The Human Eye

Anatomy of the Eye, Vision: Light & Color, Normal Vision and Vision Problems.

Eyes are organs that detect light. The human eye is wrapped in three layers of tissue. The first layer is called the sclerotic coat. This tough layer creates the "white" of the eye except in the front where it forms the transparent cornea. The cornea admits light to the interior of the eye and bends the light rays to that they can be brought to a focus. The surface of the cornea is kept moist and dust-free by secretions from the tear glands. The next layer of the eye is called the choroids coat. This middle layer is deeply pigmented with melanin. It reduces reflection of stray light within the eye. The choroids coat forms the iris in the front of the eye. This, too, is pigmented and is responsible for eye "color". The size of its opening, the pupil, is variable and under the control of the autonomic nervous system. In dim light (or when danger threatens), the pupil opens wider letting more light into the eye. In bright light the pupil closes down. This not only reduces the amount of light entering the eye but also improves its image-forming ability. Finally, there is the most well known layer of the eye: the retina. The retina is the inner layer of the eye. It contains the light receptors, called rods and cones. The retina also has many interneurons that process the signals arising in the rods and cones before passing them back to the brain. All the nerve impulses generated in the retina travel back to the brain by way of the axons in the optic nerve. At the point on the retina where the approximately 1 million axons converge on the optic nerve, there are no rods or cones. This spot, called the blind spot, is thus insensitive to light.





In the human eye, light enters the pupil and is focused on the retina by the lens. Light-sensitive nerve cells called rods (for brightness) and cones (for color) react to the light. They interact with each other and send messages to the brain that indicate brightness, color, and contour. Although cones operate only in relatively bright light, they provide us with our sharpest images and enable us to see colors. Most of the 3 million cones in each retina are confined to a small region just opposite the lens called the fovea. So our sharpest and colorful images are limited to a small area of view. Because we can quickly direct our eyes to anything in view that interests us, we tend not to be aware of just how poor our peripheral vision is. Rods are extremely sensitive to light. A single photon (the minimum unit of light) absorbed by a small cluster of adjacent rods is sufficient to send a signal to the brain. So although rods provide us with a relatively grainy, colorless image, they permit us to detect light that is over a billion times dimmer than what we see when its sunny.




Most common vision problems occur in the lens. The lens is located just behind the iris. One common problem is Farsightedness. If the eyeball is too short or the lens too flat or inflexible, the light rays entering the eye — particularly those from nearby objects — will not be brought to a focus by the time they strike the retina. Eyeglasses with convex lenses can correct the problem. Farsightedness is called hypermetropia. Another common problem is called Nearsightedness. If the eyeball is too long or the lens too spherical, the image of distant objects is brought to a focus in front of the retina and is out of focus again before the light strikes the retina. Nearby objects can be seen more easily. Eyeglasses with concave lenses correct this problem by diverging the light rays before they enter the eye. Nearsightedness is called myopia. The term color blindness is something of a misnomer. Very few (~1 in 105) people cannot distinguish colors at all. Most "color-blind" people actually have abnormal color vision such as confusing the red and green of traffic lights. As high as 8% of the males in some populations have an inherited defect in their ability to discriminate reds and greens. Abnormal blue sensitivity occasionally occurs in humans but is much rarer than abnormalities in red-green vision. The gene for the blue-cone opsin is located on chromosome 7. Thus this trait shows an autosomal pattern of inheritance being found in females as often as in males.

Work Cited

Montgomery, Ted. “Anatomy, Physiology, Pathology of the Human Eye.” Ted Mongomery.com. 2000. 25 February 2008. <>.

“Eye Anatomy.” St. Lukes. St. Lukes Cataract and Laser Institution. 1998-2008. 26 February 2008. .

“Human Eye.” The Journey. The Journey Home. 2002. February 26 2008. .