October 30, 2015
Vision: photoreceptor cells in eye
3 grps of accessory organs
1-eyebrows, eyelids, & eyelashes
eyebrows: protection fr debris & sun
eyelids: continuation of skin, protection & lubrication
eyelashes: trap dirt, help lubricate eye
2- lacrimal apparatus: produces tears to clean eye,
lubricates eye
3-extrinsic eye muscles: 3 prs of musc that anchor eye
& move eye
eyeball is elongated sphere, 2.5 cm diam
entire eyeball has 3 layers of tissues or coats
1- sclera: white fibrous connect tiss
outermost layer, encircles entire eye
anteriorly it is transparent, forms cornea
posteriorly it is opaque
protects eye, supports eyeball
October 30, 2015
3 tissue layers of eyeball
2-choroid coat: middle layer, contains blood vessels
anteriorly forms iris, pupil, ciliary body
posteriorly dark pigment, absorbs stray light rays, also
nourishes retina
3-retina: innermost layer of eye
thin membrane containing photoreceptors
attached anteriorly to ciliary body
attached posteriorly to optic nerve
only 2 attachmts, can become loosened, problem
eye parts from anterior to posterior
cornea: tough membrane through which light is admitted to e
part of sclera (tough connect tiss) but clear, protects eye
refracts light rays
aqueous humor: anterior chamber of eye, watery fluid btw
cornea & iris, protects eye, helps eye keep shape, refracts
light rays
October 30, 2015
eye parts, anterior to posterior
iris: colored part of eye, surrounds pupil, anterior to lens,
formed from choroid layer, regulates the size of pupil,
protects eye from intense light
pupil: black circular opening in center of iris, admits light into
eye
ciliary body: part of choroid layer, holds lens in place, allows
lens to accommodate
lens: hard, but flexible oval, refracts & focuses light rays on
retina, divides eye into anterior & posterior parts
vitreous humor: jellylike fluid filling posterior chamber of eye,
helps eyeball keep its shape, refracts light rays
retina: innermost layer, contains photoreceptive cells, rods &
cones, sends light info to brain
rods: photoreceptor cells on retina, responsible for black &
white vision, function in low light levels
cones: photoreceptor cells on retina, responsible for color
vision, function in high light levels
October 30, 2015
eye parts
fovea centralis: depression on retina, area of greatest
concentration of cones, greatest visual acuity
optic nerve: exits eye posteriorly, carries light info to brain
there are no rods nor cones at spot where nerve exits eye
so this a blind spot
tapetum lucidum: a beautiful layer of choroid only seen in
animals w/night vision, reflective layer that improves night
vision
October 30, 2015
Vision Physiology
Focusing: to form images, light must land on photorecept
cells on retina
lens refracts (bends) light rays to land on retina
cornea, aqueous humor, vitreous humor also refract
light rays
lens also changes shape to focus rays on retina
normal eyeball is optimal shape for light to land on retina
to focus on distant objects, lens is flattened, relaxed
to focus close up, lens must be rounded
we evolved for optimal sight distance to be 20ft away
thus eye charts measure fr. 20ft
near-sightedness, myopia: due to elongated eyeball
can see close objects, but can't see letters 20 ft away
lens focuses in front of retina for distant obj
concave lens fixes problem
October 30, 2015
far-sightedness, hyperopia: due to shortened eyeball
can see distant, but not close up
lens focuses light rays behind retina
convex lens fixes problem
accomodation: ability of the lens to change shape when
viewing things up close
when viewing close objects, ciliary musc contracts, lens
becomes more convex, rounded
when viewing distant objects, ciliary musc relaxes, lens
flattens
as you age, the lens loses its elasticity, can't round up for
close viewing
photopupillary reflex
when light entering the eye is intense, circular musc of iris
contracts, size of pupil decreases
when light entering eye is dim, circ musc of iris relaxes
allowing more light in, size of pupil increases
October 30, 2015
convergence reflex: as object being viewed moves closer to
eyes, eyeballs move medially in order for light rays to strike
equivalent spots on retina
Eye problems
astigmatism: uneven cornea, dips in cornea
light rays aren't evenly focused
fuzzy areas in field of view
glaucoma: buildup of fluid in anterior chamber due to duct
blockage
pressure compresses arteries that serve retina
nerve fibers can die, blindness can result