Eye: Accessory Structures & Layers

conjunctiva

Transparent mucous membrane lining inside of eyelids; covers front of eye (not cornea)

lacrimal apparatus

Tear gland in superolateral orbit; secretes tears → drains into nasolacrimal duct → nose

cerumen

Earwax; secreted by ceruminous glands in external auditory canal

stereopsis

Three-dimensional vision (depth perception)

extrinsic muscles

6 muscles per eye; attached to orbit wall and eyeball; move eyes in all directions

sclera

Tough, white outer layer of eyeball (fibrous layer)

cornea

Transparent, dome-shaped membrane; avascular; provides ~60% of focusing power

uvea

Middle vascular layer of eyeball (iris, ciliary body, choroid)

iris

Colored part of eye; controls pupil size (dilates/constricts)

pupil

Black opening in center of iris

lens

Transparent; changes shape (accommodation) to focus light; avascular; loses elasticity with age

aqueous humor

Watery fluid in anterior cavity (in front of lens)

vitreous humor

Gelatinous fluid in posterior cavity (behind lens); glasslike appearance

retina

Light-sensitive innermost layer; contains rods and cones

rods

~130 million; perceive light only (no color); function in dim lighting

cones

~6.5 million; perceive bright light and color; red, green, blue types

macula lutea

Circular yellowish region lateral to optic disc

fovea centralis

Small pit in center of macula; 4,000 tiny cones, no rods; sharpest vision (each cone has own nerve fiber)

optic disc

Where optic nerve connects to retina; no rods or cones = "blind spot"

optic nerve

Bundle of >1 million nerve fibers; transmits impulses to visual cortex

emmetropia

Normal vision – image focused sharply on retina

refraction

Bending of light rays (cornea first, then lens)

accommodation

Process of changing lens shape to focus on near/distant objects