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2. A Few Notes on Visual Impairment and its Causes Visual Impairment and its Causes The eye is the peripheral organ of the visual system. Its main role is On the nasal side of the fovea is the optic disc, the starting point A About the Eye and Vision of the optic nerve, sometimes called the optic nerve head. The to focus the image of scenes looked at onto the retina by converging retina is interrupted at this spot, creating a small natural scotoma, the rays of light and to translate the light message into a nervous message, which is sent to the central nervous system. known as the blind spot. 1. Description of the Eye is essentially made up of rod cells. The number of photoreceptors connected to a ganglion cell is much greater here than in the central The external envelope of the eye, the sclera, is spherical. It sur- The peripheral retina covers the whole back of the ocular globe. It rounds and protects the middle layer, the choroid, a pigmented retina. This has the effect of allowing movement detection and and vascular layer that covers the back of the eye and supplies increasing sensitivity to light but at the cost of poorer visual acuity. blood to the retina and more particularly to the macula, which is deprived of blood vessels. The internal face of the eye is covered by the retina composed of two layers: Figure 4: Anatomy of the eye: Practical Low Vision a) The superfcial layer is nervous tissue that includes the light- sensitive cells, the cones and rods cells, which provide the “transduction”, i.e. the encoding of the visual information in the nervous message and also the initial processing of the information. b) The Retinal Pigment Epithelium (RPE) of the retina contains melanin pigments that absorb unwanted light rays and ensure the renewal of the light-sensitive pigments. The anterior and internal parts of the eye are transparent. The main focusing element of the eye is the cornea, which forms the interface with the surrounding air. After passing through the cor- nea, the incident light crosses the anterior chamber flled by the aqueous humour. Part of this light rays passes through the pupil formed by the diaphragm of the iris and then the crystalline lens, © Essilor International a variable-power convergent lens, which, together with the cornea, serves to focus the image on the retina. The image thus formed, may be encoded by the visual receptors. a) Cross-section of the eye. 2. The Retina, Nerve Tissue The retina comprises several layers of nerve cells. The light passes through the various layers of the cells before being captured by photoreceptors. The density of neurons varies, depending on the sector of the macula optic disc retina: in the central part, the macula, which measures approximately 1.5 mm in diameter, is completely free of blood vessels but is supplied by those in the choroid. At the bottom of the macular depression is the fovea. In this location, aligned with the optical axis, light rays fall directly onto the photoreceptors, exclusively cones. This is the region of the retina where visual acuity is at its highest. It measures approximately 300 microns (0.3 mm) in diameter. As you move away from the optical axis, the integration neurons © Essilor International are arranged in successive layers. Horizontal, bipolar, amacrine and ganglion cells only appear from the foveal rim and cone cells are then progressively replaced by rod cells. These anatomical b) Fundus of a normal eye. characteristics are behind the main functional differences between the “central retina” and the “peripheral retina”. 10 Copyright © 2013 ESSILOR ACADEMY EUROPE, 13 rue Moreau, 75012 Paris, France - All rights reserved – Do not copy or distribute.yright © 2013 ESSILOR ACADEMY EUROPE, 13 rue Moreau, 75012 Paris, France - All rights reserved – Do not copy or distribute. Copyright © 2013 ESSILOR ACADEMY EUROPE, 13 rue Moreau, 75012 Paris, France - All rights reserved – Do not copy or distribute. Cop