Webster's New World Medical Dictionary

extremity
An uttermost part of the body, such as
a hand or a foot.
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extremity
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eye
The organ of sight. The eye has a number of
components, including the cornea, iris, pupil, lens,
retina, macula, optic nerve, and vitreous humor.
The cornea is the clear front window of the eye that
transmits and focuses light into the eye. The iris is
the colored part of the eye, and regulates the
amount of light that enters the eye. The size of the
pupil, the dark aperture in the iris, determines how
much light is let into the eye. The lens is the trans-
parent structure inside the eye that focuses light rays
onto the retina. The retina is the nerve layer that
lines the back of the eye, senses light, and creates
impulses that travel through the optic nerve to the
brain. The macula is a small area in the retina that
contains special light-sensitive cells and allows peo-
ple to see fine details clearly. The optic nerve is the
nerve that connects the eye to the brain. It carries
the impulses formed by the retina to the visual cor-
tex of the brain to interpret vision. The vitreous
humor is a clear, jelly-like substance that fills the
middle of the eye.
eye chart test
A test that measures vision ability
at various distances. An eye chart is imprinted with
block letters that line-by-line decrease in size, cor-
responding to the distance at which each line of let-
ters is normally visible. See also 
chart, Snellen.
eyedrop test
A test that involves putting certain
liquids into the eye to produce a particular
response. There are many types of eyedrops and
many types of eyedrop tests. One of the most com-
mon eyedrop tests is pupil dilation. See also 
dila-
tion, pupil.
eyelid
The lid or cover of the eye, a movable fold
of skin and muscle that can be closed over the eye-
ball or opened at will. Each eye has an upper and a
lower lid. Also known as a palpebra.