1. Eye structure & function
January 10, 2020

2. Do Now Which fact is false?
Your eye muscles are the most active muscles in your body.
Newborns don’t produce tears
Corneal scratches heal in about 48 hours.
If you sit too close to a TV , computer, or tablet, you will damage your eyes.

3. External Eye & Accessory Structures
Lacrimal refers to tears. Tears cleanse and lubricate the eyes, and also fight bacteria.

4. 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

5. External Eye & Accessory Structures
Conjunctiva are the membranes the line the eyelid and eyeball. Conjunctivitis is inflammation of these membranes, caused by irritants, allergies, or infection (e.g. “pink eye”).

6. Extrinsic eye muscles
Control movement of the eyes. Locations and functions can be reasoned out! Remember: rectus = straight, oblique = slanting
Name
Action
Lateral rectus
Medial rectus
Superior rectus
Inferior rectus
Inferior oblique
Superior oblique

7. 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

8. 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

9. 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.
Glaucoma is a common cause of blindness among the elderly. It can be corrected with eye drops that help drain the aqueous humor or with surgery to enlarge the canal Schlemm.

10. Pathway of light
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
Fun facts: cornea heals remarkably well, and can be transplanted without any rejection since it is avascular
Protects from both mechanical stress and UV light

11. Pathway of light
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.

12. Pathway of light
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.
More on focusing later

13. 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.
More on focusing later

14. Photoreceptors Rods Cones more abundant
sensitive to low levels of light
do not discriminate colors
Cones
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
Responsible for night and peripheral vision – that’s why colors seem to be lost in the dark
Watch me!
More on focusing later
Responsible for color and fine detail vision – including reading

15. 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
More on focusing later

16. Color Blindness
More on focusing later

17. Color Blindness
More on focusing later

18. Color Blindness
More on focusing later

19. Color Blindness
More on focusing later

20. Color Blindness
More on focusing later

21. 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.
At rest, our eyes naturally focus
on far-away objects.
However, by contracting the ciliary
body muscles, we can make the
lens bulge so that it has greater
refractive ability – allowing us to
focus on close items.
As we get older, our lens loses elasticity – making it harder to focus on nearby items.
This condition is called presbyopia (old eyes)
More on focusing later

22. 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.
More on focusing later
Watch me!

23. 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?

24. Exit Ticket
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?
Name two structures that help us to see in low light conditions.
Why do you often have to blow your nose after crying?
Name 4 substances that refract light? Which refracts light the most? Which is responsible for accomodation?
More on focusing later