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THE EYE

The mammalian eye is spherical; fluid filled and has three layers;

  • Outer layer or sclera
  • Middle layer or choroid
  • Inner layer or retina

 Adaptations of the eye

PartPropertiesFunctions
ScleraTough, white opaque layer.Protects delicate inner parts of the eye.Maintain shape of the eye.
CorneaTransparent and front layer of the sclera.Allow light into the eye.Refract light entering the eye
ConjunctivaThin and transparent layer before the cornea.Allow light into the eye.Protects the cornea.
ChoroidHave cells that have melanin, arteries & veinsProvide nourishment to the eye.Pigment prevents reflection of light within the eyeball.
Ciliary bodyHave thin rings of thickened tissue arising from choroids.Produce the aqueous humour 
Ciliary muscleHave circular and radial muscles which are antagonistic.Muscles alter the tension of suspensory ligaments.
Suspensory ligaments.fibrousAlter the shape of the lens.
PupilHole at the center of iris.Regulates the amount of light entering the eye.
IrisContain melanin, circular and radial muscles.

Give the eye its colour.Regulates the amount of light entering the eye
LensTransparent, biconvex and elastic structure. Found behind pupil.Refracts light onto retina.Involved in accommodation of the eye.Separates the aqueous humor from vitreous humor.
Aqueous/        Vitreous humorTransparent fluidsallow light pass / refract light, Hydrostatic pressure – maintain shape of eyeball. Contains sugars / proteins / salts – provide nutrients to eye.
RetinaContains photoreceptors (Cones & Rods) and blood vessels.Rods are sensitive to low light intensity and detect black and white and more in nocturnal animals. Have photochemical pigment called rhodopsin. Cones are sensitive to high light intensity, detect colour and present in large numbers in diurnal animals. Have photochemical pigment called iodopsin.
Fovea centralisContain mainly the cones.It’s the most sensitive part of the retina.Image is formed here.
Blind-spotHas no photoreceptorsOptic nerve leaves the retina.Blood vessels emerge here to nourish the eye.
Optic nerveMade of nerve fibresTransmits nerve impulses to the brain for interpretation.
Lachrymal glandSecrete tearsTears moisten the cornea and washout foreign particles.Tears have antiseptic effect.
Nictitating membraneTransparent membrane in birds, reptiles and fish.Draw across the eye cleaning and protecting it.
External eye musclesContractilemove eyeball within socket
others  -externally- orbit- protectiveeye lids-protect the eye by closingEye lashes-prevent entry of small foreign particles.Eye brows-prevent dust & sweat from entering eye.

Image formation and interpretation

  • Light from an object is refracted by cornea, aqueous humour, the lens, through the vitreous humour and focused on the yellow spot of the retina.
  • Image formed is recorded as real, inverted and small.
  • Photoreceptors are stimulated and generate a nerve impulse which is transmitted by the optic nerve to the cerebrum of the brain for interpretation.
  • In the brain the image is interpreted as real, upright and normal.
  • Images from the right eye are interpreted by the left hemisphere of cerebrum while those from the left eye by the right hemisphere.

Diagram  pages 100 KLB

Binocular vision or Stereoscopic Vision

  • This is the ability of both eyes to look straight ahead but see the same scene from a slightly different angle.
  • The eyes’ visual fields overlap in the center, and the brain merges these images to create a sense of depth important for judging distance.
  • Humans and other mammals have stereoscopic vision.
  • Birds, fish, and snakes have monocular vision in which each eye sees a separate image covering a wide area on each side of the head.

Advantages

  1. Larger field of view
  2. Provide much accurate assessment of distance, height or depth of objects.
  3. Damage to one eye is compensated by the other
  4. Cancels the effect of blind spot

Accommodation of The Eye

  • This is the refraction of light in order to fall on the fovea centralis (yellow spot) for clear focus of image regardless of the distance of the object.

Accommodation of a close object

  • Ciliary muscles contract thereby relaxing the tension on suspensory ligaments.
  • Curvature of the lens increases.
  • The close objects are greatly refracted by the lens focusing them on the retina.

Diagram

Accommodation of a distant object

  • Ciliary muscles relax increasing the tension on suspensory ligaments.
  • The lens is stretched decreasing its curvature i.e. lens become thinner.
  • Light rays from the object are less refracted and focused on the retina.

 Diagram

Control of Light Entering the Eye

  1. Bright Light
  2. Circular muscles of iris contract and the radial muscles relax.
  3. Diameter of pupil decrease and less light enters.
  4. This protects retina from damage by too much light.

Diagram

  • Dim Light.
  • Circular muscles relax and radial muscles contract.
  • Diameter of pupil increase and more light enters the eye.
  • This allows in enough light to stimulate photoreceptors on the retina.

Diagram

Defects of the Eye

  1. Short sightedness(Myopia)
  2. This is the ability to view near objects clearly but distant objects are blurred.
  3. This is due to a long eyeball and image is formed before the retina.

Correction

  • Wearing concave (Diverging lens)

Diagram

  • Long sightedness (Hypermetropia)
  • This is the ability to view distant objects clearly but near objects are blurred.
  • This is due to a short eyeball and image is formed behind the retina.

Correction

  • Wearing convex (converging lens)

Diagram

  • Astigmatism
  • The curvature of the cornea is uneven hence the image is formed on different planes.
  • It’s corrected by wearing cylindrical lens.
  • Squintedness
  • Eyeballs face different directions due to defective muscles which move the eye left and right.
  • Corrected surgically
  • Old sight (Presbyopia)
  • Cataracts
  • Colour blindness