Anatomy and Physiology: Special Senses – The EYES
3. Sensory Tunic – this is the innermost tunic of the eye and is called the retina. This structure extends anteriorly only to the ciliary body. It converts images into electrical impulses that are sent along the optic nerve to the brain where the images are interpreted. The retina can be compared to the film of a camera. It contains millions of receptor cells, the rods and cones. Rods and cones are called photoreceptors because they respond to light.
The rods and cones are not distributed evenly in the retina. The rods are most dense at the periphery or edge of the retina and decrease in number as the center of the retina is approached. It is more numerous, some 120 million, and are more sensitive than the cones. However, they are not sensitive to color. The 6 to 7 million cones provide the eye’s color sensitivity and they are much more concentrated in the central yellow spot known as the macula. In the center of that region is the fovea centralis a 0.3 mm diameter rod-free area with very thin, densely packed cones. Consequently, this is the area of greatest visual acuity or the point of sharpest vision and anything we wish to view critically is focused on the fovea centralis.
The photoreceptor cells are distributed over the entire retina, except where the optic nerve leaves the eyeball. This is the site called the optic disc or blind spot. When light from an object is focused on the optic disc, it disappears from our point of view and we cannot see it.
The crystalline lens is located just behind the iris. Light entering the eye is focused on the retina by the lens. The nucleus, the innermost part of the lens, is surrounded by softer material called the cortex. The lens is encased in a capsular-like bag. It is held upright in the eye by a suspensory ligament, the ciliary zonule, attached to the ciliary body. Together, the lens and the ciliary body help control fine focusing of light as it passes through the eye. The lens is divided into two segments namely:
- Anterior (aqueous) segment – located anterior to the lens and contains a clear wayetry fluid called aqueous humor. The aqueous humor helps to nourish the cornea and the lens. It is continually produced by the ciliary body.
- Posterior (vitreous) segment – located posterior to the lens and is filled with a gel-like substance called the vitreous humor or the vitreous body. The vitreous is a thick, transparent substance that fills the center of the eye. It is composed mainly of water and comprises about 2/3 of the eye’s volume, giving it form and shape. The viscous properties of the vitreous allow the eye to return to its normal shape if compressed. The vitreous humor helps maintain the shape of the eye.
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