10.2 Structure of The Human Eye

• The wall of the eye has 3 layers: Schlerotic coat (sclera), Chloroid and the Retina
• Sclerotic Coat (Sclera) is a tough, white outer covering of the eyeball (continuous with the cornea)

  It maintains the shape of the eyeball

  Eye muscles attached to this layer facilitates the movement of the eyeball
• The cornea is a dome-shaped transparent layer.
• The iris, which is the coloured part of the eye, is a circular sheet of muscles that contain pigment which gives the eye its colour.
• Rods and Cones, which are light-sensitive cells, are stimulated.
• Nerve impulses reach the optic centre of the brain.
• The pupil is a hole in the centre of the iris that allows light to enter the eye.
• Light rays are refracted when they pass through the cornea and aqueous humour.
• The inverted image is formed on the retina.
• The eyelid protects the cornea from mechanical damage and prevents excessive entry of light.
• The brain has a corrective function, as the image in the retina is upside down, but the brain makes it upright.
• The image formed in the retina is vertically and laterally inverted and smaller than the object.
• The sclera, also known as the white part of the eye, refracts light rays into the eye.
• Light rays are further refracted when they pass through the lens.
• The pupil reflex is a response to changes in light intensity that protects the eye from excessive light exposure, which could damage the retina.
• The conjunctiva is a mucus membrane that covers the sclera and secretes mucus to keep the front of the eyeball moist.
• The image on the retina either stimulates rods/cones, depending on the light intensity.
• Nerve impulses generated are transmitted through the optic nerve.
• Low light intensity causes the radial muscles of the iris to contract, causing the circular muscles of the iris to relax, resulting in the pupil dilating and increasing the amount of light entering the eye.
• The brain interprets the information and forms an upright image.
• High light intensity causes the circular muscles of the iris to contract, causing the radial muscles of the iris to relax, resulting in the pupil constricting and reducing the amount of light entering the eye.
• If the blind has his sight restored, objects appear upside down to him at first → brain has to learn how to correct the images that fall on the retina.
• Choroid is a layer between sclera and retina that contains blood vessels to supply oxygen and nutrients to the retina.
• Sclerotic Coat (Sclera) is the tough, white outer covering of the eyeball, continuous with the cornea, which maintains the shape of the eyeball.
• Eye muscles are attached to the Sclerotic Coat, facilitating the movement of the eyeball.
• Chloroid is a black pigmented middle layer in the eye, which prevents internal reflection of light.
• Chloroid contains blood vessels that carry oxygen and nutrients to the eyeball and remove metabolic waste from the eyeball.
• Retina is the innermost layer of the eye wall.
• Retina contains light-sensitive cells, known as photoreceptors, which consist of rods and cones.
• Photoreceptors in the Retina are connected to nerve fibres from the optic nerves.
• How an eye focuses on a nearby object:

  1. Ciliary muscles contract, relaxing their pull on suspensory ligaments
  2. Suspensory ligaments slacken, relaxing their pull on lens
  3. Lens become thicker and more convex, decreasing the focal length
  4. Light rays are sharply focused on the retina stimulating the photoreceptors
  5. Nerver impulses produced are transmitted to the brain via the optic nerve
  6. The brain interprets the impulses and the person sees the nearby object
• How an eye focuses on a distant object:

  1. Ciliary muscles relax, pulling on suspensory ligaments
  2. Suspensory ligaments becomes taut, pulling on lens
  3. Lens become thinner and less convex, increasing the focal length
  4. Light rays are sharply focused on the retina stimulating the photoreceptors
  5. Nerver impulses produced are transmitted to the brain via the optic nerve
  6. The brain interprets the impulses and the person sees the distant object
• Near vision → Light rays from a near object are divergent (further away from each other) — greater angle of refraction needed to bring them into focus = thicker lens (more curved lens)

  — ciliary muscles contract, suspensory ligaments slacken, lens more convex/thicker
• Far vision → Light rays from a far object are parallel — smaller angle of refraction needed to bring them into focus = thinner lens (less curved lens)

  — ciliary muscles relax, suspensory ligaments become taut, lens less convex/thinner
• Presbyopia is when the lens loses its elasticity with age, making accommodation difficult or impossible.
• Reading glasses can be used to correct presbyopia by providing a converging lens that compensates for the loss of elasticity in the natural lens.
• Focusing — curvature (more/less convex) and thickness (thicker/thinner) of lens is adjusted, allowing light rays to be focused on the retina
• Accomodation — Lens of the eye is adjusted so that clear images of objects at different distances are formed on the retina