Saturday, 4 July 2015

Eyes | Anatomy, Defects and Colours explained

Let us first understand the structure of the eye.


Structure of the Eye.
Image Courtesy: Science Easy Learning


Now we will define all the parts of the eye:

Sclera:

  • Outermost, white, tough layer covering whole part of the eyeball for its protection.

Cornea:
  • Front Portion of Sclera is called Cornea which is covered by a thin, transparent membrane known as conjunctiva. (As a layman you can say that Cornea is transparent Sclera)
  • Conjunctiva helps the eye to be moist and prevent the dryness.
Iris:
  • It is circular, colored area of the eye that surrounds the pupil. It is found in different colours like brown, black, blue, green, etc.
  • It controls the amount of light that enters the eye by allowing more light when it is dark and less light when it is bright.

Pupil:
  • It is a hole located in the center of the iris of the eye that allows light to strike the retina
  • It is an aperture which enlarges and shrinks like a diaphragm of a camera lens. 
  • It allows the light to enter through the eye lens.
Choroid:
  • Middle layer of the eyeball made up of connective tissue
  • It is red brown in colour
  • Provides nutrition to eye
Eye lens:
  • Behind the iris, there is a transparent biconvex structure holed by cilliary muscles are known as 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.

Retina:
  • It is the innermost layer of the eyeball. Retina contains blood vessels that nourish them. 
  • Retina also has two types of photoreceptors: 
    • Rods
      • They are responsible for night and side vision. 
      • These cells are more sensitive to light. 
      • Rods and Cones. Image Source: Science Easy Learning
      • These are mainly clustered in peripheral areas of the retina.
      • Rods are more numerous than cones
    • cones
      • Most sensitive part of retina is a small area known as Macula where millions cones are tightly packed.
      • High density of cones in macula makes the visual images detailed.
      • Cones are responsible for sharp, detailed central and colour vision. 
      • These cells are less sensitive to light.
Image Source: Wikipedia

Two Chambers of eye ball
  • 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.
Image Source: Science Easy Learning
  • 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.
Please note:
  • Both these fluids generate pressure to fill out the eye ball.
  • It also helps to maintain its shape.
For anyone still having confusion about the structure of the eye you may refer to the following video:

Working of eye
Light entering through Pupil (controlled by Iris) and lens (which focuses with the help of Ciliary muscles) focus the object on the rods or cones of retina. Then, it is converted into an electric signal by the photoreceptors in the retina. These signals carried to the brain by optic nerves and nerve fibers. The brain then translates the electrical signal into images we see.
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.
Defects of the eye
A clear image is formed only when the light rays from the object is focused on the retina, if not, then eye cannot form a clear image. This is known as the defect of the eye.
Nearsightedness/Myopia
  • Far objects appear blurred but near objects are seen clear.
  • Image of the distant object is formed in the front of retina.
  • Reasons
    • The power of the eye increases.
    • The focal length of the lens decreases.
    • Elongation of the eyeball
    • Excessive curvature of the cornea.
  • Correction:
    • Glasses with concave lenses.
Farsightedness:
  • In this type of the defect of vision, near objects appear blurred but far objects are seen clear.
  • This is also known as hyperopia or hyermetropia.
  • In such a defective eye, the image of the near objects is formed behind of the retina.
  • The cause of Arisen:
    • The power of the eye decrease.
    • The focal length of the lens increases.
    • The eyeball becomes too shorter.
  • Correction:
    • Glasses with convex lens.
Astigmatism:
  • Light rays focus at multiple points on retina like - some focus on the retina and some focus in the front of the retina and some focus on the behind of the retina --> Person unable to simultaneously focus on both horizontal and vertical lines --> This results in objects in one direction being well focused, while others in the perpendicular direction not well focused.
  • This is usually caused by a non uniform curvature of the cornea.
  • This defect can be corrected by using eye glasses with special spherical cylindrical lenses.
Colour blindness:
  • Images are clear but colors are not.
  • Reasons
    • Absence of cones photoreceptors in retina.
    • Can be hereditary
    • cannot be corrected completely.

Glaucoma:
  • There are two chambers in the eye ball. Both are filled with fluid. This fluid is produced in the posterior chamber and drain out from the anterior chamber through a complex drainage system. In this way, a balance is maintained between production and drainage of the fluid. This determines the Intra Ocular Pressure (IOP).
  • Due to malfunctioning of drainage of fluid, Intra Ocular Pressure (IOP) increases in the posterior chamber which damages the optic nerve. If it cannot be treated in time, it causes permanent loss of vision.
Reason of different colour of eyes
  • Eye colour eyes depends on 
    • The amount of melanin in the iris, 
    • Density of proteins
    • The frequency of the scattering of light by the turbid medium in the stroma of the iris. 
  • The colour of the iris is determined by the amount of melanin pigments, the way the melanin is distributed in the eye and the ratio of eu-melanin to pheo-melanin. 
  • Eye colours have been divided into nine categories and as many as 16 genes have been associated with eye colour inheritance. 
    • The main genes associated with eye colour include OCA2 and HERC2 and both are located in Chromosome 15. 
      • Unwanted information: The HERPC2 gene generally regulates OCA2 expression and a specific mutation within HERC2 gene is partly responsible for blue eyes. Different single-nucleotide polymorphism within OCA2 are associated with blue and green eyes.
    • Eye hues
      • Brown eyed genes are dominant and blue eyed genes tend to be recessive. (In Genetics, the term recessive refers relating to or denoting heritable characteristics controlled by genes which are expressed in offspring only when inherited from both parents)
      • In humans brown eyes are predominant eye color worldwide whereas blue eyes have become increasingly rare. 
        • Blue eyed people share one common ancestor and it is believed that the mutation that caused blue eyes happened between 6,000-10,000 years ago.
      • Grey eyes are darker than blue eyes and have less melanin pigments. 
      • Only 2% of the world has green eyes and have moderate or low amounts of melanin. 
      • Hazel eyes have colour midway between the lightest blue and the darkest brown eyes and have large amount of melanin in the anterior border of the iris. 
According to some scientists eye colour can change dramatically in the first few years of life, as many babies are born with blue eyes , later develop green or brown eyes.

References: The Hindu, Science Easy Learning, My own notes, Talk with a local doctor.

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