Human eyes are special sense
organs for the sense of vision and are called as photoreceptor organs. They are
spherical structures present in the bony socket of the skull. The bony socket
of skull is called as orbital cavity and it remaining filled with fatty tissues
the movement of eye ball in the orbital cavity is regulated by rectus muscles
and oblique muscles. The rectus muscles are anterior rectus, posterior rectus,
superior rectus and inferior rectus while the oblique muscles are superior
oblique and inferior oblique. Each eye is of about 2.5 cm diameter.
The eye ball remains protected by upper and lower
eye lids. The eyebrows and eyelashes also protect the eyeball.
The eye ball gets secretions of meibomian gland,
harderian gland, glands of Zeis and lacrimal gland. The secretion of meibomian
and harderian glands is oily and lubricates the eyelids and cornea while the
lacrimal gland secretes watery fluid, the tears. It makes eye ball moist, soft,
clean and free from bacteria. The glands of Zeis are present at the base of eye
lashes and their secretion keep eye lashes soft and smooth.
Internal
Structure
The
internal structure of eye can be studied with the help of its vertical section.
It consists of three concentric layers of tissues which are outer layer, middle
layer and inner layer.
1. The outer layer :
It
is fibrous layer and it consists of 3 parts; sclera or sclerotic, cornea and
conjunctiva.
a) Sclera: It is an opaque, tough,
fibro-elastic external covering of eye. It is made up of fibrous
connective
tissue. It is white in colour. It protects inner parts of eyeball and maintains
the shape of
eyeball. It from 5/6th part of eye ball.
b) Cornea: It is thin &
transparent front part of outer layer. It forms a slight bulge at the front and
covers about 1/6th part of outer layer. It allows light to pass into
the eye.
c) Conjunctiva:
It is thin transparent layer present over the Cornea. It protects the cornea.
It is made up of single layer of stratified squamous epithelium.
2.The Middle layer:
It is a vascular layer and it
consists of 3 parts:
a) Choroid: It is pigmented layer present
beneath the sclera. It consists of many blood vessels
and pigmented cells. It
is dark coloured due to the presence of melanin pigment. This pigmented
layer prevents
reflection within the eye and prevent from seeing blurred and confused image.
It
provides nutrition to the retina.
b) Ciliary body: The anterior end of
choroid forms ciliary body at the point of junction of sclera and
cornea. It
contains ciliary muscles. The contraction and relaxation of these muscles
changes the
shape of lens. It remains attached with the lens by fine thread
like ligaments called as suspensory
lignments which hold the lens in position.
c) Iris :
It is a muscular diaphragm attached with the ciliary body and remains infront
of the lens. Its
pigment gives colour to eye. It regulates amount of light entering inside the eye by controlling the
size of pupil, the round opening at
the centre of iris. The pupil becomes smaller in bright light while becomes
larger in dim light.
3. The inner layer or retina: The
innermost layer is called as retina and is sensitive to light. It consists of
two types of photoreceptor cells, rods and cones. The photoreceptors are
special cells by which light rays are converted into nerve impulses. The rods
help to see in dim light and consists of pigment rhodopsin which is formed with
the help of vitamin-A. The Rods are absent in fovea centralis and are more
numerous in peripheral area of retina. The rods are more in nocturnal animals.
In Owl, only rods are present and cones are absent. The cones help to see in
bright light and consist of pigment iodopsin. The fovea centralis contains only
cones and are more numerous in central region of retina. The cones are more in
diurnal animals. In fowls, only cones are present and rods are absent. It also
consists of photosensitive nerve cells bipolar neurons and ganglion cell layer.
The bipolar nerves remain joined with rods and cones and forms optic nerve. The
image of an object is formed on the retina. A slightly depressed area of retina
which contains cones only is called as fovea centralis where rods are absent.
The sharpest image is formed at fovea. The area of retina from where the optic
nerve leaves the eye, doesn't have rods and cones and is not sensitive to light
where no image is formed. This area is called as blind spot. The optic nerve
carries impulses from retina to brain.
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| Fig. Structure of retina |
The lens
is a transparent, elastic and biconvex structure which remains in the cavity of
eye ball and held in position by suspensory ligaments. It is non-nucleated,
transparent and elongated cell. It does contain blood supply. It provides fine
adjustment for focusing of light on retina. The ciliary muscles control the
thikness of lens.
The position
of lens divides the chamber of eye ball into two chambers: aqueous chamber and
vitreous chamber.
The aqueous
chamber is a small chamber present between cornea and lens. It consists of
clear, watery, salt solution called as aqueous humour. It supplies nutrition to
cornea and lens. It also provides support to cornea.
The
large chamber lying between lens and retina is called as vitreous chamber and
it remains filled with clear, transparent jelly like substance called as
vitreous humour. It maintains the shape of eyeball and supports lens and
retina.
Working
The rays of
light from any object enters inside eye through cornea and passes through
aqueous humour, pupil, lens, vitreous humour and finally on retina where image
is formed.
The lens makes
the fine adjustment to bring a sharp focus on retina. The lens or eye can focus
distant or near object on retina by changing the shape of lens. The ability of
eye to focus near and distant objects on the retina is called as accommodation.
To focus the rays of distant objects the ciliary muscles relax and lens become
thinner while to focus the rays of nearer objects, the ciliary muscles contract
and lens become thicker.
The image
formed on the retina is picked up by optic nerve and takes it to brain and is
visible to the individual.
Defects of Eye
1. Myopia or Near-sightedness: – A defect
of eye in which near objects are visible while distant objects are not visible.
it is due to either elongation of eyeball or increase in curvature of lens. It
can be corrected by using concave lens.
2. Hypermetropia of Far-sightedness – In
it distant objects are visible while near objects are invisible. It is due to
shortened eyeball or low convexity of lens. It can be corrected by using convex
lens.
3. Astigmatism: In It, the image become
blurred. It is due to irregular curvature of cornea or lens. It can be
corrected by using cylindrical lens.
4. Cataract: In it, the objects are
invisible. It is due to opacity of lens. It can be corrected by surgical
removal of opaque lens and use of biconvex lens.
5. Presbyopia: In it, near objects are
invisible. It is due to loss of elasticity of lens after the age of 40. It is
corrected by using biconvex lens.
6. Glaucoma: In it, there is gradual loss
of sight. It is due to increased secretion of aquous humour
and increase in
intraocular pressure. It can be corrected by using drugs.
7. Colour blindness: In it, a particular
kind of colour cannot be perceived. It is due to the absence of particular type
of cone cells. It is hereditary disease.