 7 to 13 inches, and it is so arranged that plus and minus lenses can be rotated into position be- hind the small central opening. The ophthalmoscope is used to deter- mine the refraction of the eye as well as to examine into the condition of the interior of the eye. How is the ophthalmoscope employed? There are two methods for its employment, known as the direct and the indirect. For the direct method the source of light, which may be a lamp or Argand burner, is placed at the side and back of the patient on a level with the eyes. The light is reflected into the eye by the mirror, the observer occupying a position in front of the patient. If both eyes are emmetropic and the accommodation at rest in both, a clear picture of the fundus of the patient's eye may be seen by the observer. If hypermetropia exists, the fundus may also remain clear : the addition of a plus lens will render the image of the fundus more distinct, and plus lenses should be placed back of the aperture until the image of the fundus begins to blur very slightly. This last lens indicates the degree of hypermetropia present. If myopia exists, a clear image of the fun- dus of the eye will not be obtained until minus glasses are placed behind the aperture in the mirror of sufficient strength to render parallel the THE OPHTHALMOSCOPE. Ill rays of light that come from the eye of the patient. The weakest glass that will give a clear image of the interior is the approximate estimate of the myopia present. How is this method applied to astigmatism? In the determination of refraction with the ophthalmoscope the small vessels in the region of the fovea centralis, or the fine stippled appearance of the fundus in the region of the posterior pole of the eye, should be selected as the object of which to obtain a clear and distinct image. If astigmatism is present, the small vessels in one meridian will be seen to be more distinct than vessels of a similar size which pass in another direction. The strongest plus or the weakest minus glass with which the vessels in the meridians of greatest and of least curvature can be seen indicates the refraction of these two meridians, and the difference between the glasses required for the two principal meridians indicates the degree of the astigmatism. What is the value of the ophthalmoscope in determining refrac- tion? If the accommodation of the patient and of the observer is relaxed, it may be relied upon to give results that are approximately correct. If the accommodation is not under control, it is liable to great error. What is the indirect method for the use of the ophthalmoscope ? It consists in placing a biconvex lens of the focal distance of 2J to 3 inches between the eye of the patient and the ophthalmoscope employed by the observer at its focal distance from the patient's eye .(Fig. 37). The ophthalmoscope, held at a distance of 12 to 14 inches from the ob- served eye; is made to reflect the light through the lens and into the eye of the patient. The rays of light which return from the eye pass again into the lens, and are brought to a focus at a point corresponding to the principal focal point of the lens. This area of image is visible to and is seen by the observer. Since it is desirable that the accommodation should be at rest in all work with the ophthalmoscope, many occulists employ a lens for a focal distance of 10 inches behind the ophthalmo- scopic mirror, for the purpose of rendering the rays of light parallel as they enter the eye from the aerial image. The direct method in the use of the ophthalmoscope produces an image by projection, a virtual up- right image. The ophthalmoscope by the indirect method produces a true aerial image, which is inverted. The indirect method is employed but little for the purpose of determining refraction, and cannot be de- pended upon to give an accurate estimation of the degree of refraction. What is retinoscopy or skioscopy? It is the study of the conduct of the disk of light which fills the pupil when light is reflected into the eye by a plane or concave mirror. The observer's eye is so placed that it may intercept the rays of light reflected 112 THE EYE. from the fundus. The source of light is placed behind and a little above the patient's head. The ob- server stations himself at a distance of 1 or 1 J m. in front of the patient, and, holding the mirror before his eye, reflects the light into the eye of the patient. A circular reddish disk will be seen to fill the pupil, which moves in various directions as the mirror is rotated on it axis. If the patient is emmetropic, on movement of the mirror the reddish disk disap- pears suddenly, apparently leaving the pupil in all its parts at about the same time. If a plane mirror is used and the eye is hypermetropic, the disk of light moves in the same di- rection across the area of the pupil with the mirror, followed by a rela- tively deep shadow. If the eye is myopic, the image moves in the op- posite direction, also followed by a shadow. In determining the refrac- tion of the eye a plus or minus lens is placed before the eye until the shadow conducts itself as in emme- tropia. The glass which produces this condition is approximately the one required to corrrect the error of refraction. In examining the re- fraction of the eye by retinoscopy the different meridians of the eye must be tested separately. In this way astigmatism, if any exists, may be detected, the difference between the glass required to produce an emmetropic condition in different meridians at right angles with each other being the estimate of the astigmatism present. This method of examination determines the total astigmatism. What is the ophthalmometer of Javal and Schiotz ? It is an instrument devised for the purpose of determining the curva- ture of the cornea in its various meridians. It is so constructed that the THE MUSCULAR APPARATUS. 