Afferent axons (AF-uh-rent AK-sonz): The axons that carry the action potential away from the sensory receptor and towards the central nervous system.

Ampulla (am-PUL-uh): A slight bulge at the base of each of the three semicircular ducts; it contains the crista ampullaris, the sensory structure responsible for detecting rotational head movements.

Anterior chamber (an-TIR-ee-er CHAYM-ber): The space filled with aqueous humor located between the posterior surface of the cornea and the anterior surface of the iris and lens.

Auditory tube (Eustachian tube) (AW-duh-tor-ee toob): A tube lined with respiratory epithelium that connects the middle ear cavity to the nasopharynx; its function is to equalize pressure between the middle ear and the atmosphere.

Auricle (pinna) (OR-ih-kul): The visible, external flap of the ear composed of elastic cartilage covered by skin; it acts as a funnel to gather and localize sound waves.

Basal complex (BAY-sul KOM-pleks): The collective term for the RPE’s basement membrane and the adjacent inner collagenous layer of Bruch’s membrane (the membrane separating the RPE from the choroid); this complex is critical for maintaining the metabolic exchange between the RPE and the choroid.

Bony labyrinth (BOH-nee LAB-uh-rinth): A series of interconnected bony cavities within the temporal bone; it contains the perilymph fluid and houses the membranous labyrinth.

Choroid (KOR-oyd): The thin, highly vascular middle layer of the eye, located between the retina and the sclera; its primary function is to supply blood and nutrients to the outer layers of the retina.

Ciliary muscles (SIL-ee-er-ee MUS-ulz): A ring of smooth muscle within the ciliary body that controls the tension on the suspensory ligaments to change the shape of the lens (a process called accommodation), allowing the eye to focus on objects at different distances.

Ciliary processes (SIL-ee-er-ee PRAH-ses-iz): Highly vascular, finger-like folds extending from the ciliary body towards the lens; they are lined by a double layer of epithelium that actively secretes the aqueous humor.

Cochlea (KOK-lee-uh): A spiral-shaped, bony chamber within the inner ear that houses the spiral organ (of Corti); it is the sensory organ for hearing.

Cochlear duct (endolymph) (KOK-lee-er dukt EN-doh-limf): The central, triangular, epithelial-lined space within the cochlea (also called scala media); it is filled with endolymph and contains the spiral organ.

Cones (koh-nz): The photoreceptor cells in the retina that require bright light and are responsible for high-acuity, color (photopic) vision.

Conjunctiva (kon-junk-TY-vuh): A thin, transparent mucous membrane that lines the inner surface of the eyelids and the anterior surface of the sclera (but not the cornea); its epithelial layer contains goblet cells that secrete mucus to lubricate the eye.

Cornea (KOR-nee-uh): The transparent, avascular anterior-most layer of the eye that covers the iris and pupil; it is the eye’s main light-refracting surface, with a highly organized layered structure of epithelium, connective tissue (stroma), and endothelium.

Crypt (lingual papilla) (kript): A deep, trench-like invagination of the epithelium that encircles the base of the vallate and foliate papillae; serous gustatory glands (von Ebner’s) secrete into these spaces to flush food particles toward the taste buds.

Cupula (KYOO-pyoo-luh): A flexible, gelatinous mass that sits atop the crista ampullaris within the ampulla of the semicircular ducts; movement of the endolymph bends the cupula, stimulating the underlying hair cells.

Episcleral (ep-ih-SKLER-ul): Pertaining to the episclera, which is the loose connective tissue layer located immediately superficial to the sclera that contains a high concentration of blood vessels.

External auditory canal (ik-STUR-nul AW-duh-tor-ee kuh-NAL): The tube-like passage lined with skin that extends from the auricle to the tympanic membrane (eardrum); it is lined with hairs and ceruminous glands (wax-producing) for protection.

Filiform papilla (FIL-uh-form puh-PIL-uh): The non-gustatory (i.e., not involved in the detection of taste) papillae that appear as pointed projections on the anterior surface of the tongue; these papillae increase surface area and add grip to the tongue’s surface for the manipulation of things in the mouth (think of them as the fingerprints of the tongue).

Foliate papilla (FOH-lee-ayt puh-PIL-uh): The leaf-like, parallel ridges of lingual papillae found on the postero-lateral edges of the tongue; they contain numerous taste buds (especially in children) and are typically associated with underlying serous gustatory glands.

Fovea (FOH-vee-uh): A small depression in the center of the macula (part of the retina) where the highest density of cones is found; it is responsible for the sharpest, most detailed, color vision.

