The Eye and Vision Most of eye protected by cushion of fat and bony orbit Accessory Structures protect eye and aid eye function Eyebrows, Eyelids, Conjunctiva, Lacrimal apparatus Extrinsic eye muscles

Conjunctiva: Transparent mucous membrane Produces lubricating mucous secretion Conjunctiva covers white of eyes but not cornea Lacrimal Apparatus Lacrimal glands secrete tears and drain them via ducts into nasolacrimal duct Lacrimal gland in orbit above lateral end of eye Tears are dilute saline solution containing mucus, antibodies, and lysozyme Blinking spreads tears toward medial commissure

Six extrinsic eye muscles attach to scleral layer and move eyeball Originate from bony orbit; insert on eyeball Enable eye to follow moving objects; maintain shape of eyeball; hold in orbit Four rectus muscles originate from common tendinous ring; names indicate movements Superior, inferior, lateral, medial rectus muscles Two oblique muscles move eye laterally and rotate eyeball Superior and inferior oblique muscles

Structure of Eye: Outermost layer; dense avascular connective tissue Two areas: sclera and cornea 1. Sclera--Opaque region Protects, shapes eyeball; anchors extrinsic eye muscles Continuous with dura mater of brain posteriorly 2. Cornea--Transparent anterior area Bends light as it enters eye Pain receptors contribute to blinking and tearing reflexes Corneal transplants typically are not rejected by immune system because cornea has no blood vessels

1. Choroid Choroid---Middle pigmented layer Contains: choroid, ciliary body, and iris 1. Choroid Supplies blood to all layers of eyeball Black pigment absorbs light to prevent it scattering within eye which makes focusing impossible 2. Ciliary body Ring of smooth muscle surrounding lens Ciliary muscles relax and contract to control lens shape to allow lens to focus light Capillaries of ciliary processes secrete fluid Suspensory ligament holds lens in position

3. Iris-anterior to ciliary body Colored part of eye Pupil—central opening that regulates light entering eye Close vision and bright light —pupils constrict Distant vision and dim light —pupils dilate Changes in emotional state—pupils dilate when subject matter is appealing or requires problem-solving skills Intrinsic eye muscles = ciliary muscle and iris

Biconvex, transparent, flexible, and avascular Lens Biconvex, transparent, flexible, and avascular Changes shape to precisely focus light on retina filled with transparent protein crystallin becomes more dense, convex, less elastic with age Cataracts (clouding of lens) consequence of aging, diabetes mellitus, heavy smoking, frequent exposure to intense sunlight

Innermost Layer: Retina Delicate 2-layered membrane Outer pigmented layer Absorbs light and prevents its scattering Phagocytize photoreceptor cell fragments Stores vitamin A Inner Neural layer Transparent Composed of 3 types of neurons Photoreceptors, bipolar cells, ganglion cells Signals spread from photoreceptors  bipolar cells  ganglion cells Ganglion cell axons exit eye as optic nerve

Optic disc (blind spot) Lacks photoreceptors The Retina Optic disc (blind spot) Lacks photoreceptors Site where optic nerve leaves eye Photoreceptors (quarter-billion) of two types Rods-black and white vision Cones-color vision

Rods Cones Dim light, peripheral vision receptors More numerous, more sensitive to light than cones Used for peripheral vision because numbers greatest at periphery Cones Receptors for color vision which function better in bright light (daytime vision) Macula lutea Mostly cones Fovea centralis Tiny pit in center of macula with all cones; best vision

Internal Chambers and Fluids Anterior area contains Aqueous humor Plasma like fluid continuously formed Drains via canal of Schlemm (at sclera-cornea junction) Supplies nutrients and oxygen mainly to lens and cornea but also to retina, and removes wastes Helps maintain shape and pressure of eye Glaucoma: blocked drainage of aqueous humor increases pressure and causes compression of retina and optic nerve  blindness

Posterior area contains vitreous humor that Transmits light Supports posterior surface of lens Holds neural layer of retina firmly against pigmented layer Contributes to intraocular pressure and maintains shape of eye As you get older, may get floaters— caused by age-related changes and/or changes in vitreous consistency  becomes more liquid  causes shrinking and pulling away from interior surface of eye. This can cast shadows on your retina, which you interpret as eye floaters. If you notice a sudden increase in number of floaters, contact an eye doctor immediately — especially if you see flashes of light or lose your peripheral vision; may be caused by retinal detachment.

