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The Ear is used by the human for hearing and balance. It consists of 3 parts: 1. External Ear 2. Middle Ear (Tympanic Cavity) 3. Internal Ear (Labyrinth) which contains the organs of hearing and balance.
The External Ear It consists of an auricle and an external auditory meatus. The auricle (Pinna): is a thin plate of elastic cartilage covered by skin and arranged in specific pattern of elevations & depressions. It has both extrinsic and intrinsic muscles, which are supplied by the facial nerve. The external auditory meatus: is a 1 inch (2. 5 cm) curved tube connecting auricle to tympanic membrane to conduct sound waves. It is composed of an outer one third of elastic cartilage and inner two thirds of bone (tympanic plate). The meatus is lined by skin containing wax producing glands (ceruminous glands). Hairs and wax form sticky barrier to prevent entrance of foreign bodies. Nerve supply of the lining skin is derived from auriculotemporal nerve and auricular branch of vagus nerve. Lymph drainage is to the superficial parotid, mastoid, and superficial cervical lymph nodes.
Middle Ear (Tympanic Cavity) It is a narrow, oblique, slit-like air-containing cavity in the petrous part of temporal bone & is lined with mucous membrane. It contains auditory ossicles. Communications: in front with nasopharynx through auditory tube, and behind with mastoid antrum through the aditus. Function? • It consists of two parts: Tympanic Cavity & Epi-tympanic recess (Attic).
Boundaries of Middle Ear Roof: tegmen tympani; separates tympanic cavity from MCF. Floor: Thin bone separates tympanic cavity from superior bulb of IJV. Anterior wall: Thin bone; separates tympanic cavity from ICA and at its upper part are openings into two canals (auditory tube & canal for tensor tympani). Posterior wall: Aditus to the mastoid antrum superiorly & Pyramid inferiorly ( for stapedius) Lateral wall: tympanic membrane inferiorly & Lateral wall of attic superiorly. Medial wall: Lateral wall of the inner ear.
Medial Wall features Promontory: is a rounded projection formed by first turn of underlying cochlea. Oval Window (fenestra vestibule): is an oval shaped opening that lies above & behind promontory and closed by base of stapes. On the medial side of this window is perilymph of scala vestibuli of internal ear. Round Window (fenestra cochleae): is around shaped opening that lies below & behind promontory and closed by secondary tympanic membrane. On its medial side, is perilymph of blind scala tympani. Processus Cochleariformis: is the posterior upward curve of the bony shelf that extends from anterior wall of TC above the promontory. It forms a pulley to support tendon of tensor tympani muscle to bends laterally reaching its insertion site. Prominence of the facial nerve canal: is a rounded ridge runs horizontally backward above promontory & fenestra vestibuli. On reaching the posterior wall, it curves downward behind the pyramid.
Tympanic Membrane (TM) • It is a thin, pearly grey, concave & circular fibrous membrane that is obliquely placed, facing downward, forward, and laterally on the lateral wall of middle ear. • At the centre of its concavity, is a small depression (Umbo) produced by tip of the handle of the malleus. • When TM is illuminated through an otoscope, its concavity produces a cone of light ( radiated anteroinferiorly from the umbo). • TM is about 1 cm in diameter and is thickened at the periphery and slotted into a groove in the bone (tympanic sulcus) which is deficient superiorly forming a notch. From the sides of this notch, two bands, termed Anterior & Posterior malleolar folds, pass to the lateral process of the malleus. • Above these folds, small triangular part of TM is slack (Pars Flaccida) and rest of TM is tense (Pars Tensa). • TM is extremely sensitive to pain & is innervated on its outer surface by auriculotemporal nerve & auricular branch of vagus; while its inner surface is innervated totally by glossopharyngeal nerve.
