The complete eye, which includes
the eyeball, muscles nerves, blood
vessels and the structures that produce
and drain tears are all housed in
a bony cavity called the orbit. Each
orbit is made up of several bones
and is shaped like a pear.
The eyeball consists mainly of the
The sclera is a relatively tough
white outer layer of the eye. The
sclera is covered near the front of
the eye by the conjunctiva. It is
the thickest layer made from fibrous
The conjunctiva is a thin mucous
membrane, which lines the inside of
the eyelids but which also stretches
to the edge of the cornea.
The cornea is a transparent, clear
dome-like structure on the front surface
of the eye over the black circle called
the pupil. The cornea serves as a
protective covering for the eye and
also functions to focus light on the
retina located at the back of the
eye. The cornea has numerous nerve
endings and is therefore very sensitive
The iris is a circular colored ring
on the front portion of the eye. It
is a thin layer of fine muscles. After
the light passes through the cornea
it goes through the pupil which is
actually a hole in the iris. The main
function of the iris is to control
the amount of light entering the eye.
In doing so, the iris makes the pupil
dilate when less light enters and
shrink when the amount of light is
excessive, as in a camera.
The lens is located behind the iris.
It is a small clear disc and is relatively
dry containing only 65% water. It
is made of a transparent elastic protein.
Its main function is to focus light
on the retina. It does this by using
small muscles called the ciliary muscles
to increase its thickness when focusing
on near objects and thinning itself
when focusing on far objects. The
lens is attached to the ciliary muscles
by fine thread like structures.
The retina is a kind of screen at
the back of the eye on which an inverted
image of the object being viewed is
formed. The retina is a thin transparent
membrane and is the part of the eye
that actually sees. It represents
the film in a camera. The retina contains
millions of light-sensitive sensory
cells or photoreceptors and blood
vessels to nourish them. These sensory
cells absorb light.
The most sensitive part of the retina
is called the macula where the density
of photoreceptors is the highest.
The macula helps in producing detailed
images, as in high resolution films
where the grain density is high. The
macula is responsible for the critical
function of focusing. It is the part
of the retina most used for normal
Each photoreceptor is connected to
a nerve fiber which when bunched together
form the optic nerve. The photoreceptors
convert the light into electrical
impulses and the optic nerves carry
these electrical impulses to the brain.
There are two types of photoreceptors,
cones and rods. Cones contribute in
producing sharp, detailed, central
and color vision and are mainly present
in the macula. The rods are responsible
for night and peripheral vision. In
short, cones function best in daylight
and rods at night or in dark environments.
The rods are mainly present in the
peripheral regions of the retina.
They are more numerous than cones
and are more sensitive to light but
do not register color vision. Rods
can only differentiate between shades
of grey. That is why we do not see
colors in the dark.
The eyeball is divided into two chambers
each of which is filled with fluid.
The front chamber stretches from the
inside of the cornea to the front
part of the lens and is filled with
a fluid called the aqueous humor which
nourishes the internal structure.
The rear chamber extends from the
back of the lens to the retina. It
is filled with a fluid called the
vitreous humor. These fluids create
a pressure which fills out the eyeball
and helps maintain its shape.
The front chamber is also divided
into two sections. The front section
extends from the cornea to the iris
and the rear section extends from
the iris to the lens. Usually the
aqueous humor is produced in the rear
section and flows slowly through the
pupil into the front section. The
fluid then drains out of the eye through
channels present where the iris meets