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Light passes through the cornea, a transparent structure that protects the eye. By
contracting and dilating, muscles in the iris regulate the amount of light that enters the
eye. At the back of the eye is the retina, which contains photoreceptors (rods and cones).
-The lens focuses the image on the retina, adjusting for the distance of the object.
-The photoreceptors then translate light energy into action potentials in the optic
Cones are responsible for color vision, whereas rods only transmit information in black
and white although they are more sensitive to light. Cones are most numerous in the
macula, which is in the center of gaze in a direct l ine from the cornea; rods are typically
found in the more peripheral regions of the retina.
The whole structure is supported by gel-like substances called humors in the chambers
in the eye.
- Aqueous (watery) humor not only provides support but also transports nutrients
and wastes, whereas the more gel-like vitreous humor protects the eye against
shock. The conjunctiva, a clear mucus membrane inside the eyelids, also
The complexity and sensitivity of the eye make it quite vulnerable to aging effects.
- The cornea, lens and vitreous humor all diminish in transparency, reducing the
amount of li ght entering the eye and also scattering the light that does come in,
making the eye more sensitive to glare. Yellowing of the lens also decreases its
transparency and makes it difficu lt to dif ferentiate blues, greens, and violets.
The lens loses elasticity with age, making it more dif ficult to focus on near objects.
-This is called presbyopia and is the most common age related visual problem. .
oThe curvature of the cornea becomes more irregular, resulting in
astigmatisms that distort vision by doubling the edges of objects and/or
causing starry halos around lights.
oThe muscles in the iris decrease in number and strength with age, and its
collagen stiffens, reducing the ability of the pupil to enlarge.
-This process begins at about age 20 and steadily reduces the amount of light
available to the eye.
oThese changes in the iris also decrease the speed at which it responds to
light, so it stays open fractionally too long when the eye is exposed to
bright light and does not expand quickly enough when the light dims. This
makes older adults more vulnerable to glare and reduces their ability to
see at night.
The f luids in the eye also show age-related changes. The lacrimal (tear) gland and
conjunctive produce less of their protective fluids, which can result in inflammation and
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