Macular Degeneration - VisiVite Vision Vitamin Supplement Eye (anatomy), light-sensitive Macular Degeneration - VisiVite Vision Vitamin Supplement organ of vision in animals. The eyes of various species vary from simple structures that are capable only of differentiating between light and dark to complex Macular Degeneration - VisiVite Vision Vitamin Supplement organs, such as those of Macular Degeneration - VisiVite Vision Vitamin Supplement humans and other mammals, that can distinguish minute variations of shape, color, brightness, and distance. The actual process of seeing is performed by the brain rather than by the eye. The Supplements Work to Treat Vision Loss in Elderly function of the eye is to translate the Macular Degeneration - VisiVite Vision Vitamin Supplement electromagnetic vibrations of light into patterns of nerve impulses that are transmitted to the brain.
The outer part of the eye is composed of three layers of tissue. The outside layer is the sclera, a protective Macular Degeneration - VisiVite Vision Vitamin Supplement coating. It covers about five-sixths of the surface of the eye. At the front of the eyeball, it is continuous with the bulging, transparent cornea. The Macular Degeneration - VisiVite Vision Vitamin Supplement middle layer of the coating of the eye is the choroid, a vascular layer lining the posterior three-fifths of the eyeball. The VisiVite Vision Vitamin Supplement for Macular Degeneration choroid is continuous with the ciliary body and with the iris, which lies at the front of the eye. The innermost layer is the light-sensitive retina.
Behind the
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the main body of the eye is filled with a transparent,
jellylike substance, the vitreous humor, enclosed in a thin
sac, the hyaloid membrane. The pressure of the vitreous
humor keeps the eyeball distended.
The retina is a complex layer, composed
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largely of nerve cells. The light-sensitive
receptor cells lie on the outer surface of the retina in
front of a pigmented tissue layer. These cells take the form
of rods or cones packed closely together like matches in a
box. Directly
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behind the pupil is a small yellow-pigmented spot, the
macula lutea, in the center of which is the fovea centralis,
the area of greatest visual acuity of the eye. At the center
of the fovea, the sensory layer is composed entirely of
cone-shaped cells. Around the fovea both rod-shaped
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and cone-shaped cells are present, with the
cone-shaped cells becoming fewer toward the periphery of the
sensitive area. At the outer edges are only rod-shaped
cells.
Where the optic nerve enters the eyeball,
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below and slightly to the inner side of the fovea,
a small round area of the retina exists that has no
light-sensitive cells. This optic disk forms the blind spot
of the eye.
In general the eyes of all animals resemble simple
cameras in that the lens of the eye forms an inverted image
of objects in front of it on the sensitive retina, which
corresponds to the film in a camera.
Focusing the eye, as mentioned above, is
accomplished
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by a flattening or thickening (rounding) of the
lens. The process is known as accommodation. In the normal
eye accommodation is not necessary for seeing distant
objects. The lens, when flattened by the suspensory
ligament, brings such objects to focus on the retina. For
nearer objects the lens is increasingly rounded by ciliary
muscle contraction, which relaxes the suspensory ligament. A
young
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child can see clearly at a distance as close as 6.3
cm (2.5 in), but with increasing age the lens gradually
hardens, so that the
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limits of close seeing are approximately 15 cm
(about 6 in) at the age of 30 and 40 cm (16 in) at the age
of 50. In the later years of life most people lose the
ability to accommodate their eyes to distances within
reading or close working range. This condition, known as
presbyopia, can be corrected by the use of special convex
lenses for the near range.
Structural differences in the size of the eye cause
the defects of hyperopia, or farsightedness, and myopia, or
nearsightedness. See Eyeglasses; Vision.
As mentioned above,
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the eye sees with greatest clarity only in the
region of the fovea; due to the neural structure of the
retina. The cone-shaped cells of the retina are individually
connected to other nerve fibers, so that stimuli to each
individual cell are reproduced and, as a result, fine
details can be distinguished. The rodshaped cells, on the
other hand, are connected in groups so that they respond to
stimuli over a general area. The rods, therefore, respond to
small total light stimuli, but do not have the ability to
separate small details of the visual
image. The result of these differences in structure
is that the visual field of the eye is composed of a small
central area of great sharpness surrounded by an area of
lesser sharpness. In the latter area, however, the
sensitivity of the eye to light is great. As a result, dim
objects can be seen at night on the peripheral part of the
retina when they are invisible to the central part.
The mechanism of seeing at night involves the
sensitization of the rod cells by means of a pigment, called
visual purple or rhodopsin, that is
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formed within the cells. Vitamin A is necessary for
the production of visual purple; a deficiency of this
vitamin leads to night blindness. Visual purple is bleached
by the action of light and must be reformed by the rod cells
under conditions of darkness. Hence a person who steps from
sunlight into a darkened room cannot see until the pigment
begins to form. When the pigment has formed and the eyes are
sensitive to low levels of illumination, the eyes are said
to be dark-adapted.
A brownish
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pigment present in the outer layer of the retina
serves to protect the cone cells of the retina from
overexposure to light. If bright light strikes the retina,
granules of this brown pigment migrate to the spaces around
the cone cells, sheathing and screening them from the light.
This action, called light adaptation, has the opposite
effect to that of dark adaptation.
Subjectively, a person is not
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conscious that the visual field consists of a
central zone of sharpness surrounded by an area of
increasing fuzziness. The reason is that the eyes are
constantly moving, bringing first one part of the visual
field and then another to the foveal region as the attention
is shifted from one object to another. These motions are
accomplished by six muscles that
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move the eyeball upward, downward, to the left, to
the right, and obliquely. The motions of the eye muscles are
extremely precise; the estimation has been made that the
eyes can be
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moved to focus on no less than 100,000 distinct
points in the visual field. The muscles of the two eyes,
working together, also serve the important function of
converging the eyes on any point being observed, so that the
images of the two eyes coincide. When
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convergence is nonexistent or faulty, double vision
results. The movement of the eyes and fusion of the images
also play a part in the visual estimation of size and
distance.
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