By the direct method, if both the eye of the observer and of the patient be normal in refraction, and the accommodation at rest in both, the details of the fundus are readily seen. If, however, either the surgeon or the patient be myopic, or if hypermetropic in excess of the power of accommodation to overcome, the refractive error must first be corrected. The power of relaxing one`s accommodation comes by practice. The primary objective point in the examination is, as by the indirect method, the optic disc, and this is brought into view by having the patient look straight forward while the surgeon looks into the eye slightly from the temporal side.
The Fundus of the Eye as Seen by the Ophthalmoscope.-(See Figures I and 2, Plate II, Chromo-Lithographs.) As already mentioned, the first objective point in all examinations of the interior of the eyeball is the optic disc, or papilla. The tern papilla is somewhat inaccurate, as the inference drawn from the word papilla would be that it was an elevation or something protruding from the surface of the fundus. This is not the case, as there is no prominence, and hence the term papilla is misleading; as, however, it is so generally employed, we shall use the word interchangeably with the more correct term disc. The optic nerve appears usually as a circular or slightly oval shaped disc, but may be quite irregular in outline. Its color varies from a pinkish white to a deep red, and may vary in different parts of the disc, often paler at the centre than at the circumference, or the nasal side a more decided red than the temporal. The tint also varies with the age and complexion of the patient, and the contrast with the color of the surrounding fundus.
The white appearance of some portion of the disc is due to a depression at that point, the floor of which is composed of an interlacing opaque fibrous tissue called the lamina cribrosa, through which the nerve fibres pass, and it is here they lose their medullary sheath and become transparent axis cylinders. This white spot, varying in size, is seen usually at the centre of the papilla, or rarely, more at the temporal side is called the physiological cup or excavation. Care must always be taken to differentiate this physiological cupping from the excavation found in glaucoma and in optic nerve atrophy. A description of the different forms of cupping of the disc will be found under the study of glaucoma. The border of the optic disc is well defined, being sharply outlined by a double ring. The inner, or scleral ring, appears as a faint white streak, especially distinct in elderly people, and indicates the opening of the sclerotic coat through which the optic nerve enters the eyeball. Jaeger has called this the connective tissue ring, formed by the junction of the connective tissue elements of the inner sheath of the nerve with layers of the sclera. The outer, or choroidal ring, usually seen as a slight black crescent upon one side of the disc and often wholly absent, bounds the opening in the choroid.
The next most noticeable feature in the examination of the fundus is the blood-vessels. The arterial trunk usually divides, just before emanating from the bottom of the disc, into an upward and downward branch, each of these branches generally dividing again as they pass off from the optic disc. These arteries as they spread out above and below continue to divide dichotomously into numerous branches, supplying all parts of the fundus, excepting a small area at the temporal side of the optic nerve. This area is called the macula lutea, or yellow spot, and at its centre is the point of most distinct vision, the fovea centralis. The temporal half of the retina is more freely supplied with blood-vessels than is the nasal side. The retinal veins follow the same general course and parallel to the arteries, and empty by two large branches into centre of the disc.
From this general arrangement of the retinal vessels we may have many variations in the normal fundus. The arteries and veins are distinguishable by their size and color, the veins being larger in proportion of about three to two and of a dark red as contrasted with the bright color of the arteries. The veins are also tortuous in their course and spontaneous pulsation is not frequently seen in the veins. The so-called reflex or light streak, which runs along the crest of the vessels, covering about one-third of their diameter, is of a pale straw color, and is more brilliant, broader and more sharply defined upon the arteries than veins and may be entirely absent in the veins. The cause of this reflex is unsettled, some claiming it to be a reflex from the vessel wall, others from the blood column.
The appearance of the macula lutea is as difficult to describe as it is to the student to see. NO two observes seem to illustrate or describe it in the same coloring. In many cases while we examine the muscular region we see nothing, and often we are but conscious of a luminous oval ring, the centre of which is marked by a small spot of a darker color. This phantom-like reflex, or, as it is sometimes called, halo, varies in size, though usually of an oval or circular shape. The inclosed space seems to be more of a grayish or brown color than the yellow we should naturally expect from the macula lutea being commonly spoken of as the yellow spot. The examination of the region of the macula lutea should always be practiced, for while in the normal eye the halo is often absent and the coloration of this spot variable, in diseased states an accurate picture of the macula is often of the utmost importance. The location of the yellow spot is about one and one-half optic nerve diameters to the outer side of the disc and is usually best seen by the indirect method.
The retina, being a transparent membrane, is practically invisible and reveals nothing of its delicate structure excepting the retinal vessels, which are readily seen ramifying within its inner layers. Some, however have claimed to have seen, especially in the deeply pigmented eye of the negro, with a weak illumination, the presence of the retina as a very faint grayish tinge in the neighborhood of the disc. To the observer, especially when inexperienced, the retinal vessels seem to course over and from a part of the background of the eye. They should, however, always remember that they lie some little distance in front of the underlying choroid. This can be more easily appreciated in the slightly pigmented eye, especially the albino, where they are readily seen passing over the choroidal vessels. Recognition of the choroid varies with the pigmentation of the eye. The bright red color from the pupil when the eye is illuminated with the ophthalmoscopic mirror arises from the choroid. The choroidal vessels appear as flat curvilinear stripes of a light pink hue interlacing in distinct meshes. The pigment stroma shows as irregular patches within the meshes of the choroidal vessels. The pigmentation is often more dense around the optic nerve and posterior part of the fundus. The visible choroidal vessels are always broader than the retinal trunks, and no distinction can be made between the arteries and veins.
Professor of Ophthalmology in the College of the New York Ophthalmic Hospital; Surgeon to the New York Ophthalmic Hospital. Visiting Oculist to the Laura Franklin Free Hospital for Children; Ex-President American Homoeopathic Ophthalmological, Otological and Laryngological Society. First Vice-President American Institute of Homoeopathy : President Homoeopathic Medical Society of the State of New York ; Editor Homoeopathic Eye. Ear and Throat Journal : Associate Editor. Department of Ophthalmology, North American Journal of Homoeopathy, etc.