Fungiform papilla (FUNG-guh-form puh-PIL-uh): A mushroom-shaped projection on the surface of the tongue that may or may not contain taste buds (i.e., a gustatory papilla); approximately one-quarter of all taste buds reside within the fungiform papillae. Found on the anterior surface of the tongue as rounded projections that sit above the epithelial surface, but do not typically contain deep crypts.

Ganglion cell layer (GANG-glee-un sell LAY-er): The layer of the retina containing the cell bodies of the ganglion cells; their axons form the optic nerve that transmits visual information to the brain.

Gelatinous membrane (jel-uh-TIN-us MEM-brayn): A thick, glycoprotein layer in the maculae and ampullae into which the tips of the hair cells’ stereocilia are embedded; it transmits force to the hair cells.

Hair cell (hair sell): The mechanoreceptor sensory cell found in the spiral organ and macula/crista; the deflection of its stereocilia by fluid movement or force generates the electrical signal for hearing or balance.

Incus (ING-kus): The second of the three ossicles in the middle ear; it is an anvil-shaped bone that connects the malleus to the stapes.

Inner ear (in-er eer): The innermost part of the ear, housed within the temporal bone, containing the sensory organs for hearing (cochlea) and balance (semicircular canals, utricle, and saccule); it is composed of the bony and membranous labyrinths.

Inner nuclear layer (IN-er NOO-klee-er LAY-er): The layer of the retina containing the cell bodies of the bipolar cells, horizontal cells, amacrine cells, and Müller cells (support cells).

Inner phalangeal cell (IN-er fuh-LAN-jee-ul sell): A type of supporting cell in the spiral organ (of Corti) that encloses the base of the single row of inner hair cells.

Inner plexiform layer (IN-er PLEK-suh-form LAY-er): The synaptic layer of the retina where the axons of the bipolar cells synapse with the dendrites of the ganglion cells and amacrine cells.

Intermediate cell (in-ter-MEE-dee-ut sell): An epithelial cell type found in the middle layer of the stria vascularis in the cochlea; these, along with marginal cells, are essential for generating the electrical potential difference in the endolymph.

Iris (EYE-ris): The thin, contractile structure composed of connective tissue and two sets of smooth muscles (sphincter and dilator) that forms the colored part of the eye; it regulates the amount of light entering the eye by changing the diameter of the central opening, the pupil.

Kinocilium (ky-noh-SIL-ee-um): A single, true cilium found on the apical surface of a hair cell in the vestibular system (utricle, saccule, semicircular ducts); its orientation establishes the directionality of the hair cell’s response.

Lens (lenz): A biconvex, transparent structure located immediately behind the iris and pupil; composed of highly organized, elongated epithelial cells (lens fibers), its function is to refract light and focus it onto the retina.

Macula (MAK-yuh-luh): The sensory patch of neuroepithelium found in the walls of the utricle and saccule; it contains hair cells embedded in a gelatinous membrane with otoliths, detecting linear acceleration and gravity.

Malleus (MAL-ee-us): The first and largest of the three ossicles in the middle ear; its handle attaches to the tympanic membrane, and it transmits vibrations to the incus.

Marginal cell (MAR-jih-nul sell): The epithelial cell type that lines the cochlear duct surface of the stria vascularis; they are responsible for secreting the potassium-rich endolymph.

Membranous labyrinth (MEM-bruh-nus LAB-uh-rinth): A series of epithelial-lined ducts and sacs suspended within the bony labyrinth; it is filled with endolymph and contains the sensory organs for hearing and balance.

Middle ear (mid-ul eer): An air-filled cavity (tympanic cavity) located between the tympanic membrane and the oval window; it contains the three small bones (ossicles: malleus, incus, stapes) that transmit sound vibrations from the eardrum to the inner ear.

Mucous acinus (MYOO-kus AS-ih-nus): The secretory endpiece of an exocrine gland characterized by a cluster of cells that secrete a thick, viscous, lubricating fluid (mucus); cells typically appear pale or foamy in H&E preparations due to mucus washing out, with flattened nuclei pushed toward the base.

Nerve fiber layer (nerv FY-ber LAY-er): The innermost layer of the neural retina composed primarily of the unmyelinated axons of the ganglion cells as they travel toward the optic nerve head.

Neuroepithelial sensory cell (noor-oh-ep-uh-THEE-lee-ul SEN-suh-ree sell): The bipolar neuron whose dendrites act as sensory receptors that are found amongst epithelial cells; e.g., Taste cells, Olfactory cells.

