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The visual system provides a supremely efficient means for the rapid assimilation of information from the environment to aid in the guidance of behavior.(1) The importance of eye examinations cannot be over emphasized, as an eye examination often reveals signs of systemic disease and intrinsic ocular disorders, as well as the effects of drugs that are either administered systemically or instilled into the eye itself. The eyes are mounted in a prominent position on the head, and thus are vulnerable to a myriad of disturbances.
Vision can be damaged by trauma, exposure, or infection. Diseases intrinsic to the eye including glaucoma, cataracts, or retinal detachment, may also affect vision. Many systemically acquired diseases may have ocular symptoms, as do some of the neurologic diseases affecting areas of the cortex, thalamus, and brainstem that are devoted to visual perception or to the execution of eye movements. Although it may be determined that a patient requires specialized eye care, the initial examination and assessment of visual acuity, pupils, eye movements, visual fields, and fundi lies within the realm of all physicians. The slit lamp and ophthalmoscope provide the best view of the transparent anatomy of the eye and afford the opportunity to directly inspect blood vessels.
The eye is composed of three anatomical subdivisions: the eyelid, the external eye structures, and the internal eye structures.(2) The eyelids have an external covering of skin and are lined internally by conjunctiva. The purpose of the eyelids is to protect the eye from foreign objects, bright light, and to distribute fluids over the surface of the eye to prevent drying. Eyelids normally cover the upper border of the iris, and contain several anatomical features that provide the protection and lubrication needed for normal vision. The eyelashes help protect the eyes from foreign material; and Mebomian glands secrete sebaceous materials that lubricate the inner eyelids. The eyelids are controlled by the oculomotor nerve (cranial nerve 3), the facial nerve (cranial nerve 7), and the sympathetic nervous system. The conjunctiva, a thin translucent membrane, is richly supplied with blood vessels, covers the anterior eye, and folds into the inner eyelid. The palpebral conjunctiva covers the inner aspect of the eyelids, and the bulbar conjunctiva coats the sclera. It joins the corneal margin at the limbus.
The external eye includes the lacrimal glands and lacrimal sac located in the temporal region of the bony orbits that house the eyeball. Ophthalmic lacrimal, sebaceous, and mucous glands produce tears, which flow over the corneal surface, then collect in the conjunctival cul-de-sac and drain through the puncta. The lacrimal drainage system then directs tears into the nasal cavity. Disruption in any of these systems may cause an abnormality of tear formation, composition, or flow process.
Tears are necessary to provide nutrition for anterior surface cells; aid in metabolism, secretion, waste removal, and maintenance of optical clarity; and exert an antibacterial action. The preocular tear film is actually a three-layer system. Mucin is the innermost layer and adheres to the corneal and congunctival cell microvilli. Mucin, produced by goblet cells, changes the normally hydrophobic surface of the corneal epithelium to hydrophilic by a surfactant action, enabling it to become wet.(3, 4) The intermediate layer is aqueous and can spread more easily over the conjunctiva as a result of the action of mucin. The aqueous layer composes about 90 percent of the thickness of the tear film, contains chemicals responsible for corneal metabolism (e.g., glucose, inorganic ions) and the antimicrobial agent lysozyme. The outermost layer of tears is a lipid layer, which helps retard evaporation and prevents instability or rupture of tear film. Aging an
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