Chapter 19 Special Senses: Vision

Accessory Structures of the Eye 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 1 2 3 4 5 Nostril Nasal cavity Nasolacrimal duct Lacrimal sac Lacrimal canaliculi Lacrimal caruncle Lacrimal puncta Lacrimal gland (orbital part) (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

Fig. 19.11a Anatomy of the Internal Eye Fibrous tunic Sclera Cornea Vascular tunic Iris Ciliary body Choroid Retina Pigmented layer Neural layer Fig. 19.11a 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

Accessory Structures of the Eye Fig. 19.9b Accessory Structures of the Eye 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

Accessory Structures of the Eye Fig. 19.9b Accessory Structures of the Eye Eyebrow conjunctiva contains blood vessels does not cover surface of cornea so blood vessels and nerves don’t block vision Ocular conjunctiva Palpebral conjunctiva Superior tarsal plate Superior eyelid Cornea Eyelashes Inferior eyelid Inferior tarsal plate

Central Ciliary muscle artery of Ciliary body Central retina vein of Fig. 19.11 Central artery of retina Ciliary muscle Ciliary body Central vein of retina Ciliary process Suspensory ligaments Limbus CN II (optic) Lens Iris Cornea Optic disc Pupil Fovea centralis Sphincter pupillae Dilator pupillae Posterior cavity Retina Anterior chamber Anterior cavity Choroid Posterior chamber Sclera (b)

Fig. 19.11a Anatomy of the Internal Eye Fibrous tunic Sclera Cornea Vascular tunic Iris Ciliary body Choroid Retina Pigmented layer Neural layer Fig. 19.11a 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

Within iris, two layers of muscles 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. 19.11a Anatomy of the Internal Eye Fibrous tunic Sclera Cornea Vascular tunic Iris Ciliary body Choroid Retina Pigmented layer Neural layer 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

Ciliary muscles in ciliary body Fig. 19.11 Ciliary muscles in ciliary body Suspensory ligaments Lens

Fig. 19.11a Anatomy of the Internal Eye Fibrous tunic Sclera Cornea Vascular tunic Iris Ciliary body Choroid Retina Pigmented layer Neural layer 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

Fig. 19.11a Anatomy of the Internal Eye Fibrous tunic Sclera Cornea Vascular tunic Iris Ciliary body Choroid Retina Pigmented layer Neural layer 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

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 optic disk creates blind spot Sclera Choroid Optic disc Optic nerve Fovea centralis

Organization of the Retina Fig. 19.13a & b Organization of the Retina Choroid Retina Sclera Choroid Rod Photoreceptor cells Optic disc Cone Pigmented layer Horizontal cell Bipolar cells Optic nerve Retina Amacrine cell Light travels through neural layer and is received by photoreceptor cells on deepest layer of retina Neural layer Ganglion cells Axons of ganglion cells to optic nerve Incoming light Nerve signal

Organization of the Retina Fig. 19.13a & b Organization of the Retina Choroid 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 high-intensity light, most numerous within fovea centralis Rod Photoreceptor cells Cone Horizontal cell Bipolar cells Amacrine cell Ganglion cells Axons of ganglion cells to optic nerve

Axons of ganglion cells LM 250x Posterior cavity Fig. 19.11 Choroid Pigmented layer Rods and cones Retina Bipolar cells Neural layer Ganglion cells Axons of ganglion cells LM 250x Posterior cavity

Organization of the Retina Fig. 19.13a, 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 produces crispest vision Optic disc Optic nerve Fovea centralis Macula lutea

Medial Lateral Optic disc Blood vessels Fovea centralis Macula lutea

(c) Amsler grid, seen with normal vision (a) Normal vision (b) As viewed by a person with macular degeneration (d) Amsler grid, as viewed by a person with macular degeneration 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

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)

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

Central Ciliary muscle artery of Ciliary body Central retina vein of Fig. 19.11 Central artery of retina Ciliary muscle Ciliary body Central vein of retina Ciliary process Suspensory ligaments Limbus CN II (optic) Lens Iris Cornea Optic disc Pupil Fovea centralis Sphincter pupillae Dilator pupillae Posterior cavity Retina Anterior chamber Anterior cavity Choroid Posterior chamber Sclera (b)

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