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Accessory structures of the eye • Eyelids (palpebrae) separated by the palpebral fissure – protect the anterior surface of the eye • Conjunctiva covers most of eye – stratified squamous ET • Palbebral conjunctiva – lines the inner surface of the eyelid • Ocular conjunctiva – lines the anterior surface of the eyball Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings Accessory Structures of the Eye – lacrimal apparatus • Structures that are associate with the production, secretion and removal of tears • Secretions from the lacrimal gland contain lysozyme (to destroy bacteria) • Tears form in the glands wash across pass through the canaliculi, lacrimal nasolacrimal duct lacrimal the eye, lacrimal sac and • Lacrimal apparatus – keeps the surface of the eye moist • Lacrimal gland – produces lacrimal fluid • Lacrimal sac – fluid empties into nasal cavity Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings Figure 16.5b Extrinsic Eye Muscles Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings Lens • A biconvex, transparent, flexible, avascular structure that: • Allows precise focusing of light onto the retina • Is composed of epithelium and lens fibers • Lens has 2 regions • Lens epithelium – cuboidal cells found at the anterior surface of the lens. These cells differentiate into lens fibers • Lens fibers – cells filled with the transparent protein crystallin. These cells are packed in layers and contain no nuclei. • New lens fibers are added continuously the lens enlarges, become denser, less elastic. • The lens is held in place by its ciliary zonule (suspensory ligaments) • The lens and ciliary zonules divide the eye posterior and anterior cavities Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings Internal Chambers and Fluids • Posterior segment (cavity) • Filled with vitreous humor • Clear, jelly-like substance • Transmits light • Supports the posterior surface of the lens • Helps maintain intraocular pressure • Anterior segment • Filled with aqueous humor • Renewed continuously • Formed as a blood filtrate • Supplies nutrients to the lens and cornea Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings Eye’s tunics (layers) Fibrous tunic Sclera and Cornea Vascular tunic Choroid, Ciliary body, and Iris Sensory tunic Pigmented layer Neural layer Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings The Fibrous Tunic • Most external layer of the eyeball • Composed of two regions of connective tissue • Sclera – “white of the eye” • White, opaque region • Provides shape and an anchor for eye muscles • Cornea – transparent part that allow passage of light • Limbus – junction between sclera and cornea Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings Cornea • Clear window in the anterior part of the eye that allows the entrance of light. • It is a major part of the light-bending apparatus of the eye • 4 layers • External layer – stratified squamous ET – for protection; merges with the ocular conjuntiva • absorbs oxygen and other needed cell nutrients that are contained in tears. • filled with thousands of tiny nerve endings that make the cornea extremely sensitive to pain when rubbed or scratched. • Deep epithelial layer –single layer of cells • primary task is to pump excess water out of the stroma (by having sodium pumps). • Bowman’s layer • a tough layer consisting of irregularly-arranged collagen fibers • Stroma • Located behind the external epithelium • A thick, transparent middle layer, consisting of regularly-arranged collagen fibers Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings http://dels.nas.edu/ilar_n/ilarjournal/40_2/40_2Cowellfig1.jpg Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings http://www.siumed.edu/~dking2/intro/IN022b.htm Vascular Tunic (Uvea) Choroid - highly vascular, dark brown (melanin) membrane that forms the posterior portion of the uvea Ciliary body - thickened ring of tissue encircling the lens, composed of smooth muscle bundles (ciliary muscles) which control lens shape. Suspensory ligament extends from ciliary processes to the lens to hold it in place Iris - colored part of the eye between cornea and lens Pupil - central opening of the iris that regulates amount of light entering the eye Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings The Sensory Tunic (Retina) • Composed of two layers • Pigmented layer – single layer of melanocytes • Neural layer – sheet of nervous tissue • Contains three main types of neurons • Photoreceptor cells • Bipolar cells • Ganglion cells Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings The Retina: Photoreceptors Rods: Respond to dim light, more sensitive to light than cones, but do not provide sharp images or color vision. Used for peripheral vision. Cones: Respond to bright light, have high-acuity color vision. Found in the macula lutea (lateral to optic disc). Concentrated in the fovea centralis of the macula lutea. Macula lutea – contains mostly cones Fovea centralis – contains only cones, Region of highest visual acuity Optic disc – blind spot – no photo receptor, exit of optic nerve Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings How the eye focus light • The eye must focus light onto a precise spot on the retina (macula lutea) • The cornea accounts for about 80% of the focusing that the eye does. • The lens does the rest of the focusing. • The lens is also able to change its shape to focus so things at different distances can come into focus (like an auto-focusing camera). • The closer that an object is to the eye, the more focusing the lens has to do in order to make the image clear Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings Nearsightedness (myopia) • Light is focused in front of the retina rather than directly on it. This leads to a naturally closer point of focus. • People with myopia can see close objects without difficulty, but distant objects are blurred. • The correction requires a concave lens (minus power), which causes the light reaching the eye to diverge. Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings Farsightedness • Light is focused behind the retina. • These people do not have problems with distant vision but need glasses to with convex lenses to augment the converging power of the lens to close vision. • There is a natural decline in the ability of the lens to focus as one ages. Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings • The notation of visual acuity is written as a fraction, with normal vision being 20/20 (twenty/twenty vision) • At a 20 foot distance, (the top number in the fraction, or testing distance), a person with normal vision should be able to read the small 20/20 line on an eye chart. Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings