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Myambutol (Ethambutol)

The ratio of 20 to 20 refers to the difference between the eye being measured and a theoretical normal-seeing eye. For example, if you are told by your eye doctor that you have 20/40 vision in the left eye, that eye has to be as close as 20 feet to see what a normal eye can see at 40 feet. It has about one half normal vision.
On the other hand, if your right eye is 20/15, that means it can see at 20 feet what a normal eye sees when placed within 15 feet of an object. It has vision approximately 25 percent better than normal. When the anatomy of the eye is functioning physiologically correctly, you are able to see with 20/20 vision.
The central area in the eye furnishing the best sight is the macula, which is some l/20th of an inch in diameter. The macula contains most of the 6.25 million cones that join together with 125 million rods which make up the approximately 130,000,000 light-sensitive cells in the retina. Retinal cones are specialized visual cells responsible for sharpness of vision and color vision. Retinal rods respond to light, dark, movements, shapes, but not to colors.
The very tightest concentration of cones (147,000 per square millimeter [mm]) is in the fovea, a tiny depression in the center of the macula. When you look at something, you turn your eyes so that the light rays are focused precisely on the fovea. There is very little overlying tissue to block the light rays.
The retina is the lining at the back of the eye where the image is formed. It is composed of those specialized light-sensitive cells that we’ve mentioned, rods and cones, plus various typical brain-type cells, and a network of arteries and veins. Moreover, the retina is comprised of a formidable complex system of interconnected nerve cells. Its rods share a common line to the brain, while its cones each have an almost direct nerve line to the brain. The more numerous the rods are distributed throughout the retina, the number getting fewer at the extreme edges.
Along with the retina, crammed into the eyeball, which is a sphere about an inch long, are other specialized parts. The cornea is a transparent tissue covering the front of the eye much as a watch crystal covers a watch, except that it is living tissue. The cornea is clear as crystal and situated in front of the iris and pupil.
The iris is a thin circular curtain which is the colored part of the eye. A person’s eye color depends on the amount of pigment in the iris; deep brown eyes have the most pigment and light blue have the least.
The pupil is a hole in the center of the iris. It is black because the inside of the eye is dark. The pupil’s size varies with the amount of light entering, for it can get smaller with increased light and larger with lessened light.
The tough, but delicate and sensitive outer coat of the eye actually is composed of two parts. As mentioned, the cornea is one, and the sclera is the other. The sclera is opaque (impervious to light) and protects the rest of the eye: it’s referred to as the “white of the eye.”

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