Download Vision and the Eye

Vision and
Structure of the Eye
General Info
• Our eyes give us our most important
sense—sight or vision
• Estimated that 90% of the data processed
by our brains about the environment is
related to vision
• Processed in the visual cortex at the
posterior of the brain
• Eyes contain photoreceptors that are
sensitive to various qualities of light
Accessory Structures
• Several structures assist with the proper
functioning of the eye, though not directly
related to perceiving light
• These include the eyelids, extrinsic
muscles, and the lacrimal apparatus which
is responsible for producing tears
Eyelids
• Protect anterior surfaces of the eyes
• Eyelashes help protect from debris and
shade the eyes slightly
• The conjunctiva is a mucous membrane
which lines the inside of the eyelid and
folds back to cover the anterior portion of
the eye; secretes mucus to help moisten
and lubricate the eye
Lacrimal Apparatus
• Consists of a lacrimal gland, ducts, and
the nasolacrimal duct which channels
excess fluid into the nasal cavity
• Tears produced by the lacrimal gland are
slightly salty and are released
continuously
• Help to moisten, lubricate, and clear eye of
debris
• Also help to create a smooth surface for
refraction of light
Structure of the Eye
• Approx. 1 inch in diameter
• Made of three distinct layers, or tunics, on
its outer surface
• These are the fibrous tunic, vascular tunic,
and nervous tunic listed from superficial to
deep
• Inside is filled with fluid which is divided
into chambers
Fibrous Tunic
• Made up of two parts; the posterior sclera
and the anterior cornea
• Sclera is also known as the “white of the
eye”
• Dense with blood vessel; penetrated on
the posterior surface by the optic nerve
• Cornea is the clear “window” that bulges
out slightly
Vascular Tunic
• Very dense with blood vessels that supply
nourishment to various structures of the
eye
• Components include the choroid, the
ciliary body, and the iris. Associated with
the lens
• Choroid is a thin, dark brown pigment that
absorbs light bouncing around within the
eye; reduces distortion of images
Vascular Tunic cont’d
• Anterior of the choroid are the ciliary body
and the iris which are made of smooth
muscle tissue
• Ciliary body connects to the lens by way of
a suspensory ligament
• Contraction of muscles in the ciliary body
changes the shape of the lens to adjust
focus, called accommodation
• The lens provides separation of the interior
of the eye into two chambers
• The anterior chamber, located between
the lens and the iris, is filled with a fluid
called the aqueous humor, which
circulates and is continuously recycled
• The posterior chamber, located between
the lens and the retina, is filled with a fluid
called the vitreous humor which is not
recycled
Vascular Tunic cont’d
• The iris is the
beautifully colored,
donut-shaped portion
of the eye.
• Contraction of it’s
muscle changes the
size of the pupil (its
blackened middle)
which controls how
much light enters into
the eye
The Nervous Tunic
• Made up of the retina, which is imbedded
with numerous photoreceptors and nerves
which transmit the data detected to the
brain; rely on photo-reactive pigments
• Rod cells – contain an elongated,
cylindrical dendrite, sensitive to small
levels of light
Photoisomerization
• Cone cells – contain a dendrite that tapers
to a point, color sensitive and produce a
sharper image
Rods and Cones
The Nervous Tunic cont’d
• The axons from the rods and cones
converge at a spot near the center of the
retina called the optic disc
• There are no photoreceptors at the optic
disc which forms a blind spot
• The macula lutea and fovea centralis are
areas of the retina which have the highest
concentration of cones which provide
optimal visual acuity
Extrinsic Muscles
• Humans have six
• Move and rotate the eyeball within its
socket
• Originate from the walls of the orbit and
insert upon the outer surface of the eye
Muscles that Move the Eye
Superior rectus
Turns eye upward
Inferior rectus
Turns eye downward
Lateral rectus
Turns eye laterally
Medial rectus
Turns eye medially
Inferior oblique
Rotating movements of
eye
Rotating movements of
eye
Superior oblique
MR = Medial Rectus
LR = Lateral Rectus
SR = Superior Rectus
IR = Inferior Rectus
SO = Superior Oblique
IO = Inferior Oblique
Common Vision Conditions
Emmetropia
Normal vision
Hypermetropia Cannot see close objects clearly;
farsightedness
Astigmatism
Nyctalopia
Inability to focus an image clearly
on the retina due to unequal
curvature of the lens, cornea, or
retina
Inability to adjust vision to
darkness; nightblindness
Common Vision Conditions
Cont’d
Strabismus
Myopia
Inadequate muscle action or an
increase in the strength of one
muscle over another; cross-eyed
Cannot see distant objects clearly;
nearsightedness
Cataract
Reduced vision due to the presence
of opaque spots on the lens
Glaucoma
Increase in intraocular pressure that
interferes with vision