Ora serrata (OR-uh suh-RAH-tuh): The scalloped anterior junction where the non-photosensitive ciliary portion of the retina meets the photosensitive neural retina.

Otoliths/otoconia (OH-tuh-liths oh-tuh-KOH-nee-uh): Tiny, dense calcium carbonate crystals found on the surface of the gelatinous membrane over the maculae of the utricle and saccule; their inertia provides the mass required to bend the hair cells in response to gravity or linear movement.

Outer ear (ow-ter eer): The external part of the ear, composed of the auricle (pinna) and the external auditory canal; its function is to collect sound waves and channel them toward the tympanic membrane.

Outer nuclear layer (OW-ter NOO-klee-er LAY-er): The layer of the retina containing the cell bodies and nuclei of the rods and cones (the photoreceptor cells).

Outer phalangeal cell (OW-ter fuh-LAN-jee-ul sell): A type of supporting cell in the spiral organ (of Corti) that extends processes to enclose the cell bodies and base of the outer hair cells.

Outer plexiform layer (OW-ter PLEK-suh-form LAY-er): The synaptic layer of the retina where the rods and cones form synapses with the dendrites of bipolar cells and horizontal cells.

Oval window (OH-vul WIN-doh): An oval-shaped, membrane-covered opening in the bone that connects the middle ear to the scala vestibuli of the inner ear; it is where the footplate of the stapes pushes to transmit sound energy.

Papilla (lingual) (puh-PIL-uh): General term for the epithelial and connective tissue projections on the dorsal surface of the tongue; these may be gustatory (containing taste buds) or non-gustatory (providing friction).

Perilymph (PER-ih-limf): The fluid found in the space between the bony labyrinth and the membranous labyrinth (within the scala vestibuli and scala tympani); it is similar to cerebrospinal fluid (CSF).

Pillar cell (PIL-er sell): Large, rigid supporting cells (inner and outer) in the spiral organ (of Corti) that lean against each other to form the triangular inner tunnel, providing structural integrity to the organ.

Posterior chamber (pah-STIR-ee-er CHAYM-ber): The small space filled with aqueous humor located behind the iris and in front of the suspensory ligaments and lens.

Retina (RET-ih-nuh): The innermost, light-sensitive layer of the eye containing the photoreceptor cells (rods and cones) and the complex neural circuitry that translates light into electrical signals sent to the brain.

Retinal pigmented epithelium (RPE) (RET-ih-nul PIG-mun-ted ep-uh-THEE-lee-um): A single layer of cuboidal cells located between the choroid and the neural retina; its functions include absorbing stray light, maintaining the blood-retina barrier, and phagocytizing the shed outer segments of the photoreceptors.

Rods (rodz): The photoreceptor cells in the retina that are highly sensitive to low light levels and are responsible for black-and-white (scotopic) vision.

Round window (rownd WIN-doh): A small, membrane-covered opening located just below the oval window that connects the middle ear to the scala tympani; it acts as a pressure relief valve for fluid movement in the cochlea.

Saccule (SAK-yool): The smaller of the two membranous sacs located in the vestibule of the inner ear; along with the utricle, it detects linear acceleration and gravity/head tilt.

Scala media (SKAH-luh MEE-dee-uh): Alternate name for the cochlear duct, the central endolymph-filled space of the cochlea that contains the spiral organ (of Corti).

Scala vestibuli (SKAH-luh ves-TIB-yoo-ly): The upper, perilymph-filled duct of the cochlea, separated from the scala media by the vestibular membrane; it begins at the oval window.

Sclera (SKLER-uh): The dense, tough, white outer layer of the eyeball; it is composed of thick, irregular connective tissue (fibrous layer) that protects the internal structures and serves as an attachment point for the extrinsic eye muscles.

Semicircular canals (sem-ih-SUR-kyoo-ler kuh-NALZ): Three bony loops within the bony labyrinth of the inner ear oriented at right angles to each other; they contain the semicircular ducts which detect angular (rotational) movements of the head.

Semicircular duct (endolymph) (sem-ih-SUR-kyoo-ler dukt EN-doh-limf): The epithelial-lined duct inside the semicircular canal that contains the endolymph fluid; movement of this fluid stimulates the hair cells for rotational balance.

Serous acinus (SIR-us AS-ih-nus): The secretory endpiece of an exocrine gland characterized by a cluster of pyramidal cells that secrete a watery, protein-rich fluid (often containing enzymes, e.g., amylase or lipase); cells have spherical nuclei and basophilic basal cytoplasm.

