sense organs: the eye

• Eye structure


  • Cross section of the eye
  • Light hits the cornea
  • Cornea is transparent with no blood vessels
  • Cornea refracts light
  • Iris controls pupil size
  • Pupil allows light to pass through to the lens
  • Lens refracts light and can change shape
  • Retina has cone cells for color vision and rod cells for low light black and white vision
  • Fovea is a spot on the retina with only cone cells for clearest vision
  • Optic nerve transmits impulses from receptor cells to the brain
• Iris reflex


  1. Pupil constricts in bright light

  2. Pupil dilates in low light

  3. Circular muscles in iris contract to constrict pupil

  4. Radial muscles in iris contract to dilate pupil
• Bright light conditions


  Pupil constricts to prevent retina damage
• Low light conditions


  Pupil dilates to allow more light in
• Constricted pupil is called pupil constriction
• Dilated pupil is called pupil dilation
• Accommodation


  1. Reflex that changes the refractive power of the lens

  2. Allows seeing both near and distant objects
• Glasses


  • Help when the accommodation process doesn't work properly
• Cornea


  Refracts or bends light
• Lens


  Refracts or bends light
• Ciliary muscles


  Control the shape of the lens
• Suspensory ligaments


  Control the shape of the lens
• Focusing light on the fovea

  1. Light from object is refracted by cornea

  2. Light is further refracted by lens to converge on fovea
• Object is close


  Lens needs to be short and fat to refract light strongly
• Object is distant


  Lens needs to be stretched out to reduce refractive power
• Changing lens shape


  1. Ciliary muscle contracts/relaxes

  2. Suspensory ligaments slacken/become taut

  3. Lens changes shape
• Ciliary muscle contracts inwards towards the lens, not outwards
• Suspensory ligaments can only be pulled taut or slackened, they cannot contract or relax
• Long-sightedness (hyperopia)


  Lens cannot refract enough, light focuses behind retina
• Short-sightedness (myopia)


  Lens refracts too much, light focuses in front of retina
• Glasses for long-sightedness


  • Contain convex lenses to provide extra refracting power
• Glasses for short-sightedness


  • Contain concave lenses to counteract over-refraction of the lens
• The virus-infected cells can be destroyed by T-lymphocytes that secrete chemicals to kill the infected cells
• The two eyes are positioned in bony eye sockets, at the front of the skull
• Shape of the eyeball

  • More or less spherical
• Eyelids and eyelashes


  • Protect the eye from foreign objects
• Conjunctiva


  • Thin mucous membrane covering the front of the eye
  • Contains pain receptors stimulated by dust and foreign particles, resulting in the blink reflex
  • Lines the eyelids on the inner surface
• Tear gland


  • Positioned above the eyeball
  • Secretes antiseptic tears that protect the conjunctiva from bacteria and dehydration
• Eye muscles


  • Each eye is equipped with six eye muscles, making eye movement possible in all directions - from side to side, up and down as well as diagonally
• Internal structure of the eye

  • Outer fibrous layer
  • Middle vascular layer
  • Inner light-sensitive retina
• Sclera


  • Strong, white, inelastic layer of connective tissue that forms the outer layer of the eyeball and extends to the transparent cornea at the front
  • Protects the internal parts of the eye
  • Helps to maintain the spherical shape of the eyeball
  • Serves for muscle attachment
• Cornea


  • Transparent front part of the sclera, which is more convex than the rest of the eyeball
  • Allows light rays through to the inner layers of the eye
  • Causes the refraction of incoming light rays
• Choroid


  • Thin, darkly pigmented, vascular layer
  • Pigment absorbs excess light rays to prevent internal reflection which may cause blurred images
  • Blood vessels supply oxygen and nutrients to cells
• Ciliary body


  • Thickening of the front part of the choroid, consisting of involuntary muscles called the ciliary muscles
  • Attached to the lens by suspensory ligaments
  • Contracts and relaxes to change the curvature of the lens during accommodation
• Iris


  • Round, coloured structure suspended towards the centre of the eye like a circular curtain
  • Situated in front of the lens and is a continuation of the choroid
  • Contains pigments that give colour to the eye
  • Contains two sets of involuntary muscles, the radial and circular muscles
  • Has an opening in the middle called the pupil, through which light rays enter the eye
  • Regulates the amount of light that enters the eye by controlling the size of the pupil
• Lens


  • Rubbery, elastic, transparent, biconvex structure that can change shape
  • Kept in position by suspensory ligaments attached to the ciliary body
  • Changes shape to refract light rays from near and far objects to form a clear image on the retina
• Retina


  • Lines the inside of the eyeball
  • Consists of a pigment layer and a nervous tissue layer with two types of photoreceptors: rods and cones
  • Rods are stimulated in low light intensity and give rise to black and white vision
  • Cones are stimulated in high light intensity and give rise to intense, bright, colour vision
  • Yellow spot contains mostly cones and very few rods, with the fovea centralis in the middle containing cones only
  • Nerve fibres from the photoreceptors form a synapse with sensory bipolar neurons, which in turn form a synapse with optic neurons that join to form the optic nerve
• Optic nerve


  • Conducts nerve impulses to the cerebral cortex of the brain where they are interpreted, giving rise to the sensation of sight
• Blind spot


  • Place where the optic nerve leaves the eyeball, has no rods or cones
  • When light rays fall on the blind spot, no nerve impulses are conducted to the brain and no image is formed
• Anterior cavity


  • In front of the lens, filled with watery fluid (aqueous humour)
  • Further subdivided by the iris into the anterior chamber (between cornea and iris) and posterior chamber (between iris and lens)