The Human Eye

5 08 2009

 

 

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Over this week  Tui reading group from R17 has been learning and researching about the human eye. 

 The Human Eye

The eyes are undoubtedly the most sensitive and
 delicate organs we possess,
and perhaps the most amazing. They present us
with the window through which
we view the world, and are responsible for four
 fifths of all the information
our brain receives – which is probably why we
rely on our eyesight more than
any other sense.

Here are some definitions for the parts of the human eye:

Anterior Chamber
The cavity in the front part of the eye between the lens
 and cornea is called the Anterior
Chamber. It is filled with Aqueous, a water-like fluid.
 This fluid is produced by the ciliary
body and drains back into the blood circulation through
 channels in the chamber angle. It is
turned over every100 minutes.

Chamber Angle
Located at the junction of the cornea, iris, and sclera,
 the anterior chamber angle extends 360
degrees at the perimeter of the iris. Channels here
allow aqueous fluid to drain back into the
blood circulation from the eye. May be obstructed in
 glaucoma.

Ciliary Body
A structure located behind the iris (rarely visible)
 which produces aqueous fluid that fills the
front part of the eye and thus maintains the eye
 pressure. It also allows focusing of the lens.

Conjunctiva
A thin lining over the sclera, or white part of the eye
. This also lines the inside of the eyelids.
Cell in the conjunctiva produce mucous, which helps to
 lubricate the eye.

Cornea
The transparent, outer “window” and primary focusing
 element of the eye. The outer layer of
the cornea is known as epithelium. Its main job is
 to protect the eye. The epithelium is made
up of transparent cells that have the ability to
 regenerate quickly. The inner layer of the cornea
is also made up of transparent tissue, which allows
 light to pass.

Hyaloid Canal
A narrow channel that runs from the optic disc to the
 back surface of the lens. It serves an
embryologic function prior to birth but none afterwards.
Iris
Inside the anterior chamber is the iris. This is the
 part of the eye which is responsible for one’s
eye color. It acts like the diaphragm of a camera,
 dilating and constricting the pupil to allow
more or less light into the eye.

Pupil
The dark opening in the center of the colored iris
 that controls how much light enters the eye.
The colored iris functions like the iris of a camera
, opening and closing, to control the amount
of light entering through the pupil.

Lens
The part of the eye immediately behind the iris that
 performs delicate focusing of light rays
upon the retina. In persons under 40, the lens is soft
 and pliable, allowing for fine focusing
from a wide variety of distances. For individuals over
 40, the lens begins to become less
pliable, making focusing upon objects near to the eye
 more difficult. This is known as
presbyopia.

Macula
The part of the retina which is most sensitive, and is
 responsible for the central (or reading)
vision. It is located near the optic nerve directly at
 the back of the eye (on the inside). This
area is also responsible for color vision.

Optic Disc
The position in the back of the eye where the nerve
(along with an artery and vein) enters the
eye corresponds to the “blind spot” since there are
 no rods or cones in these location.
Normally, a person does not notice this blind spot
since rapid movements of the eye and
processing in the brain compensate for this absent
 information. This is the area that the
ophthalmologist studies when evaluating a patient
for glaucoma, a condition where the optic
nerve becomes damaged often due to high pressure
within the eye. As it looks like a cupwhen viewed with an ophthalmoscope, it is sometimes
 referred to as the Optic Cup.

Optic Nerve
The optic nerve is the structure which takes the
information from the retina as electrical signals
and delivers it to the brain where this information
is interpreted as a visual image. The optic
nerve consists of a bundle of about one million nerve
 fibers.

Retina
The membrane lining the back of the eye that contains
 photoreceptor cells. These
photoreceptor nerve cells react to the presence and
intensity of light by sending an impulse to
the brain via the optic nerve. In the brain, the
multitude of nerve impulses received from the
photoreceptor cells in the retina are assimilated into
 an image.

Sclera
The white, tough wall of the eye. Few diseases affect
 this layer. It is covered by the episclera
(a fibrous layer between the conjunctiva and sclera )
 and conjunctiva, and eye muscles are
connected to this.

Vitreous
Next in our voyage through the eye is the vitreous.
 This is a jelly-like substance that fills the
body of the eye. It is normally clear. In early life
, it is firmly attached to the retina behind it.
With age, the vitreous becomes more water-like and may
 detach from the retina. Often, little
clumps or strands of the jelly form and cast shadows
which are perceived as “floaters”. While
frequently benign, sometimes floaters can be a sign of
 a more serious condition such as a
retinal tear or detachment and should be investigated
with a thorough ophthalmologic
examination.

By:Tommy.Hou & Sumayya from Owairaka Primary School

What do you know about the eye?


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