113 difference between the curvatures of the principal meridians may be de- termined and indicated in dioptres. By this instrument the corneal astigmatism may be accurately estimated, but the total astigmatism when lenticular astigmatism is present cannot be determined. It is an aid, in a limited sense, for the determination of refraction. What is the ultimate test for refraction? The trial-lenses must be depended upon for determining the glass which is best suited for the correction of errors of refraction in every case, and the prescription must be given according to the results arrived at by use of these lenses. THE MUSCULAR APPARATUS. Give the names of the muscles that are attached to the exterior of the eye. These are six in number : four recti muscles, known as the internal, external, superior, and inferior recti ; two oblique muscles, known as the superior and inferior oblique. What are their origin and insertion? All of the muscles of the eye, except the inferior oblique, take their origin from the apex of the orbit encircling the optic-nerve canal. The recti muscles extend forward and are inserted as follows : The internal rectus is inserted into the sclerotic at a distance of about 6 mm. from the sclero-corneal junction by a thin spreading tendon which extends from the lower to the upper tangent of the perpendicular meridian of the cor- nea. The external rectus is inserted in a like manner at the outer side of the globe 7 to 8 mm. from the sclero-corneal junction. The superior rectus is inserted in a like manner into the superior portion of the scle- rotic about 7 mm, from the sclero-corneal junction. The inferior rectus has a similar insertion into the lower part of the sclerotic. The superior oblique passes forward to the upper inner angle of the orbit, where it passes through a tendinous pulley, is reflected on to the globe below the superior rectus, and is inserted into the sclerotic a little back of the equator of the globe at the upper margin of the external rectus. The inferior oblique takes its origin from the inner angle of the margin of the orbit, passes backward and outward beneath the inferior rectus, and is inserted into the sclerotic slightly back of the equator and beneath the external rectus. What is the source of the motor nerve-supply to these muscles ? The superior, inferior, and internal recti and the inferior oblique are supplied with motor nerve-filaments from the motor oculi ; the superior oblique, from the fourth cranial nerve ; the external rectus, by the sixth cranial or abducens. S— Eye. 114 THE EYE. What is the action of the extrinsic muscles of the eye ? The external and internal recti muscles produce the lateral movements of the eye on its horizontal plane. The superior rectus rotates the cornea upward and inward, turning the perpendicular meridian of the cornea inward above ; the inferior rectus rotates the cornea downward and in- ward, rotating the perpendicular meridian of the cornea inward below; ; the superior oblique rotates the cornea downward, and turns its perpendicular meridian inward above; the inferior oblique rotates the cornea upward and outward, and turns its perpendicular meridian inward below. In the production of the lateral movements the external and internal recti alone are involved. In upward movement the superior rectus and the inferior oblique are employed ; in downward movements, the inferior rectus and the superior oblique. What are the associated movements of the globes ? They are the movements which the eyes make together for the accom- plishment and continuation of binocular single vision. These movements are very complex. The one of greatest consequence is that of con- vergence, which is employed for seeing all objects within infinity. With it are closely associated the accommodative effort and the movements of the pupil. Convergence assists accommodation, and on strong conver- gence the pupils become contracted. What is necessary for binocular single vision? The visual axis of each eye must be directed toward the same point, and the image of the object must fall on a corresponding part of the retina of each eye. The accomplishment of this is termed binocular fixation. The sensorium interprets the images as one, and single vision is the result. Deviation of the visual lines produces diplopia. The deviation may be very slight or it may be very pronounced. In the slighter forms the images may be fused by an effort on the part of the muscles