© 2000 PATTS
How we see is best understood by comparison with a camera. The camera uses a shutter with a small, central opening to admit light. If the object is bright, the shutter closes the opening; if dim, the shutter enlarges the opening to collect more light. The camera lens focuses the light, bringing it to a sharp image, on photographic film. Because this film is at a fixed distance behind the lens and because viewed objects lie at various distances, different-shaped lenses are needed to bend light rays to a focus. For close objects, a more "bulging," or convex ("zoom"), lens is needed for greater bending. A thinner lens, producing less bending, is needed for more distant objects. Film images "develop" because the focused light patterns react chemically with light-sensitive "receptors" on the film. A camera case encloses and protects the shutter, lens, and film.
In the eye, the iris, the colored part of the eye, performs the shutter function on the light-admitting opening - the pupil. In bright light, the iris makes the pupil small, thus admitting less light; and in dim light, the iris makes it large, admitting more. The camera lens corresponds to the eye's lens. The lens assumes a fat or thin shape, as needed to focus close or distant objects, through the action of the ciliary muscle to which it is attached. The retina, at a fixed distance behind the lens, corresponds to the film. The "developing process" involves light-sensitive receptor cells on the retina - the rods and cones. As light strikes them, they break down chemically and initiate nerve impulse conduction. These impulses reach the occipital lobe of the brain, which interprets them as the final image. The protective camera-case function is performed by the sclera. Tears secreted by the lacrimal glands, a cushion of fat, and the surrounding skull bones afford further protection for the eye.
Just inside the sclera is
the next coating, the choroid, through which many blood vessels
run. It functions not only as a source for blood supply for the rest of
the eye, but as a light-absorbing layer to prevent internally-reflected
light from blurring the image. Its anterior portion is modified into three
separate structures: the iris, the ciliary body (muscles),
and the suspensory ligament. It is the pigmented cells of
the iris that give the eye its color. The iris contains smooth-muscle fibers,
arranged in circular and radial directions, which act
to control the size of the pupil.
When the circular muscle fibers contract, the pupil constricts, admitting
less light; when the radial muscles contract, the pupil dilates to admit
more light. The pupil appears black because all the light striking the
retina is absorbed and none is reflected out of the eye. (See also: Oculomotor
The third, innermost coating is the retina. The retina contains the light-sensitive receptor cells, called rod cells and cone cells, which convert the light energy into nerve impulses. These nerve impulses then travel along the optic nerves to the occipital lobe -- i.e. visual areas of the cerebral cortex.
The ciliary body, suspensory
ligament, and the lens divide the eye into two cavities, anterior and posterior,
filled with transparent fluid, or "humor." The anterior cavity is
filled with aqueous humor, a clear, watery substance, which often
leaks out when the eye is injured. Too much aqueous humor and too little
of its absorption increase eyeball pressure. This increased pressure, if
undetected, can lead to retinal damage and loss of vision (glaucoma).
posterior cavity is filled with vitreous humor, a gelatin-like
substance which maintains sufficient intraocular pressure to prevent the
eyeball from collapsing. The vitreous
humor may also develop opacities which affect vision.
When both eyes are gazing on an object, its image focuses on corresponding portions of each retina. The left field of vision, here in blue, focuses on the right side of each retina; the right field of vision, in red, focuses on the left sides. But notice that the image focuses in different portions in each retina relative to the nose. The left visual field focuses on the left retina on the side nearest the nose - the nasal portion, but focuses on the right retina on the side farthest from the nose - the temporal portion. The right field of vision focuses on the nasal portion of the right retina, but the temporal portion of the left retina.
Merging these "fields of vision" into a meaningful whole involves a cross-over process at the optic chiasma. There, optic fibers from the nasal portion of each retina cross over and join the fibers from the temporal portion of the retina on the opposite side. These combined fibers form the optic tracts. Thus, the left optic tract contains impulses of images from the right visual field, and the right optic tract contains those from the left visual field. Synapsing at the left/right thalamus, the fibers continue as optic radiations to terminate in the cortex of the right and left occipital lobes. Location of a lesion in the visual pathway determines the resulting visual defect. For example, destruction of an optic nerve produces blindness in the same eye. Complete loss of the right optic radiations, such as may occur in stroke, blocks vision from the left visual field, and vice versa.
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© PATTS, Last update: September 2000, Maintained by Lane Price Rose