EYE STRUCTURE &
FUNCTION
6-7 January 2015
Do Now
Which fact is false?
1. Your eye muscles are the most active muscles in your body.
2. Newborns don’t produce tears
3. Corneal scratches heal in about 48 hours.
4. If you sit too close to a TV , computer, or tablet, you will
damage your eyes.
External Eye & Accessory Structures
Lacrimal refers to tears. Tears cleanse and lubricate the
eyes, and also fight bacteria.
External Eye & Accessory Structures
The lacrimal ducts empty into the nasal cavity
this is why nose and eye irritation is often linked.
•
you get the sniffles if cry and
•
you get watery eyes if congested
External Eye & Accessory Structures
Conjunctiva are the membranes the line the eyelid and
eyeball.
Conjunctivitis inflammation of these membranes, caused by
irritants, allergies, or infection (e.g. “pink eye”).
Extrinsic eye muscles
Control movement of the eyes.
Locations and functions can be reasoned out!
Remember: rectus = straight, oblique = slanting
Name
Lateral rectus
Medial rectus
Superior rectus
Inferior rectus
Inferior oblique
Superior
oblique
Action
Extrinsic eye muscles
Control movement of the eyes.
Locations and functions can be reasoned out!
Remember: rectus = straight, oblique = slanting
Name
Action
Lateral rectus
Moves eye laterally
Medial rectus
Moves eye medially
Superior rectus Moves eye up
Inferior rectus
Moves eye down
Inferior oblique
Moves eye up and laterally
Superior
oblique
Moves eye down and
laterally
Eyeball
The eye has three tunics, or coats.
• Sclera – “whites of the eye” , outermost, thick connective tissue.
• Choroid – has blood vessels, middle layer
• Retina – contains the photoreceptors (rods & cones), inner layer
Eyeball
The eye is divided into two fluid-filled chambers:
• The anterior chamber is filled with aqueous humor
• The posterior chamber is filled with vitreous humor
• Both fluids maintain eye pressure, and the aqueous humor nourishes the
cornea and lens.
Glaucoma
occurs when
the aqueous
humor
doesn’t drain
properly,
resulting in
increased
eye pressure
and
blindness.
Pathway of light
1.
Light enters the eye at the cornea – a clear, hard part of the sclera.
Functions: protects eye and focuses light
Fun fact: the cornea is responsible for ~70% of the eye’s
focusing ability
Pathway of light
2.
Light passes through the pupil which is the opening in front of the
lens.
•
•
The size of the pupil is controlled by the muscles of the iris (the
colored part of the eye).
The pupil dilates or contracts to vary the amount of light hitting the
retina.
Pathway of light
3.
The light passes through the lens, which focuses the light onto the
retina.
•
The ciliary body are muscles which change the shape of the lens to
focus on nearby items, a process called accomodation.
Pathway of light
4. The light passes through the vitreous humor to land on the retina,
which contains the photoreceptors.
There are no photoreceptors on the optic disc, which
is where the optic nerve exits the eye – this causes a
small blind spot.
Photoreceptors
Rods
• more abundant
• sensitive to low levels of light
• do not discriminate colors
Cones
Responsible for night and
peripheral vision – that’s
why colors seem to be lost
in the dark
• 3 types, each sensitive to a different wavelengths
• triggering of more than one cone is
interpreted by brain as different colors
e.g. if both red and green are activated,
the brain will interpret the light as yellow
or orange
• greater resolution than cones
• mostly found in fovea centralis
Responsible for color and fine
detail vision – including reading
Color Blindness
Color blindness is usually caused by the absence of one or more
cones.
• Occurs in ~5% of population
• X-linked trait … much more common in men
Color Blindness
Color Blindness
Color Blindness
Color Blindness
Color Blindness
Refraction & Accomodation
Light is bent – or refracted – by nearly every eye structure that it
passes through on the way to the retina.
However the lens is the only structure that can vary how much the light
is bent in order to allow us to focus on different objects – a process
called accomodation.
As
getour
older,
lens loses
• Atwe
rest,
eyesour
naturally
focus
elasticity
– making
it harder to
on far-away
objects.
focus on nearby items.
• However, by contracting the ciliary
body
muscles,iswe
can make the
This
condition
called
lens bulge(old
so that
it has greater
presbyopia
eyes)
refractive ability – allowing us to
focus on close items.
Fun fact:
Refraction flips and reverses the light rays, forming an upside
down and reversed image on the retina … but the brain learns to
interpret visual information correctly.
Watch me!
Closure
• What were our objectives, and what did you learn about
them.
• What was our learner profile trait and how did we
exemplify it?
• How does what we did today address our unit question?
Exit Ticket
1. Nine children attending the same day-care center developed
red, inflamed eyes and eye lids. What is the most likely
cause and name of this condition?
2. Name two structures that help us to see in low light
conditions.
3. Why do you often have to blow your nose after crying?
4. Name 4 substances that refract light? Which refracts light the
most? Which is responsible for accomodation?