Wednesday, March 18, 2015
Human Eye How does the human eye work
Human Eyes are part of the organ of sight, with almost perfect sphere shape, about one inch in diameter.
Each eye is protected at its back and sides by the bones of the skull and at the front by two lashed eyelids.
The outer covering of the eye, the sclera or "white," is both protective and structural. Light penetrates the sclera only at the front of the eye, where the outer surface bulges into the transparent cornea, a delicate structure overlaid with a thin defensive membrane, the conjunctiva.
Under each upper eyelid is a tear-secreting lacrimal gland whose constant activity keeps the conjuctiva moist and free from germs.
Light entering the eye passes through the cornea and then through a watery fluid, the aqueous humor, in the front of the eye. Behind the fluid is the iris, a ring of muscle with a central hole, the pupil. The cornea focuses light rays so that they pass through the pupil.
The iris determines how much light enters the eye. In dim light its muscles relax to let in more light; in bright light its muscles contract to reduce pupil size and restrict light entry.
The fine focusing of light is achieved by the lens, a soft, transparent structure lying behind the iris. The lens is held in place by ligaments attached to internal eye muscles. The actions of these muscles bring about changes in the shape of the lens so that close and distant objects can be focused upon. For viewing near objects, the muscles make the lens shorter and fatter; for viewing distant objects, the lens becomes longer and thinner. This process is known as accommodation.
From the lens, light passes through the thick jelly (vitreous humor) that fills the center of the eye. The light is projected onto the retina, a light-sensitive layer inside the sclera from which it is separated by the choroid, a dark layer of tissue rich in blood vessels.
The retina contains two sorts of light-receptor cells: rods, which detect shades of black and white; and cones, which are sensitive to color.
In response to light, the rods and cones generate nerve impulses that pass along the optic nerve to the brain to be interpreted as vision. The concentration of cones is densest at a single spot called the fovea. The fovea is the region that gives the greatest visual sharpness.
Visual sharpness (acuity) depends on the number and density of the rods and cones, since each cell can record only the presence of light and, in cones, its color. There are about 10 million cones and 100 million rods in each eye.
Where the optic nerve leaves the back of the eye, there are no rods or cones; this is called the blind spot.
