1. The Special Senses allow the human body to react to the environment. 2. The body is able to see, to hear, to taste, to smell, and to maintain balance. 3. Senses occur because the body has structures that receive the sensation, nerves that carry the sensory message to the brain, and a brain that can interpret and respond to the message.

 Organ that controls the special sense of sight.  Receives light rays and transmits the rays to the optic nerve, which carries the rays to the brain where they are interpreted as vision or sight.

 Eye is well protected a. Partially enclosed in a bony socket of the skull. b. Eye lids and lashes help keep out dirt and pathogens. c. Lacrimal glands produce tears -Tears constantly moisten and cleanse out the eye. -Tears flow across eye and drain through the lacrimal duct into nasal cavity. d. Conjunctiva -Mucous membrane that protects the eye. -Lines the eyelids and covers the front of the eye. -Provides protection and lubrication.

Three main layers of the eye 1. Sclera - Outermost layer -Tough connective tissue - Frequently referred to as the white of the eye. - Maintains the shape of the eye. - Extrinsic muscles, responsible for moving the eye within the socket, are attached to the outside of the sclera.

2. Choroid Coat -Middle layer of the eye -Interlaced with many blood vessels that nourish the eyes -Pupil a. Hole in the front of the choroid coat. b. Allows light rays to enter. -Iris a. Special part of the choroid coat. b. Colored portion of the eye. c. It is a muscle that controls the size of the pupil and regulates the amount of light entering the eye.

3. Retina -Innermost layer of the eye. -Made of many layers of nerve cells that transmit the light impulses to the optic nerve. Two special types of cells in retina a. Cones: used mainly for light vision, are sensitive to color, and are located in a depression on the back surface of the retina called the fovea centralis, the area of sharpest vision. b. Rods: used for dark or dim vision

 Lens 1. Circular structure located behind the pupil 2. Suspended in position by ligaments 3. Refracts or bends light rays so the rays will focus on the retina  Aqueous humor 1. Clear, watery fluid 2. Fills the space between the cornea and iris 3. Helps maintain the forward curvature of the eyeball. 4. Bends or refracts light rays

 Vitreous humor 1. Jellylike substance 2. Fills the area behind the lens 3. Helps maintain the shape of the eyeball 4. Also bends or refracts light rays  Series of muscles located in the eye provide for eye movement.

 When light rays enter the eye they pass through a series of parts that bend or refract the rays.  Allows the rays to focus on the retina  Rays pass through the cornea, the aqueous humor, the pupil, the lens, and the vitreous humor to focus on the retina  In the retina, the rays or image are picked up by the rods and cones, changed into nerve impulses, and transmitted by the optic nerves to occipital lobe of cerebrum, where sight is interpreted  If rays are not refracted correctly by the various parts, vision can be distorted or blurred

 Amblyopia or “lazy eye” 1. Commonly occurs in early childhood 2. Results in poor vision in one eye caused by the dominance of the other eye 3. Treatment -Covering good eye to stimulate development of “lazy eye” -Exercises to strengthen the weak eye -Corrective lenses and surgery 4. If not treated before 8 to 9 years of age, blindness of the affected eye may occur

 Astigmatism 1. Abnormal shape or curvature of the cornea that causes blurred vision 2. Corrective lenses (glasses or contact lenses) correct the condition

 Cataract 1. Normally clear lens becomes cloudy or opaque 2. Occurs gradually and is usually a result of aging, but may be the result of trauma 3. Symptoms -Blurred vision and halos around lights -Gradual loss of vision -Milky white pupil in later stages 4. Treatment -Surgical removal of the lens -Implanting of an intraocular lens or

 Conjunctivitis or pink eye 1. Contagious inflammation of the conjunctiva 2. Usually caused by a bacteria or virus 3. Symptoms: redness, swelling, pain, and pus formation 4. Treatment: antibiotics, frequently as eye ointment

 Glaucoma 1. Condition resulting from an increased intraocular (inside the eye) pressure 2. Caused by an excess amount of aqueous humor 3. Common after age 40 and a leading cause of blindness 4. Tonometer, an instrument for measuring intraocular pressure, is usually used during every eye examination to check for this condition 5. Symptoms: loss of peripheral (side) vision, halos around lights, limited night vision, and a mild aching 6. Treatment -Controlled with medications that decrease amount of fluid produced or improve drainage - In severe cases, surgery to create an opening for the flow of aqueous humor

 Hyperopia or farsightedness 1. Occurs when light rays are not refracted properly and the image focuses behind the retina 2. Vision is corrected by the use of convex lenses

 Myopia or nearsightedness 1. Occurs when light rays are refracted too sharply and the image focuses in front of the retina 2. Vision is corrected by the use of concave lenses 3. Radial keratotomy also used to treat myopia -Surgical procedure - Small incisions made in cornea to flatten it so it refracts light rays -Laser can be used to flatten cornea without cutting -Can correct myopia and eliminate need for corrective lenses

 Presbyopia 1. Farsightedness caused by a loss of elasticity in lens 2. Occurs as a result of the normal aging process 3. Treated with corrective lenses or “reading glasses”

 Strabismus 1. Condition in which eyes do not move or focus together 2. Eyes may move inward (cross-eyed), outward, or up or down 3. Caused by muscle weakness in one or both eyes 4. Treatment: eye exercises, covering the good eye, corrective lenses, and surgery on muscles moving the eye