8th Grade Cow Eye Dissection
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8th Grade Cow Eye Dissection |
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The retina is a very thin layer of tissue that lines the innermost coat of the eye. It is located in the back of the eye. It is made up of light-senstivie nerve endings that can carry the visual impulse to the optic nerve, and then on to the brain. The retina is very similar to the film of a camera. The retina contains millions of photoreceptors that capture light rays and convert them into electrical impulses, as briefly mentioned above. The two kinds of photoreceptors in the retina are rods and cones. There are about 6 million cones and 125 million rods.
The macula can be found in the center of the retina. It is a small and extremely sensitive part of the retina. It is responsible for detailed central vision. The macula allows us to appreciate detail and perform tasks that require central vision such as reading.
When one looks directly at something, the light from that object forms an image on one’s macula. A healthy macula ordinarily is capable of achieving at least 20/20 (“normal”) vision or visual acuity, even if this is with a correction in glasses or contact lenses. Not uncommonly, an eye’s best visual acuity is 20/15; in this case, that eye can perceive the same detail at 20 feet that a 20/20 eye must move up to 15 feet to see as distinctly. Some people are even capable of 20/10 vision, which is twice as good as 20/20. Vision this acute may be due to there being more cones per square millimeter of the macula than in the average eye, enabling that eye to distinguish much greater detail.
There are no rods in the macula, only cones. Because there are no rods, small dim objects in the dark cannot be seen if one looks directly at them. For this reason, to see faint stars in the sky, one must look just to the side of them so that their light falls on a retinal area, containing numerous rods, outside of the macular zone (the area that they macula is located).
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