LABWORK 1. Dissect sheep eye 2. Using eye models and charts, locate the following: sclera, choroid, cornea, retina, iris, pupil, ciliary body, lens, aqueous humor, vitreous body, optic disk, optic nerve. 3. Perform eye tests for visual acuity, astigmatism, and other tests in your lab book, time permitting.

The Ear: Hearing and Equilibrium (Balance) Three major areas of ear External (outer) ear – hearing only Middle ear (tympanic cavity) – hearing only Internal (inner) ear – hearing and equilibrium Receptors for hearing and balance respond to separate stimuli

External Ear Auricle (pinna) Funnels sound waves into auditory canal Sound is form of energy transmitted in waves; different sounds have different wavelengths External acoustic meatus (auditory canal) Short tube lined with skin ceruminous glands Transmits sound waves to eardrum

Tympanic membrane (eardrum) Boundary between external and middle ears Connective tissue membrane that vibrates in response to sound Transfers sound energy to bones of middle ear Middle Ear Small, air-filled, mucosa-lined cavity in temporal bone Eardrum at lateral border and medially is bony wall containing oval (vestibular) and round (cochlear) windows

Eustachian or Pharyngotympanic (auditory) tube—connects middle ear to nasopharynx Equalizes pressure in middle ear cavity with external air pressure Ear Ossicles Three small bones in tympanic cavity: the malleus (hammer), incus (anvil), and stapes (stirrup) Suspended by ligaments and joined by synovial joints Transmit vibratory motion of eardrum to oval window

Inner Ear (has sound and equilibrium receptors) Bony labyrinth --filled with perilymph Three regions: vestibule, semicircular canals, and cochlea Membranous labyrinth -filled with endolymph Series of membranous sacs and ducts Vestibular apparatus Equilibrium receptors in semicircular canals and vestibule Vestibular receptors monitor static equilibrium Semicircular canal receptors monitor dynamic equilibrium

Vestibule Central cavity of bony labyrinth Contains two membranous sacs Saccule: continuous with cochlear duct Utricle: continuous with semicircular canals Sacs responsible for static or linear equilibrium Maculae contain equilibrium receptor regions Responds to gravity and changes in position of head

Semicircular Canals Three canals (anterior, lateral, and posterior) lie in three planes of space Membranous ducts line canals and communicate with utricle Ampulla of each canal houses equilibrium receptor region called crista ampullaris Receptors respond to angular (rotational) movements of head

spiral, conical, bony chamber; size of split pea The Cochlea--hearing spiral, conical, bony chamber; size of split pea Extends from vestibule Coils around bony pillar Contains cochlear duct, which contains Organ of Corti (spiral organ) "roof" of cochlear duct is vestibular membrane External wall secretes endolymph "Floor" of cochlear duct composed of basilar membrane which supports organ

Auditory Processing Pitch perceived by impulses from specific hair cells in different positions along basilar membrane Loudness detected by increased numbers of action potentials that result when hair cells experience larger deflections Localization of sound depends on relative intensity and relative timing of sound waves reaching both ears

Sensory receptors for static equilibrium Maculae Sensory receptors for static equilibrium One in each saccule wall and one in each utricle wall Monitor position of head in space, necessary for control of posture Respond to linear acceleration forces, but not rotation Contain supporting cells and hair cells Stereocilia and kinocilia are embedded in otolith membrane studded with otoliths (tiny CaCO3 stones)

The Ear: 1. Find the following structures in your lab book and the human ear models: Pinna, pharyngotympanic (auditory) tube, Tympanic membrane (eardrum), malleus, Incus, Stapes, Cochlea, Vestibule, Semicircular canals, Auditory canal, Round/oval windows, Vestibulocochlear nerve. 2. View slide of the cochlea and note the three fluid-filled chambers and the organ of Corti. 3. Perform hearing/balance tests in lab book.