Chapter 19 Special Senses Vision Fig 19 9

Chapter 19 Special Senses: Vision

Fig. 19. 9 Accessory Structures of the Eye • Structures that prevent foreign objects from entering eye: • eyebrows • eyelashes (length ideally 1/3 width of eye opening) • eyelids (AKA palpebrae) include • thin layer of skin, muscle, fibrous core

Fig. 19. 10 Lacrimal Apparatus Lacrimal gland (orbital part) Lacrimal gland (palpebral part) • Produces, collects, and drains lacrimal fluid (tears) • lubricates anterior surface of eye • cleanses and moistens eye surface • contains antibacterial enzyme to help prevent infection Lacrimal puncta Lacrimal caruncle Lacrimal canaliculi 1 2 Lacrimal sac 3 4 Nasolacrimal duct 5 Nasal cavity Nostril

Fig. 19. 11 a Anatomy of the Internal Eye • Human eye is about 2. 5 cm in diameter • Orbital fat cushions eye against bone • provides blood vessels and supports nerves • Fibrous tunic is tough external layer • Sclera is “white” of eye, made of dense, irregular connective tissue • Cornea is clear surface of anterior eye • convex shape bends light coming into eye • continuous with sclera but clear Fibrous tunic Sclera Cornea Vascular tunic Iris Ciliary body Choroid Retina Pigmented layer Neural layer

Fig. 19. 9 b Accessory Structures of the Eyebrow • Conjunctiva produces mucus and (some) tears • Palpebral conjunctiva covers inner surface of eyelid • Ocular conjunctiva covers sclera Ocular conjunctiva Palpebral conjunctiva Superior tarsal plate Superior eyelid Cornea Eyelashes Inferior eyelid Inferior tarsal plate

Fig. 19. 9 b Accessory Structures of the Eye • conjunctiva contains blood vessels • does not cover surface of cornea so blood vessels and nerves don’t block vision Eyebrow Ocular conjunctiva Palpebral conjunctiva Superior tarsal plate Superior eyelid Cornea Eyelashes Inferior eyelid Inferior tarsal plate

Fig. 19. 11 Central artery of Central retina vein of retina Ciliary muscle Ciliary body Ciliary process Suspensory ligaments Limbus CN II (optic) Lens Iris Cornea Pupil Optic disc Fovea centralis Posterior cavity Retina Choroid Sclera (b) Sphincter pupillae Dilator pupillae Anterior chamber Posterior chamber Anterior cavity

Fig. 19. 11 a Anatomy of the Internal Eye • Vasclar tunic (AKA uvea) • 3 regions • Iris is colored portion of eye • black hole at center is pupil • two layers of pigment-forming cells give eye color • contains two groups of smooth muscle fibers, controls size of pupil Fibrous tunic Sclera Cornea Vascular tunic Iris Ciliary body Choroid Retina Pigmented layer Neural layer

Fig. 19. 11 • Within iris, two layers of muscles • sphincter pupillae is in concentric circles, constricts pupil • dilator pupillae extends in radial pattern, dilates pupil Lens Iris Cornea Pupil Sphincter pupillae Dilator pupillae

Fig. 19. 11 a Anatomy of the Internal Eye • Vascular tunic (AKA uvea) • 3 regions • Ciliary body is continuous with iris • composed of ciliary muscles and ciliary processes that cover muscles • suspensory ligaments extend to lens, focus eye by contracting or relaxing Fibrous tunic Sclera Cornea Vascular tunic Iris Ciliary body Choroid Retina Pigmented layer Neural layer

Fig. 19. 11 Ciliary muscles in ciliary body Suspensory ligaments Lens

Fig. 19. 11 a Anatomy of the Internal Eye • Vascular tunic (AKA uvea) • 3 regions • Choroid is most posterior region, black color • prevents reflection of excess light back into retina • in cats, cows, etc. , choroid covered with tapetum lucidum that reflects light Fibrous tunic Sclera Cornea Vascular tunic Iris Ciliary body Choroid Retina Pigmented layer Neural layer

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Fig. 19. 11 a Anatomy of the Internal Eye • Retina is most internal layer • composed of 2 layers • pigmented layer is attached to choroid • provides Vitamin A for photoreceptor cells • transports nutrients and oxygen to photoreceptor cells, removes waste • neural layer is internal to pigmented layer • houses photoreceptors and other neurons involved in receiving and processing light signals • not attached to other layers, except at optic nerve Fibrous tunic Sclera Cornea Vascular tunic Iris Ciliary body Choroid Retina Pigmented layer Neural layer

Tapetum lucidum Cow eye Human eye

Organization of the Retina • Blood vessels and neurons leave eye through optic disc, travel through center of optic nerve out back of eye Sclera Choroid Optic disc – optic disk creates blind spot Optic nerve Fovea centralis

Fig. 19. 13 a & b Organization of the Retina Choroid Retina Sclera Choroid Optic disc Optic nerve Rod Photoreceptor Cone cells Pigmented layer Retina • Light travels through neural layer and is received by photoreceptor cells on deepest layer of retina Neural layer Incoming light Nerve signal Horizontal cell Bipolar cells Amacrine cell Ganglion cells Axons of ganglion cells to optic nerve

Fig. 19. 13 a & b Organization of the Retina • Photoreceptor cells – rods are most sensitive in dim light, process black and white, most numerous outside fovea centralis – cones process color, are most sensitive in highintensity light, most numerous within fovea centralis Choroid Rod Photoreceptor Cone cells Horizontal cell Bipolar cells Amacrine cell Ganglion cells Axons of ganglion cells to optic nerve

Fig. 19. 11 Choroid Pigmented layer Retina Rods and cones Bipolar cells Neural layer Ganglion cells LM 250 x Axons of ganglion cells Posterior cavity

Fig. 19. 13 a, 19. 14 Organization of the Retina • Area around fovea centralis is macula lutea • Contains numerous rods and cones, no bipolar or ganglion cells • Light doesn’t have to pass through other cells to get to rods and cones Optic disc Optic nerve Macula lutea – produces crispest vision Fovea centralis

Macula lutea Lateral Fovea centralis Blood vessels Optic disc Medial

Macular Degeneration (a) Normal vision (b) As viewed by a person with macular degeneration • Loss of photoreceptors and thinning of pigmented layer in macula • May also involve bleeding, capillary proliferation, scar tissue formation • Major cause of blindness – caused by diabetes, infection, smoking, hypertension, trauma to the eye • Laser surgery can slow degeneration, but not restore lost sight (c) Amsler grid, seen with normal vision (d) Amsler grid, as viewed by a person with macular degeneration

• Space between lens and cornea is anterior cavity • divided into anterior chamber between iris and cornea, and posterior chamber between iris and lens • filled with aqueous humor (clear liquid) 24

• Space behind lens is posterior cavity • filled with vitreous humor • clear, jelly-like substance • helps maintain eye shape 25

Fig. 19. 11 Central artery of Central retina vein of retina Ciliary muscle Ciliary body Ciliary process Suspensory ligaments Limbus CN II (optic) Lens Iris Cornea Pupil Optic disc Fovea centralis Posterior cavity Retina Choroid Sclera (b) Sphincter pupillae Dilator pupillae Anterior chamber Posterior chamber Anterior cavity

How do these structures work together to produce vision? • Cornea bends light as it enters eye • Light passes through aqueous humor without bending • Iris controls amount of light entering eye • Lens further bends light • Light is focused on fovea centralis Fovea centralis