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The Eye & Vision The Eye and Vision 70 percent of all sensory receptors are in the eyes - only see 1/6th of eye Each eye has over a million nerve fibers Protection for the eye Most of the eye is enclosed in a bony orbit A cushion of fat surrounds most of the eye Eyelids – aka. palpebra Meets at medial and lateral canthus Protection and Shading Eyelashes Figure 8.1b Meibomian glands – tarsal glands – modified sebacious glands produce an oily secretion to lubricate the eye Chalazion (ka-lay-zee-an)- inflammation Chalazion Ciliary glands – modified sweat glands between the eyelashes Sty inflammation Conjunctiva Membrane that lines the eyelids Connects to the surface of the eye Secretes mucus to lubricate the eye Conjunctivitis – inflammation of – sometimes known as “pink eye” Conjunctivitis Lacrimal apparatus Lacrimal gland – produces lacrimal fluid Lacrimal canals – drains lacrimal fluid from eyes Figure 8.1a Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Slide 8.4b Lacrimal sac – passage of fluid towards nasal cavity Nasolacrimal duct – empties fluid the nasal cavity Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Slide 8.4c Properties of lacrimal fluid Dilute salt solution (tears) Contains antibodies and lysozyme Protects, moistens, and lubricates the eye Empties into the nasal cavity Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Slide 8.5 6 eye muscles attach to the outer surface of the eye Produce gross eye movements Rectus muscles: lateral, medial, superior, inferior Oblique muscles: inferior, superior Need to be able to label on diagram & identify the motions they are responsible for Slide 8.6 Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings The wall is composed of three tunics Fibrous tunic – Outer layer Vascular tunic (uvea) – middle layer Sensory tunic inside layer Figure 8.3a Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Slide 8.7 Sclera White connective tissue layer Seen anteriorly as the “white of the eye” Cornea Transparent, central anterior portion Allows for light to pass through Repairs itself easily The only human tissue that can be transplanted without fear of rejection Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Slide 8.8 Cornea transplant Choroid – posterior portion Blood-rich nutritive tunic Dark pigment prevents light from scattering Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Slide 8.9 Modified interiorly Ciliary body – anterior smooth muscle Attached to lens Iris Pigmented layer that gives eye color Pupil – rounded opening in the iris for light passage Circular & radial fibers regulate opening Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Slide 8.9 Neural layer contains receptor cells (photoreceptors) Rods & Cones Signals pass from photoreceptors via a two-neuron chain Photoreceptors to bipolar neurons Bipolar neurons to ganglion cells Signals leave the retina toward the brain through the optic nerve Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Slide 8.10 Neurons of the Retina Figure 8.4 Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Slide 8.11 Rods Most are found towards the edges of the retina Allow dim light vision and peripheral vision Perception is all in gray tones Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Slide 8.12a Cones Allow for detailed color vision Densest in the center of the retina Fovea centralis area of the retina with only cones Area of greatest visual acuity Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Slide 8.12b No photoreceptor cells are at the optic disk, or blind spot Site of optic nerve leaving eyeball Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Slide 8.12b There are three types of cones Different cones are sensitive to different wavelengths Color blindness is the result of lack of one cone type Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Figure 8.6 Slide 8.13 Biconvex crystal-like structure Held in place by a suspensory ligament attached to the ciliary body Figure 8.3a Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Slide 8.14 Activity of ciliary muscle of ciliary body changes lens thickness to focus light onto retina Cataracts: clouding of lens Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Slide 8.14 Cataract Removal Aqueous humor in Anterior Segment Watery fluid found in chamber between the lens and corne Similar to blood plasma Helps maintain intraocular pressure Provides nutrients for the lens and cornea Reabsorbed into venous blood Blocked drainage = glaucoma Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Slide Vitreous humor Gel-like substance behind the lens Keeps the eye from collapsing Lasts a lifetime and is not replaced Keeps retina firmly against wall of eyeball Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Slide 8.15b Vision Light Light must be refracted (bent) so that it focuses on the retina to form an image. The cornea, aqueous humor, & vitreous humor have a constant refractory power The lens is the only refractory structure that can change shape in order to change refractory power. Focus Distance vision is accomplished by: Ciliary muscles relaxation Suspensory ligaments tightening Results in: Maximum flattening of lens (decreased convexity) Decreased refraction Focus Close vision is accomplished by: Ciliary muscles tightening Suspensory ligaments relaxing Results in: Lens rounds up (increased convexity) Increased refraction Requires continuous muscle contractions Prolonged periods of close vision can cause eyestrain Lens Accommodation The eye is set for distance vision (over 20 ft away) The lens must change shape to focus for closer objects This ability decreases with age Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Figure 8.9 Slide 8.16 Real Image Real image is projected on retina: reversed from left to right upside down & smaller Brain adjusts our perception Figure 8.10 Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Slide 8.17 Correcting the Eye Correct Focus = emmetropia Nearsightedness = myopia Distant objects are blurry (near objects clear) Focus of light in front of retina Eyeball too long lens too strong – too convex Cornea is too curved – too convex Requires concave lens to correct Emmetropia Myopia Correcting the Eye Farsightedness = hyperopia Near objects are blurry – distance objects are clear Focus of light beyond the retina Short eyeball lazy lens – lens too weak – too concave Cornea too flat – too concave Requires convex lens to correct Hyperopia Astigmatism Unequal curvatures in cornea & lens Blurred vision Specially ground lenses required to compensate irregularities Presbyopia Decreasing lens elasticity so lens cannot adjust for close vision Causes difficulty focusing on near objects Typically age related – corrected with reading glasses, etc. Near point of accommodation – closest distance at which you can focus on an object