| Although it is small in size, the eye arguably provides
us with the most important of the five senses - vision. Vision occurs when light enters the eye
through the pupil; with help from other
important structures in the eye the iris and cornea the appropriate
amount of light is
directed towards the lens.
Just like a lens in a camera sends a message to produce a film;
the lens in the eye refracts incoming light onto the retina, where
messages are encoded. The retina, which is made up by millions
of specialised cells known as rods and cones, transforms the image
into electrical energy and this is sent to the optic disk on the
retina where it will be transferred via electrical impulses along
the optic nerve to be processed by the brain.
Eye Anatomy Terms
Choroid
The choroid
is the middle layer of the eye between
the retina and the sclera. It also
contains a pigment that absorbs excess
light so preventing blurring of vision.
Ciliary Body
The
ciliary body is the part of the eye that
connects the choroid to the iris.
Cone Cells
Are one of two types
of light sensitive cells in the retina
of the eye. The human retina contains
6-7 million cones; they function
best in bright light and are essential
for acute vision
(receiving a sharp accurate image).
The area of the retina called the
fovea contains the greatest concentration
of cones. It is thought that there
are three types of cones, each sensitive
to the wavelength of a different primary
colour - red green or blue. Other
colours are seen as combinations of
these primary colours.
Cornea
The transparent
circular part of the front of the eyeball.
It refracts the light entering the
eye onto the lens, which then focuses it
onto the retina. The cornea contains
no blood vessels and is extremely sensitive
to pain.
Fovea
The fovea
forms a small indentation at
the centre of the macula and is described
as the area with the greatest concentration
of cone cells, the messages encoded
at the centre of the fovea will
be interpreted by the brain in the
form of a visual image.
Iris
The iris regulates the
amount of light that enters the
eye. It forms a coloured muscular
diaphragm across the front of the
lens. Light enters through a central
opening called the pupil.
Lens
The lens is
a transparent crystalline structure
situated behind the pupil of the
eye and it is enclosed in a
thin transparent capsule. It helps
to refract incoming light and focus
it onto the retina. A cataract is
when the lens becomes cloudy, and can
be extracted through a practice known
as Phacoemulsification and replaced
with a plastic intraocular lens.
Macula
Is the yellow spot
on the retina at the back of the
eye, which surrounds the fovea, the
area with the greatest concentration
of cone cells, and is therefore the
area of greatest acuity of vision.
When the eye is directed at an
object, the part of the image
that is focused on the fovea is
the image most accurately registered
by the brain.
Optic Disk
Is
the visible portion
of the optic nerve also found
on the retina of the eye. The optic
disk identifies the start of the optic
nerve where messages from cone and
rod cells leave the eye via nerve fibres
to the optic centre of the brain.
This area is also known as the 'blind
spot'.
Optic Nerve
Is a continuation of the retina,
leaving the eye at the optic disk,
and transfers all the visual information
to the brain, via millions of nerve
fibres branching from the rods and
cones.
Pupil
The circular opening
in the
centre of the iris
through which light passes into the
lens of the eye. The iris controls
dilation and constriction of the pupil.
Retina
Is a light sensitive
layer that lines the interior of
the eye. It is composed of light
sensitive cells known as rods and
cones. The human eye contains
about 125 million
rods, which are necessary for seeing
in dim light. Cones on the other
hand function best in bright light
- there are between
6 and 7 million in the eye - they
are essential for receiving a sharp
accurate image; cones can also distinguish
colours. The retina works much
in the same way as film in a camera.
Sclera
The white part of the eye,
a tough covering with which the cornea
forms the external protective coat of the
eye.
Rod Cells
Are
one of the two types
of light-sensitive cells in the retina
of the eye. There are about 125 million
rods, which are necessary for seeing
in dim light. They contain
a pigment 'rhodopsin' (or visual
purple) which is broken
down in the light and
regenerated in the dark. Breakdown
of visual purple gives rise to nerve
impulses when all the
pigment is bleached
(i.e. in bright light) and the rods
no longer function;
this is when the cones are
activated.
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