Eye

• Eye

  A) Cornea

  B) Pupil

  C) Lens

  D) Iris

  E) Retina

  F) Fovea

  G) Optic nerve

  H) Sclera

  I) Ciliary muscle

  J) Suspensory ligament

  
• Cornea

  • Its job is the refract light.
  • It is transparent as there are no blood vessels. This means that oxygen has to diffuse into it, from the air.
• Pupil

  • Allows light to pass through it to the lens
• Iris

  • It is the muscles which surround the pupil.
  • They contract and relax to control how big or small the pupil is.
• Lens

  • Job is to refract light, so it hits the fovea.
  • It can change shape, which allows it to focus the light on the fovea.
• Optic nerve

  • Job is to take the impulses from receptor cells and transmit to the brain, so it can create an image.
• Retina

  • Is a layer of light sensitive cells, which are stimulated when light hits them.
  • Contains two types of receptor cells: cone and rod cells.
  • Cone cells; allow us to see in colour, because they are more sensitive to the colour of light. This also means they don't work well in low light.
  • Rod cells; allow us to only see in black and white, because they are more sensitive to light.
• Fovea

  • Is only made of cone cells.
  • Is found on the retina.
• Cilliary muscle and Suspensory ligamnet

  • They hold the lens in place and control it's shape and size.
• Sclera

  • Surrounds the eye, to support the structure.
  • And is strong to prevent some damage.
• Iris reflex - controls the size of the pupils.

  {This is helpful as bright light can damage the retina.}
• Constricted iris;

  • Happens when there is a bright light, so the pupil is smaller.
  • Circular muscle contracts and Radial muscle relaxes.
• Dilated iris;

  • Happens when there is low light, so pupil is bigger.
  • Circular muscle relaxes and Radial muscle contracts.
• Iris

  A) Circular muscle

  B) Radial muscle

  C) Pupil

  
• Accommodation - a reflex that changes the refractive power if lens, to see near or distant objects.
• When light from an object hits the eye it has to be focused on the fovea. To do this the light has to be refracted. First, the cornea always refracts the light by the same amount. Then, the lens fine tune the refraction. So the light rays are focused on the fovea.
• Close by objects:

  • The lens need to be short and fat (more curved), to refract more strongly.
  • This is done when the ciliary muscle contracts and the suspensory ligament slackens.
  • This means they are no longer pulling on the lens, so it can return to it's natural shape.
• Distant objects:

  • Lens don't need to refract as much, as cornea has refracted it most of the way.
  • So the lens needs to be thin and long, to weaken the refraction.
  • This happens when ciliary muscle relaxes and the suspensory ligament tauts.