evaporate flow toward the inner angle of the eye,where they drain down ducts into the nose.MIDDLE LAYER.—The middle layer of the eyeis called the choroid. This layer is a highly vascular,pigmented tissue that provides nourishment to theinner structures. Continuous with the choroid is theciliary body. The ciliary body is formed by athickening of the choroid and fits like a collar into thearea between the retina and iris. Attached to the ciliarythe body are the suspensory ligaments, which blendwith the elastic capsule of the lens and holds it in place.Iris.—The iris is continuous with the ciliary body.The iris is a circular, pigmented muscular structure thatgives color to the eye. The iris separates the anteriorcavity into anterior and posterior chambers. Theopening in the iris is called the pupil (fig. 1-49). Theamount of light entering the pupil is regulated throughthe constriction of radial and circular muscles in theiris. When strong light is flashed into the eye, thecircular muscle fibers of the iris contract, reducing thesize of the pupil. If the light is dim, the pupil dilates toallow as much of the light in as possible. The size andreaction of the pupils of the eyes are an importantdiagnostic tool.Lens.—The lens is a transparent, biconvex(having two convex surfaces) structure suspendeddirectly behind the iris. The optic globe posterior to thelens is filled with a jellylike substance called vitreoushumor, which helps to maintain the shape of theeyeball by maintaining intraocular pressure. The lensseparates the eye into anterior and posterior cavities.INNER LAYER.—The inner layer of the eye iscalled the retina (fig. 1-48). It contains layers of thenerve cells, rods, and cones that are the receptors of thesense of vision. The retina is continuous with the opticnerve, which enters the back of the globe and carriesvisual impulses received by the rods and cones to thebrain. The area where the optic nerve enters the eyeballcontains no rods and cones and is called the optic disc(blind spot) (fig. 1-50).Rods.—Rods respond to low intensities of lightand are responsible for night vision. They are locatedin all areas of the retina, except in the small depressioncalled the fovea centralis, where light entering the eyeis focused, and which has the clearest vision.Cones.—Cones require higher light intensities forstimulation and are most densely concentrated in thefovea centralis. The cones are responsible for daytimevision.Vision ProcessThe vision process begins with rays of light froman object passing through the cornea. The image is thenreceived by the lens, by way of the iris. Leaving thelens, the image falls on the rods and cones in the retina.The image then is carried to the brain for interpretationby the optic nerve (fig. 1-51). Note the image receivedby the retina is upside down, but the brain turns itright-side up.REFRACTION.—Deflection or bending of lightrays results when light passes through substances ofvarying densities in the eye (cornea, aqueous humor,lens, and vitreous humor). The deflection of light in theeye is referred to as refraction.ACCOMMODATION.—Accommodation is theprocess by which the lens increases or decreases itscurvature to refract light rays into focus on the foveacentralis.CONVERGENCE.—The movement of theglobes toward the midline, causes a viewed object tocome into focus on corresponding points of the two1-46SYMPATHETICMOTORNERVERADIALLY ARRANGED SMOOTHMUSCLE FIBERS OF THE IRISCIRCULARLY ARRANGED SMOOTHMUSCLE FIBERS OF THE IRISPUPILPARASYMPATHETICMOTOR NERVE FIBERIN DIMLIGHTIN BRIGHTLIGHTHM3F0149Figure 1-49.—Anterior view of the eye.
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