Spiral limbus (SPY-rul LIM-bus): A thickened ridge of periosteum that projects from the bony wall of the cochlea into the scala media; it is the structure from which the tectorial membrane originates.

Spiral organ (of Corti) (SPY-rul OR-gun): The principal sensory organ for hearing, located within the cochlear duct; it consists of specialized hair cells, supporting cells, and the tectorial membrane, which convert sound vibrations into nerve impulses.

Spiral prominence (SPY-rul PRAH-mih-nens): A protrusion of connective tissue and epithelium located on the outer wall of the cochlear duct, inferior to the stria vascularis.

Stapes (STAY-peez): The smallest of the three ossicles in the middle ear; its footplate fits into the oval window, transmitting vibrations from the incus into the inner ear fluid (perilymph).

Stereocilia (ster-ee-oh-SIL-ee-uh): Numerous non-motile, actin-filled microvilli of varying height that project from the apical surface of an inner ear hair cell; their deflection by fluid movement initiates the sensory signal.

Stria vascularis (STRY-uh vas-kyoo-LAIR-is): A unique, highly vascularized, stratified epithelium that lines the outer wall of the scala media; it actively maintains the high concentration of potassium ions in the endolymph (the endolymphatic potential).

Supporting cell (suh-POR-ting sell): Non-sensory epithelial cells (e.g., phalangeal cells, pillar cells) that surround and structurally support the hair cells in the spiral organ and vestibular sensory regions.

Suspensory ligaments (suh-SPEN-suh-ree LIG-uh-munts): Thin, radially arranged fibers that connect the ciliary processes to the capsule of the lens; they transmit the tension generated by the ciliary muscles to adjust lens curvature.

Taste bud (tayst bud): A small, barrel-shaped collection of specialized neuroepithelial (taste cells), supporting cells, and basal cells, found primarily on the epithelial surface of lingual papillae (fungiform, vallate, and foliate); it is the sensory organ for taste detection.

Taste pore (tayst por): The opening of the taste bud on the surface of the lingual epithelium where the taste receptors reside.

Tectorial membrane (tek-TOR-ee-ul MEM-brayn): A ribbon-like, acellular, gelatinous structure that projects over the spiral organ (of Corti); the stereocilia of the outer hair cells are embedded in it, and movement of the basilar membrane causes shear force against the hair cells.

Trabecular outflow (truh-BEK-yoo-ler OUT-floh): The primary drainage route for aqueous humor from the anterior chamber; the fluid passes through a meshwork of connective tissue (the trabecular meshwork) into the scleral venous sinus and then into the venous circulation.

Tympanic membrane (tim-PAN-ik MEM-brayn): A thin, cone-shaped membrane (eardrum) composed of three layers, separating the outer ear from the middle ear; it vibrates in response to sound waves, beginning the process of sound conduction.

Utricle (YOO-trih-kul): The larger of the two membranous sacs located in the vestibule of the inner ear; along with the saccule, it detects linear (straight-line) acceleration and gravity/head tilt (static equilibrium).

Uveoscleral outflow (yoo-vee-oh-SKLER-ul OUT-floh): A secondary, less significant drainage pathway for aqueous humor, in which the fluid passes through the ciliary muscle tissue and is absorbed into the suprachoroidal space.

Vallate papilla (VAL-ayt puh-PIL-uh): The largest of the lingual papillae, these papillae exist as 5-12 large, rounded projections that form a V-shape at the junction between the anterior two-thirds and posterior one-third of the tongue; approximately half of the taste buds in the tongue are found in the vallate (or circumvallate) papillae. Vallate papillae have large deep crypts with taste buds on the epithelial surface – serous gustatory glands release their secretions into the crypts, providing lingual lipase but also to flush out dissolved food particles from the crypts.

Vestibular membrane (ves-TIB-yoo-ler MEM-brayn): A thin barrier composed of two layers of simple squamous epithelium that separates the scala vestibuli (perilymph) from the scala media (endolymph) in the cochlea.

Vestibulo-cochlear nerve (ves-TIB-yoo-loh-KOK-lee-er nerv): The cranial nerve (CN VIII) that is divided into two parts: the vestibular nerve (carrying balance/equilibrium information) and the cochlear nerve (carrying auditory information) from the inner ear to the brainstem.

Vitreous chamber (VIT-ree-us CHAYM-ber): The large, posterior space of the eye, filled with the vitreous humor, a transparent, gel-like substance that helps maintain the eye’s shape and provides support for the retina.