Eye

The eyes are a pair of sensory organs of
vision. These are located in the orbit of skull
with a cushion of fat around them. Each eye
is spherical/rounded and called eyeball. The
eyes are protected bones, eyebrows, upper and
lower eyelids with eyelashes and the tear glands. Movement of the eyeball within
the orbit is controlled by 6 sets of muscles.
Wall of the eyeball is made up of 3 layers
(1) sclera, (2) choroid and (3) retina

1. Sclera/scelrotic : It is the outermost layer
   made of dense fibroelastic connetive tissue with
   collagen fibres. It provides attachment to the
   eyeball muscles. The anterior thick, transparent
   part of sclera is cornea. It is slightly bulged out
   for focussing light on the retina. The sclera
   is provided with blood vessels, however the
   cornea is devoid of them. Cornea is nourished
   by aqueous humour and also by lacrimal
   secretion. The exposed part of sclera and the
   entire Cornea are covered by a transparent
   membranous covering called conjuctiva. It
   provides protection and lubrication to the
   cornea.
2. Choroid /Uvea : It is the middle, vascular
   and pigmented layer. It is not a complete layer
   and can be divided into 3 regions – a. Choroid
   proper b. Ciliary body c. Iris
   a. The choroid proper : It lines the sclera.
   Due to its pigmented nature it prevents internal
   reflection. The blood vessels of choroid provide
   nutrition and oxygen to the retina.
   b. Ciliary body : It is a thick, muscular, ring
   like structure at the junction of choroid and
   iris. Its epithelium secretes aqueous humor.
   Attached to the ciliary body are suspensory
   ligaments which hold the lens. The ligaments
   and muscles of the ciliary body help in the
   adjustment of the size of lens.

c. Iris : what is iris? At the junction of the sclera and cornea ,the vascular part of choroid sharply bends
into the cavity of eyeball, forming a thin and
coloured partition called iris. It is perforated in
the middle by an aperture called pupil. Smooth
muscles of the iris help in regulating the size
of pupil depending on the intensity of light
entering the eyeball. The pigment in the iris
determines the colour of the eye.
Lens : It is a transparent, elastic, biconvex
structure. It is suspended in the eyeball by the
suspensory ligaments. The lens and suspensory
ligaments divide the cavity of the eyeball into
a small anterior aqueous chamber, filled with
a clear watery fluid aqueous humor and a
posterior large vitreous chamber, filled with a
jelly like vitreous humor. It maintains shape of
the eyeball and maintain pressure for keeping
the lens in position.

3. Retina : It is the innermost, delicate, non
   vascular light sensitive layer. It has 2 regions
   (a) single layer of pigmented non sensory part
   lining the iris and ciliary body (b) sensory part
   lining the choroid. It has an outer pigmented
   part and an inner nervous part. The inner
   nervous part is transparent and made of 3
   layers (1) outer photosensitive layer made of
   rod and cone cells. (2) middle layer of bipolar
   nerve cells (3) inner layer of ganglion cells. The
   nerve fibres from the basal end of the ganglion
   cells collectively form the optic nerve

The blind spot is an area diagonally
opposite to the lens. It is the area of retina from
where the optic nerve and blood vessels leave
the eyeball. There are no rod and cone cells in
this region. An area, lateral to the and above
the blind spot is called yellow area or macula
lutea. At its centre is a depression called lovea
centralis. It has maximum density of cone cells
and is the place of formation of sharpest vision.
The rod and cone cells lie deep in the retina,
so that light has to pass through the ganglion
and bipolar cells before reaching them.
Photo receptor cells : These are of two
types (a) Rod cells (b) cone cells.
They contain light sensitive proteins
termed as photopigments. The cones are
responsible for daylight (photopic) vision and
colour vision. While the rods function in dim
light (Scotopic) vision. The purple red protein
called rhodopsin is present in the rods which
is vitamin A derivative. The cones are of three
types, which contain their own characteristic
photo-pigments that respond to red, green
and blue lights. Various combinations of
these cones and their photopigments produce
sensation of different colours. The sensation of
white light is produced due to the simultaneous
equal stimulation of these three types of cones.
The Optic nerve consists of the fibres
arising from the base of ganglion cells. It leaves
the eye ball from the posterior side and carries
visual impulses from the retina to the brain.

Generation of image :
The light rays from the object pass through
the conjunctiva, cornea through the pupil upon
the lens and is focused on the retina to form
an image. In the visual area of cerebrum, the
nerve impulses are analysed and the image
formed is recognized. Read more