The eye is composed of 3 separate layers:

1. SCLERA (outermost layer)

2. CHOROID LAYER (middle layer)


a) Light sensitive

b) Bipolar Cells

c) Cells of the OPTIC NERVE

In the center of the retina is a small depression called the FOVEA CENTRALIS, it is the most sensitive part of the eye, contains closely packed cones (but no rods), most light rays will fall here.

The BLIND SPOT is the area where the optic nerve comes into contact with the retina, all of the nerve fibres from the rods and cones collect here.

ACCOMMODATION REFLEX: changes to the pupil and lens in response to changes in the distance from the object being viewed

(ex: nearby object ---> pupil constricts and lens bulges)

(ex: distant object à pupil dilates and lens is pulled thin)

(ex: dark à pupil dilates)

(ex: bright à pupil constricts)

- CILIARY MUSCLES attach to the lens to alter the shape of the lens



CONJUCTIVIA: a thin layer membrane that covers the eyelids and eyeball to form a seal (conjuctivitis is pink eye).


  1. GLAUCOMA: the aqueous humor does not drain properly and fluid pressure builds up, may collapse blood vessels in the retina
  2. CATARACT: the lens or cornea become clouded or opaque
  3. COLOR BLINDNESS: a genetic disorder that effects more males than females, most common is red-green color blindness due to defects in the cone cells on the retina
  4. ASTIGMATISM: caused by an uneven curvature of the lens or cornea causing a distortion of the image being seen, some parts of the image are in focus while other parts are not
  5. NEARSIGHTEDNESS (MYOPIA): caused by an eyeball that is too long or a lens that cannot flatten enough, resulting in an image that is in focus in front of the retina. Can be corrected with a CONCAVE lens. People can still often focus on near objects, but not distant objects.
  6. FARSIGHTEDNESS (HYPEROPIA): is caused by an eyeball that is too short causing the image to focus behind the retina. Can be corrected with a CONVEX lens. People can still often focus on distant objects but not objects that are near to them.