- The eye muscles
- 6 muscles per eye control eye movements
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Also linked to vestibular system (system necessary for balance located
in inner ear)
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Problems with the eye muscles lead to lazy eye or crossed eyes (strabismus)
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Sclera
- White of the eye, about 1 mm thick
- Tightly woven fivers since pressure in eye 2 times that of atmosphere
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Cornea
- Front of eye, transparent bulge
- Made of the same material as sclera, just more neatly organized and no blood vessels
- Begins to bend light to bring into focus on retina (up to 2/3 of the bending of light)
- Astigmatism If cornea is misshapen, retinal images is distorted.
- Cornea should be shaped like bowl, however, with astigmatism, cornea is shaped more like a spoon
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Iris
- Pigmented section of the eye
- Darker iris color helps keep extraneous light from entering the eye
- Forms the pupil
- Pupil size varies according to light intensity
- Smaller pupils, better acuity, clarity, or focus over a greater distance (similar to the aperture size on a camera)
- As we age, pupil doesnt open as wide in a dim lit area, 80 year olds pupil only opens to about ½ the size of a 20 year old, resulting in less light hitting the retina
- Pupil size can also change as a function of excitement, arousal, or fear
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Lens
- Shaped like a large aspirin tablet (about 9 mm in diameter and 4 mm thick)
- Important for accommodation (more bending of light)
- Ciliary muscles surround the lens.
- Objects near by, muscles contract, lens thickens so light is bent as much as possible
- Object further away, muscles relax, lens flattens so light is not bent much
- Presbyopia farsightness due to age
- As we age, lens continues to grow and becomes more difficult to bend the light as much as it should
- Cataract cloudy lens, reduced transparency, caused by injury or old age
- In severe cases, operate to remove (most common cause of blindness in world)
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Humor
- Fluid filling eye, serving to transport oxygen and nutrients and remove waste for several structures (like cornea and lens), maintain shape of eyeball
- Glaucoma excess fluid in eye causes pressure in eye which can damage the retina (most common cause of blinding eye disease in North America)
- Floaters debris can form, causing dark specks to appear, usually harmless, but can be a sign of a detached retina
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Retina (see figure 3.8)
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Region of highest acuity fovea (macula)
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Where optic nerve leaves the eye optic disk (blind spot)
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If eye is too long and image falls in front of retina, person is nearsighted
(myopia)
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Correct with concave lens
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If eye is too short and image falls behind retina, person is farsighted
(hyperopia)
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Correct with convex lens
- Rods and Cones (photoreceptors) facing the back of the eye
- Problems with cones leads to color blindness
- Type dependent on which type of cone (or cones) is missing
- Information from photoreceptors passed along to bipolar cells
- Horizontally connecting photoreceptors and bipolar cells are horizontal cells
- Information from bipolar cells passed along to ganglion cells, whos axons make up the optic nerve
- Horizontally connecting bipolar cells and ganglion cells are amacrine cells
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Ganglion Cells