The human eye has been called the most complex organ in our body. It is amazing that so small can have so many parts. Eye is a natural optical instrument. It is present in the form of eyeball in the sockets of our skull. Its diameter is approximately 2.5 cm.

Three layers of eye ball:

  1. Outermost layer – Sclera
  2. Middle layer – choroid
  3. Innermost layer – retina
  1. Sclera:

It is the outermost, white, tough layer. It covers the whole part of the eyeball. It protects the eyeball. The front portion of the sclera is known as cornea.

  • Cornea:

Cornea is covered by a thin, transparent membrane known as conjunctiva.  Conjunctiva helps the eye to be moist and prevent the dryness.

  1. Choroid:

It is the middle layer of the eyeball. It is made up of connective tissue. It is red brown in color.  It provides the nutrition to the eye.

  • Iris:



It is circular, colored area of the eye that surrounds the pupil. It is found in different colors like brown, black, blue, green, etc.

  • Function of Iris:

It controls the amount of light that enters the eye.

  • Pupil:             It is an aperture which enlarges and shrinks like a diaphragm of a camera lens.
    • Function of Pupil:

    It allows the light to enter through the eye lens.

    • The iris allows more light into the eye when environment is dark and allows less light into the eye when environment is bright.
    • The size of pupil is control by the action of papillary sphincter muscle and dilator muscle.
    • Eye lens:

    Behind the iris, there is a transparent biconvex structure holed by cilliary muscles are known as eye lens.

    • Function of eye lens:

    It helps to focus the light on the retina.

    • Cilliary muscles:

    It holds the eye lens and adjusts the focal length of the eye lens so that any object can be seen clearly.

    1. Retina:

    It is the innermost layer of the eyeball. Retina contains blood vessels that nourish them. Retina also has two types of photoreceptors: rods and cones.

    The most sensitive part of retina is a small area known as Macula where millions cones are tightly packed.

    • Important Points:
    • High density of cones in macula makes the visual images detailed.

    types of photoreceptoRs

    There are two types of photoreceptors:

    • Cones:

    Cones are responsible for sharp, detailed central and color vision. They are mainly clustered in macula. These cells are less sensitive to light.


    • Rods:

    Rods are responsible for night and side vision. These cells are more sensitive to light. These are mainly clustered in peripheral areas of the retina.

    • Important points:
    • Rods are more numerous than cones.

    Chambers of eye ball

    The eyeball is divided into two chambers:

    • Anterior chamber:

    It is the space between the inner part of the cornea and the eye lens. It has a fluid known as aqueous humor. Aqueous humor contains mainly water and a little amount of salt.

    • Posterior chamber:

    It is the space between the lens and retina. It has a jelly like fluid known as vitreous humor. Vitreous humor contains water, salt and albumin protein. It nourishes the retina.

    • Important points:
    • Both these fluids generate pressure to fill out the eye ball.
    • It also helps to maintain its shape.

    Working of eye

    When light enter into the eye it passes through pupil. Iris controls the amount of the light. The cilliary muscles help the lens to focus the object on the retina. When light strikes on either on the rods or the cones of the retina, it is converted into an electric signal. The photoreceptors in the retina convert the light into the electrical signals. These signals carried to the brain by optic nerves and nerve fibers. The brain then translates the electrical signal into images we see.

    Each eye has own optic nerve and nerve fibers. Both optic nerves meet at the optic chiasm. From the optic chiasm, half of the optic nerve from each side cross to the other side and run (continue) to the backside of the brain.

    Thus the right side of the brain receives impulses from the left optic nerve as well as right optic nerve and the left side of the brain receives impulse from the right optic nerve as well as the left optic nerve. Then the brain integrates the information to produce a complete picture.



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