1. Chapter 8 Special Senses

2. The Senses  Special senses  Smell  Taste  SIGHT  Hearing  Equilibrium Lady website

3. The Eye and Vision  Of all the sensory receptors in the body  70% are found in the eyes  Optic tracts  Carry information from the eyes to the brain  Consists of over a million nerve fibers per eye

4. The Eye and Vision  Protection for the eye  Only 1/6 th of the eye’s surface can be seen  Remaining portion of the eye  Enclosed in a bony orbit  Cushioned by a layer of fat  Accessory Structures  Eyelids  Conjuctiva  Extrinsic eye muscles  Lacrimal apparatus

5. Accessory Structures of the Eye Eyelids Meet at the medial & lateral corners of the eye = (Medial & lateral canthus)  Nictitating membrane  Eyelashes

6. Accessory Structures of the Eye

7.  Conjunctiva  Delicate membrane  Lines the eyelids & covers part of the outer surface of the eyeball  Connects to the surface of the eye  Secretes mucus to lubricate the eye  Conjuctivitis?

8. Accessory Structures of the Eye

9. Conjunctivitis  PINKEYE  Common condition  Inflammation of the conjunctiva  Clear membrane that covers the white part of the eye & lines the inner surface of the eyelids  No danger to the eye or your vision  Causes  Bacteria, viruses, allergies (hay fever), irritants

10. Accessory Structures of the Eye  Lacrimal apparatus  Consists of the lacrimal glands and ducts  drain lacrimal secretions into the nasal cavity  Lacrimal glands  Continually release tears  Tears  Antibodies  Lysozyme  Enzyme that destroys bacteria  Clean, protect and moisten the eye surface

11. Accessory Structures of the Eye  Extrinsic (external) eye muscles  6 muscles attached to the outer surface of each eye  Produce eye movements

12. Internal Structures: The Eyeball  Eyeball  Hollow sphere  Its wall is composed of three tunics (layers)  Sclera or fibrous tunic  “White of the eye”  Outermost layer  Choroid or vascular tunic  Middle layer  Sensory tunic  Innermost layer  Retina  Interior is filled with fluids called humors

13. Internal Structures: The Eyeball

14. The Fibrous Tunic  Sclera  White connective tissue layer  “White of the eye ”  Cornea  Transparent “window”- central anterior portion  Allows for light to pass through  Repairs itself easily  Only human tissue that can be transplanted from one person to another without the fear of rejection  No blood vessels  beyond the reach of the immune system

15. Choroid Layer  Middle layer  Blood-rich nutritive tunic  Pigment prevents light from scattering  Modified anteriorly into two structures  Cilliary body – smooth muscle to which the lens is attached  Iris  Pigmented layer that gives eye color  Pupil – rounded opening in the iris through which light passes

16. Sensory Tunic (Retina)  Contains receptor cells (photoreceptors )  Rods  Cones  Signals pass from photoreceptors and leave the retina toward the brain through the optic nerve  Photoreceptors  Distributed over the entire retina, except where the optic nerve leaves the eyeball  Optic disc or blind spot*

17. Neurons of the Retina

18. Neurons of the Retina & Vision  Rods  Most are found towards the edges of the retina  Allow dim light vision and peripheral vision  Test fact of rod location by looking “off- center” at objects at night…stars/glow-in- the-dark objects on ceiling  Perception is in all gray tones

19. Neurons of the Retina & Vision  Cones  Densest in the center of the retina  Fovea centralis  Area of the retina with only cones  Greatest visual acuity  3 types of cones  respond to a particular wavelength of light  Blue light  Green light  Both green & red light  ** Color blindness  lack of one to all cone types

20. Color-Sensitive Cones

21. Lens  Focuses light entering the eye on the retina  Flexible biconvex crystal-like structure  Held in place by a suspensory ligament  attached to the ciliary body  Divides the eye into 2 segments  Anterior (aqueous) segment  Posterior (vitreous) segment

22. Lens

23. Internal Eye Chamber Fluids  Anterior (aqueous) Segment  Contains a watery fluid  Aqueous humor  Found in chamber between the lens and cornea  Similar to blood plasma  Helps maintain intraocular pressure  Provides nutrients for the lens and cornea  Reabsorbed into venous blood  Blocked drainage = Glaucoma

24. Internal Eye Chamber Fluids  Posterior (Vitreous) Segment  Vitreous Fluid  Gel-like substance behind the lens  Keeps the eye from collapsing  Lasts a lifetime and is not replaced

25. Lens Accommodation  Light must be focused to a point on the retina for optimal vision  Resting eye is set for distance vision (over 20 ft away)  The lens must change shape to focus for closer objects

26. Vision  Emmetropia  Correct focus  Myopia  nearsightedness  Focus of light in front of retina  Eyeball too long or lens too strong  Distant objects are blurry  Hyperopia  farsightedness  Focus of light beyond the retina  Short eyeball or lazy lens  Near objects are blurry.

27. Emmetropia

29. Hyperopia