Vision - Vision Theeyeissupportedbythefollowing...

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Vision The eye is supported by the following  accessory organs: class="bullet"> •   The eyebrows shade the eyes and  help keep perspiration that accumulates  on the forehead from running into the  eyes. •   The eyelids (palpebrae) lubricate,  protect, and shade the eyeballs.  Contraction of the levator palpebrae  superioris muscle raises the upper  eyelid. Each eyelid is supported  internally by a layer of connective 
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tissue, the tarsal plate. Tarsal  (Meibomian) glands embedded in the  tarsal plate produce secretions that  prevent the upper and lower eyelids  from sticking together. The inner lining  of the eyelid, the conjunctiva, is a  mucous membrane that produces  secretions that lubricate the eyeball.  The conjunctiva continues beyond the  eyelid, folding back to cover the white of  the eye. •   The eyelashes, on the borders of  the eyelids, help protect the eyeball.  Nerve endings at the base of the hairs  initiate a reflex action that closes the 
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eyelids when the eyelashes are  disturbed. •   The lacrimal apparatus produces  and drains tears. Tears (lacrimal fluid)  are produced by the lacrimal glands,  which lie above each eye (toward the  lateral edge). In each eye, tears flow  across the eyeball and enter two  openings (lacrimal puncta) into lacrimal  canals that lead to the lacrimal sac.  From here, the tears drain through the  nasolacrimal duct into the nasal cavity.  Tears contain antibodies and lysozyme  (a bacteria-destroying enzyme).
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•   Six extrinsic eye muscles provide  fine motor control for the eyeballs.  These are the lateral, medial, superior,  and inferior rectus muscles and the  inferior and superior oblique muscles. The eyeball is a hollow sphere whose wall consists of three tunics (layers), shown in Figure 1. The three tunics of the eye are described below: Fibrous tunic: The outer fibrous tunic consists of avascular connective tissue called the sclera. The forward 1/6 portion of this tunic is the cornea, a transparent layer of collagen fibers that forms a window for entering light. The remainder of the fibrous tunic is the sclera. Consisting of tough connective tissue, the sclera
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and provides for the attachment of the eye muscles. The visible forward portion of the sclera is the white of the eye. Vascular tunic: The middle vascular tunic (uvea) consists of three highly vascularized (as the name implies), pigmented parts (the iris, the ciliary body, and the choroid): The iris is the colored portion of the eye that opens and closes to control the size of its circular opening, the pupil. The size of the pupil regulates the amount of light entering the eye and helps bring objects into focus. The ciliary body lies
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Vision - Vision Theeyeissupportedbythefollowing...

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