10.9 – Sense of Sight
The Eye as an Organ
• Lies w/ the Orbit
▫ Made of bone & contains fat, bl. Vessels, nerves, &
connective tissue
• Provides vision along with accessory organs
▫ Eyelids
 Thinnest skin of body
 Lined with conjunctiva – mucous membrane that folds
back to cover surface of eye as well
▫ Lacrimal apparatus
 Lacrimal gland + ducts
 Secretes tears continuously – lubrication & antibacterial
enzyme
▫ Extrinsic eye muscles
 Attach to sclera
 Move eye in all directions
Structure of the Eye
• Fluid-filled hollow sphere with 3 layers (tunics)
• Outer Tunic
▫ Fibrous tunic
 Transparent cornea
 White sclera
▫ Optic nerve & bl. vessels pierce the sclera @ back
Structure of the Eye
• Middle Tunic
▫ Choroid coat
 Vascular, darkly pigmented
 2 functions
 Nourish other tissues
 Keep inside of eye dark
▫ Ciliary body
 Forms ring around front of eye
 contains ciliary muscles & ligaments = holds lens in place
& changes its shape to focus objects
 Called accomodation
▫ Iris
 Thin, smooth muscle
 Adjusts to amount of lilght entering the pupil
Structure of the Eye
• Middle Tunic cont.
▫ Anterior Cavity consists of:
 Anterior Chamber
 b/w cornea & iris
 Posterior Chamber
 b/w iris & vitreous body housing the lens
 Filled with aqueous humor
 Fluid that circulates from one chamber to the other
through the pupil
Structure of the Eye
• Inner Tunic
▫ Covers back side of eye to the ciliary body
▫ Retina – contains photoreceptors
 Fovea Centralis = back of retina, point of sharpest
image
 Optic disk = nerve fibers leave eye; causes blind spot
▫ Open cavity of the eye in front of the retina is filled
with vitreous humor
Light Refraction
• Light waves must be focused – called refraction
• Cornea & lens bend light
▫ Bent light is focused on retina
▫ Aqueous & Vitreous humors also help to
bend/refract the light
Visual Receptors
• 2 kinds:
▫ Rods
 Elongated
 Sensitive to light; function in dim light
 Produce colorless vision
▫ Cones
 Provides sharp images in bright light
 Produces vision in color
 Fovea centralis contains the most cones
Visual Nerve Pathways
• Axons of ganglion cells leave eyes to form the
optic nerves
• Fibers from the medial half of the retina cross
over in the optic chiasma
• Impulses are transmitted to the thalamus
• Then sent to the visual cortex of the occipital
lobe for final association
Optical Device – The Eye
• Myopia – nearsighted
▫ Eyes are too long
• Hyperopia – farsighted
▫ Eyes are too short
• Astigmatism
▫ Cornea misshapen
• Cataracts
▫ Clouding of the lens
• Floaters
▫ Cell fragments break off
Lasik eye surgery