Unit 5 Part 3 – The Eye
I. Two Types of Senses
a. Special Sense Organs
i. Characterized by complex organs or specialized receptors.
1. Ex. Eye, Ear (Balance and Hearing), Nose, and Taste Buds).
b. General Sense Organs
i. Detect stimuli
Ex. Nerve endings and Corpuscles
a. Consists of 3 layers
i. Sclera – Tough, white outer layer.
1. Also composed of transparent cornea, mucous membranes (line eyelids), and lacrimal gland (tears).
2. Lacrimal & Tarsal Glands; Tear Sac & Nasolacrimal Duct
a. Lacrimal Gland – produces watery part of tears
b. Tarsal Glands – produces oily part of tears
ii. Choroid – Middle layer
1. Composed of dark pigment to prevent scattering of light (Humans vs. dogs)
2. Composed of 2 involuntary muscles
a. Iris – Gives eye color and regulates pupil size to control amount of light entering eye.
b. Ciliary muscle – Contracts and relaxes to change shape of lens to focus on near and far objects.
i. Distant objects – Muscle relaxes and lens curves slightly (Allows you to see disant objects clearly)
ii. Near objects - Muscle contracts causing lens to bulge and curve. (Allows you to see near objects clearly)
iii. Presbyopia – Weakening of ciliary muscle (Old Sightedness)
iii. Retina – Innermost portion containing receptor cells.
1. Rods – Night Vision
2. Cones – Day vision and color
- Fovea Centralis -- area in the center of the macula; dense area of cones; 50% of Optic Nerve Fibers are served by this area that makes up only 5% of the retina; Purpose is for sharp central vision. The word fovea is Latin for "pit" or "pitfall".
3. Human Eye vs. Cat Eye
Human versus Cat in Daytime Human at Night Cat at Night
b. Fluids in eye function to maintain eye shape and refract (scatter) light rays.
i. Aqueous humor – Watery fluid in front of eye.
1. Aqueous humor is constantly produced and drained, if blockage occurs, pressure increases in eye causing glaucoma.
ii. Vitreous humor – Jellylike fluid behind lens.
1. Overexposure to UV light causes milky, white spots on lens called cataracts.
c. Eye Problems and Conditions
i. Myopia – Nearsightedness caused when lens is too long and image focuses in front of retina rather than on it.
ii. Hyperopia – Farsightedness caused when lens is too short and image focuses behind the retina.
iii. Emmetropia – Normal Vision
iv. Astigmatism – irregular shaped cornea or irregular shaped lens
v. Conjunctivitis -- inflammation of the clear membrane covering the sclera (Pink Eye)
vi. Convergence – Inability to move eyes together to maintain binocular vision
vii. Photopupillary Reflex – The pupil closes when light becomes too bright
viii. Night Blindness – inability to see in dim light
1. Due to a lack of Vitamin A
2. Rods lose their ability to react to light (genetic)
ix. Pupillary Accommodation Reflex -- reduction of pupil size in response to an object coming close to the eye.
x. Accommodation -- Eye increases optical power to focus on close up objects
xi. Glaucoma -- blockage of Aqueous Humor in the anterior chamber of the eye causing increased pressure.
xii. Cataracts -- overexposure to UV radiation causing milky white spots occuring on the cornea and/or lens (hardening of the lens)