The eye

• sclera — the white of the eye. It protects the eyeball from mechanical damage.
• conjunctiva — thin transparent membrane covering the sclera in front. It secretes mucus which helps to keep the front of the eyeball moist.
• eyelashes — helps to shield the eye from dust particles
• tear glands — lies at the corner of the upper eyelid

  • wash away dust particles
  • keep the cornea moist for atmospheric oxygen to dissolve so that the dissolved oxygen can diffuse into the cornea
  • lubricate the conjunctiva, helping to reduce friction when the eyelids move
• eyelids

  • protects the cornea from mechanical damage
  • squinting prevents too much light from entering the eyelid and damaging the retina
  • blinking spreads tears over the cornea and conjunctiva, and wipes dust particles off the cornea
• iris — circular sheet of muscles

  • controls the size of pupil and the amount of light entering the eye
  • contains a pigment that gives the eye its colour
• pupil — a hole in the centre of the iris which allows light to pass through
• Retina — innermost layer of the eyeball and contains light sensitive cells or photoreceptors
• lens — transparent , circular and biconvex structure. It is able to change its thickness to focus light onto the retina.
• blind spot — the region where optic nerve leaves the eye
• optic nerve — the nerve that transmits nerve impulses to the brain when the photoreceptors in the retina are stimulated
• fovea — small yellow depression in the retina where images are focused. It contains the greatest concentration of cones but no rods
• vitreous chamber — space behind the the lens. It is filled with vitreous humour, a transparent, jelly like substance. The vitreous humour keeps the eyeball firm and helps to refract light onto the retina
• choroid — middle layer of the eyeball

  • pigmented black to prevent internal reflection of light
  • contains blood vessels that bring oxygen and nutrients to the eyeball and remove metabolic waste products
• ciliary — thickened region at the front end of the choroid

  • It contains ciliary muscles, which controls the thickness of the lens
• suspensory ligament — connective tissue that attaches the edge of the lens to the ciliary
• cornea — dome-shaped transparent layer continuous with the sclera or white of the eye. It refracts or bends light rays into the eye. The cornea causes the greatest refraction of light into the eye.
• aqueous chamber — space between the lens and the cornea. It filled with aqueous humour, which keeps the front of the eyeball firm and helps to refract light into the pupil.
• photoreceptors includes rods and cones
• pupil reflex is a reflex action which the pupil changes in size in response to changes in light intensity. This is controlled by the iris.
• the size of the pupil is controlled by two sets of involuntary muscles in the iris: circular muscles and the radial muscles.
• bright light:

  1. the circular muscles of iris contract
  2. the radial muscles of the iris relax
  3. the pupil becomes smaller or constricts. this reduces the amount of light entering the eye
• dim light:

  1. the radial muscles of the iris contract
  2. the circular muscles of the iris relax
  3. the pupil enlarges or dilates. this increases the amount of light entering the eye.
• pathway of nerve impulses in pupil reflexes:

  stimulus (change in light intensity) —> receptors (in retina) —> sensory neurons in the optic nerve —> brain —> motor neurone —> effector (iris muscles)
• how is imaged formed:

  1. the light rays are refracted through the cornea and the aqueous humour onto the lens
  2. the lens causes further refraction and the rays are brought to a focus on the retina
  3. the image on the retina stimulates either the rods or the cones, depending on the intensity of the light
• image formed on the retina:

  • upside down (inverted)
  • laterally
  • diminished (smaller in size than the actual object)
• focusing/accommodation — is the adjustment of the lens of the eye so that clear images of objects at different distances are formed on the retina
• focusing on the distant object:

  1. ciliary muscles relax, pulling on the suspensory ligaments
  2. suspensory ligaments become taut, pulling on the edge of the lens
  3. lens becomes thinner and less convex
• focusing on a near object:

  1. ciliary muscles contract, relaxing their pull on the suspensory ligaments
  2. suspensory ligaments slacken, relaxing their pull on the lens
  3. the lens, being elastic, becomes thicker and more convex