Anatomy.-The retina is the delicate membrane lying be-tween the choroid and the vitreous and extending from the optic nerve to the ciliary processes, where it terminates in a finely in dented border called the ora serrata. Microscopically the retina is divided into ten layers which are from within outward: I. The internal limiting membrane; 2. The nerve-fibre layer; 3. The layer of ganglion cells; 4. The internal molecular layer; 5. The internal granular layer; 6. The external molecular layer; 7. The external granular layer; 8. The external limiting membrane; 9. The layer of rods and cones; 10. The pigment layer.
The internal limiting membrane is a very thin, imperfect membrane, serving to separate the nerve-fibre layer from the vitreous. The fibres of Muller terminate in this layer.
The nerve-fibre layer consists of the axis cylinders of the optic nerve-fibres, which run in a radiating direction to the ora serrata, where they terminate. At the macula lutea these fibres are bent into arches and are so arranged that a large number of them reach the yellow spot than could if they approached it in a radiating direction.
The layer of ganglion cells forms, excepting in the region of the macula, several layers of multipolar cells, having both a nucleus and a nucleolus. A nerve-fibre enters each of these cells and one or more prolongations, extend outward into the inner molecular layer. These ganglionic cells are arranged more closely to each other near the optic nerve than at the ora serrata. The internal molecular layer consists of the fine fibres from the layer of ganglion cells, irregularly arranged, with an amorphous molecular substance.
The internal granular layer is composed of two kinds of cells with nuclei. The larger of these are nerve cells, similar to those in the layer of ganglion cells, and having two offshoots, one passing into the inner granular layer to anastomose with the offshoots from the ganglionic cells and the other passing outward into the external molecular layer, where, it is claimed by some, they anastomose with fibres from the layer of rods and cones. The smaller cells of this layer are connected with the fibres of Muller.
The external molecular layer is very thin and is made up of the fibres just mentioned, together with a molecular substance similar to that of the internal molecular substance similar to that of the internal molecular layer.
The external granular layer, like the internal, is composed of both nerve and connective tissue elements. The former consists of bi-polar cells, from which offshoots pass outward to the layer of rods and cones and inward to the internal granular layer.
The external limiting membrane is the expansion formed by the terminal extremities of the fibres of Muller.
The layer of rods and cones is the most important part of the retina. The rods, commencing as fine fibres in the outer molecular layer, pass through the outer granular layer and, just beneath, the external limiting membrane, begin to increase in size, forming the rod granule, and some distance after passing through this membrane they taper down into cylindrical shaped rods, which extend outward to the pigment layer. The cones also commence as a cone-shaped swelling in the outer molecular layer, where they are in direct communication with the fibres from the internal granular layer. The cone-fibre becomes thinner until, just underneath the external limiting membrane, it again swells rapidly and there forms the cone itself, which contains a large oval nucleus and nucleolus. The cones are shorter and thicker than the rods and are of a bottle-shaped appearance. The rods and cones are arranged perpendicularly to the plane of the retina and may be divided into an inner and outer part. The inner segment is thicker than the outer and appears granulated; the outer part is broken up into fine, highly refracting lamellae, appearing like superposed circular discs or a pile of coins.
The pigment layer consists of a single layer of hexagonal nucleated cells, the inner surface of which is loaded with pigment granules.
The fibres of Muller from the connective tissue framework of the retina which traverses its various layers from the internal to the external limiting membranes and spreads out in these membranes.
The macula lutea or yellow spot is about I. 25 mm. in diameter and is the most sensitive portion of the whole retina. It lies to the outer side of the antero-posterior axis of the eyeball. The shape of the macula has been almost universally described in text-books as oval; the error of this has been pointed out by Johnson, Archiv. Ophthal., vol. xxi., I, 1892. who claims that: “The macula is invariably circular, and probably corresponds to the extreme limit of macula region,” and that it is best seen with a very weak illumination the macula ring can be seen in its entire circumference in nearly every person under thirty-five years of age and frequently over that age. He concludes that the mistake of all authors who have seen the macula has also been variously described, usually as of a somewhat yellowish appearance, from which it was called the yellow spot, but Johnson (loc. cit), by means of sixteen colored drawings, shows that “, in normal eyes of Europeans the inner portion of the macula appears of a more intense or brighter red than the fundus generally, the color deepening as it approaches the fovea centralis, where it is almost always masked by the bright foveal reflex, varying in shape and intensity.”
Anatomically the macula differs from other parts of the retina in that there are no rods, and the cones are longer and narrower than in other parts of the fundus. At the centre all the other layers of the retina are thinner, forming a depression called the fovea centralis, for toward the margin the retinal layers, especially the layer of ganglionic cells, are for the most part thicker than elsewhere.
The vascular supply of the retina is derived from the arteria centralis retinae which divides on the optic disc into an upper and lower branch. These branches then turn outward, forming a large ellipse around the macular region, none of its capillaries extending into the fovea; other branches are given off to supply the inner and other parts of the retina. Each artery is generally accompanied by a vein.
The appearance of the healthy retina is that of perfect transparency. The retinal vessels are easily distinguished from those of the choroid by being more clear and well-defined and by their taking a more radiating course and branching dichotomously. Pulsation of the retinal veins and still more rarely of the arteries may occasionally be seen in normal eyes; both may be produced by pressure of the finger upon the globe during an ophthalmoscopic examination. Usually, however, pulsation of the retinal vessels is indicative of some pathological change.
Hyperaemia Retinae-It is difficult to state in any given case whether the congestion exceeds that which is physiological or not, and in making the diagnosis the relative sizes of the arteries and veins should be considered. Normally the retinal arteries are about three-quarters the size of the corresponding vein. Hyperaemia may be either active or passive. Active hyperaemia (arterial or irritation) usually results from some straining of the eyes, such as a prolonged use of the eyes fine work or by poor light. It is often associated with or caused by some refractive error and is, or course, present in the first stage of retinitis, or may be present with inflammation of the cornea, iris, etc. The relative size of the vessels in active hyperaemia is usually well maintained and the diagnosis rests upon the congestion of the optic disc, which becomes more pinkish, with less contrast between it and the surrounding fundus. It manifests itself to the patient by fatigue on using the eyes, sensitiveness to light, pain and pressure within the eye. Passive hyperaemia (venous or stasis) results from some circulatory interference which may take place in the eye, as in glaucoma, or external to the eye, as in pressure upon the optic nerve. In this form the relative normal proportions between the arteries and veins becomes lost and we find the veins tortuous and increased in size, while the arteries may either remain normal or become diminished. In hyperaemia the only symptoms complained of by the patient will be some dimness of vision; or of flashes of light before the eyes.
TREATMENT-Hyperaemia frequently depends upon some refractive error, which should be corrected by suitable glasses. Rest of the eyes of the great importance, and hence the patient should be instructed to abstain from all use of the eyes. The remedies most frequently found of service are Dubois., Belladonna, phos., Conium, Pulsatilla or Bryonia The special indications will be found under Retinitis.
Retinitis Simplex-{ Retinitis Serosa, Retinitis Diffusa, OEdema of the Retina)
PATHOLOGY-There is hyperaemia of the retinal vessels followed by an infiltration of serous fluid into all the layers of the retina. The membrane, especially in the neighborhood of the disc, becomes somewhat swollen and thickened.
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.