Chapter 14: Coordination and Response: II Animal Receptor Organs

The Mammalian Eye

• Cornea

  • A clear layer coating iris.

  • Refracts light in the eye.

• Iris

  • Colored section of eye.

  • Controls amount of light entering by contracting and dilating pupil.

• Pupil

  • Allows entrance of light.

• Lens

  • Behind iris.

  • Focuses image on retina.

• Retina

  • Contain photoreceptors; rods and cons.

• Optic Nerve

  • Each photoreceptor cell is attached to neuron.

  • Carries nerve impulses to brain.

• Fovea

  • Section in middle of retina.

  • Provides clear images.

• Ciliary Muscles

  • Alters the thickness of muscles.

• Choroid Coat

  • Contains number of blood capillaries which nourish eye.

  • Painted black (due to supply of blood vessels) to prevent internal reflection of light.

• Aqueous Humor

  • Watery fluid.

• Vitreous Humor

  • Transparent jelly like fluid.

NOTE: both aqueous humor and vitreous humor serve to keep eyeball form and refract light.

• Rods and Cons

Controlling the entry of light in eye

• Achieved by altering diameter of pupil.

• Size of pupil controlled by iris muscles - circular and radial muscles.

• When circular muscle contract, radial muscles relax, and pupil becomes smaller.

• When radial contract, circular muscle relax , and pupil enlarges (diltes).

• Pupil enlarges when surrounding light intensity is low, and smaller when the light intensity is high.

Vision

Focusing (Accomodation)

It is the adjustment of lens of the eye so that clear images of objects at different distances are formed on retina.

• Focusing on distant object

  • Circular ciliary muscles relax.

  • Suspensory ligaments are pulled tight - lens become flatter and less convex.

• Focusing on nearer object

  • Circular ciliary muscles contract.

  • Suspensory ligaments are released loosely - lens becomes thicker and more convex.