THE SPECIAL SENSES ( THE EYES)

• CORNEA- This is the clear front surface of the eye that allows light to enter. It also help to focus (or refract) the light onto the retina.
• IRIS - This is the colored part of the eye. It controls the size of the pupil and thus determines how much light enters the eye
• PUPIL- This is the opening in the center of the iris. It changes size to control the amount of light that enters the eye.
• Lens- This is a clear structure located behind the iris. It changes shape to help focus light onto the retina.
• Retina- This is the layer at the back of the eye that contains cells sensitive to light. These cells convert light into electrical signals.
• Optic Nerve- This is the nerve that transmits electrical signals from the retina to the brain, where these signals are interpreted as visual image.
• Vitreous Humor- This is the clear, gel-like substance that fills the space between the lens and the retina. It helps maintain the shape of the eye and transmit light to the retina
• Sclera- This is the white part of the eye. It provides protection and helps maintain the shape of the eye.
• COMMON EYE DISEASE : GLAUCOMA, Age-Related Macular Degeneration, AMBLYOPIA (LAZY EYE), ASTIGMATISM, CATARACTS, COLOR BLIND, DIABETIC RETINOPATHY
• Both eyes are set in bony sockets, called orbits, in the skull.
• The adult human eyes are globe - shaped structures, each approximately one inch in diameter.
• The white outer layer of the eyeball is called the SCLERA . Composed of collagen and other connective tissue, the sclera forms a tough
• The cornea has a curved surface that serves to focus light coming into the eye. In addition, the cornea protects that front portion of the eye from injury.
• Lying immediately below the sclera is the vascular layer of the eye, which is called the uvea.
• Choroid- is a layer of darkly pigmented tissue that lies just below the sclera. In addition to housing the many tiny blood vessels that deliver nutrients to throughout the eye. This prevents the blurring of visual images by light present within the eye.
• Just below the junction of the sclera and cornea, the choroid becomes thicker, forming a structure known as the "ciliary body". As you will see below, smooth muscles that are embedded in the ciliary body adjust the shape of the lens to enable the eyes to focus on objects at varying distances .
• Another extension of the choroid is the IRIS , a ring of pigmented tissue that protrudes into the cavity of the eye. The pigmentation of the iris is what determines eye
• The opening in the center of the iris is called the PUPIL.
• Contraction of the pupils is referred to as miosis while dilation of the pupils is called mydriasis
• REFRACTORY MEDIA- Light entering the eye passes through several liquid and solid structures that refract ( bend ) the light rays slightly, bringing them into sharper focus.
• lens- is transparent, elastic disk which is attached to the ciliary body by tiny fibers called suspensory ligaments.
• The chamber between the cornea and the lens is filled with a watery fluid called the aqucous humor.
• The chamber behind the lens is filled with a more gelatinous fluid called the vitreous humor.
• Light - sensitive receptor cells in the eye ate located in the retina, a thin layer of tissue located on the inner surface of the choroid.
• retinal cells are classified as rods and cones.
• Rods- which are most concentrated on the periphery of the retina, detect shades of gray area of the retina, detect color but operate only at high light levels
• cones generally detect visual stimuli with greater precision.
• Nerve fibers (and blood vessels ) enter and leave the retina through an area called the OPTIC DISK . Because it has no rods or cones; it is also referred to as the blind spot.
• The optic nerve- carriers impulses from the retina to areas of the brain that are responsible for rocessing visual informtion.
• eyelids - Although not directly involved in vision, other structures help protect the eye from injury
• The inside of each eyelid is lined by a thin, usually transparent membrane known as the CONJUCTIVI, which also extends over the white of the eye.
• achromatopsia- color blindness
• amblyopia- reduced or dulled vision
• ametropia- general term for any error of refraction in which light is not properly focused on the retina
• anisocoria- inequality in the size of the pupils; often a sign of neurologic disease
• blepharoptosis- drooping of the upper eyelid; often called ptosis for short
• cataract- condition in which the crystalline lens becomes cloudy or opaque as a result of protein deposits on its surface
• hemianopia- blindness in one half of the visual field
• Hyperopia- farsightedness; refractive disorder in which light is focused on a point behind the retina
• keratitis- inflammation of the cornea, usually due to infection or other type of damage