Auditory Ossicles • They are malleus, incus & stapes. Malleus: (largest ossicle) has head, neck, long process (handle), anterior and lateral processes. • Head articulates posteriorly with incus. • Neck is constricted part below the head. • Handle is firmly attached to medial surface of TM. It can be seen through TM on otoscopic examination. • Anterior process is a spine-like projection that is connected to anterior wall of tympanic cavity by a ligament. • Lateral process is attached to anterior & posterior malleolar folds of TM Incus: has a large body & 2 processes (Long & Short). • Body is rounded and articulates anteriorly with head of malleus. • Long process descends parallel to the handle of malleus & its lower end articulates with head of stapes. • Short process is attached to posterior wall of tympanic cavity by ligament. Stapes: has head, neck, two limbs, and base. • Head articulates with long process of incus. • Neck is narrow and receives insertion of Stapedius Muscle. • Two Limbs are attached to the oval base. • Base is attached to margin of Fenestra Vestibuli (oval window) by a ring of fibrous tissue, Anular ligament.
Auditory Tube • Also called Eustachian Tube or Pharyngotympanic tube • It connects anterior wall of tympanic cavity to nasal pharynx. • Its posterior third is bony and its anterior two thirds is cartilaginous. • As the tube descends it passes over upper border of superior constrictor muscle of pharynx. • It serves to equalize air pressures in the tympanic cavity and the nasal pharynx.
Mastoid Antrum It lies behind the middle ear &communicates with middle ear by the aditus. Relations of the Mastoid Antrum: • Anterior wall is related to the middle ear and contains the aditus. • Posterior wall separates the antrum from the sigmoid venous sinus and the cerebellum. • Lateral wall is thick plate of bone and forms the floor of the suprameatal triangle. • Medial wall is related to the posterior semicircular canal. • Superior wall is the thin plate of bone (tegmen tympani) • Inferior wall is perforated with holes, through which the antrum communicates with the mastoid air cells. Mastoid Air Cells: are a series of communicating cavities within mastoid process. They are continuous above with mastoid antrum. They are lined with mucous membrane.
Facial Nerve • It passes through internal acoustic meatus then enters facial canal. The nerve runs laterally above vestibule until it reaches medial wall of middle ear where it expands to form sensory geniculate ganglion. • On arriving at posterior wall of middle ear, it curves downward on medial side of aditus. Then descend behind the pyramid, and finally emerges through the stylomastoid foramen into the neck. • Important Branches of facial nerve within petrous bone: 1 - Greater Petrosal Nerve: contains preganglionic parasympathetic fibers that pass to lacrimal glands of mucous membranes of nose and palate. It is joined by Deep Petrosal Nerve from the sympathetic plexus around ICA and they both form Nerve of Pterygoid Canal. This nerve enters pterygopalatine fossa & ends in pterygopalatine ganglion. 2 - Nerve to Stapedius muscle: arises behind the pyramid &supplies Stapedius within the pyramid. 3 - Chorda Tympani: supplies: • Taste fibers from anterior 2/3 of tongue that anastomose at geniculate ganglion. • Preganglionic parasympathetic secretomotor fibers for submandibular & sublingual glands.
Chorda Tympani arises above stylomastoid foramen & enters the middle ear close to posterior border of the tympanic membrane. It then runs forward over the TM & crosses root of the handle of malleus. It lies between mucous membrane & fibrous layers of TM. It leaves middle ear through petrotympanic fissure & enters infratemporal fossa, where it joins lingual nerve.
Tympanic Nerve It arises from the glossopharyngeal nerve, just below the jugular foramen and enter the middle ear and onto the promontory to form the tympanic plexus. The tympanic plexus supplies the lining of the middle ear and gives off the lesser petrosal nerve, which sends secretomotor fibers to the parotid gland via the otic ganglion.
The Internal Ear (Labyrinth) The labyrinth is situated in the petrous part of the temporal bone, medial to the middle ear. It consists of bony labyrinth (BL) & membranous labyrinth (ML).
Bony Labyrinth (BL) • It is a series of bony cavities within petrous part of temporal bone. They are lined by endosteum & contain clear fluid (perilymph) in which membranous labyrinth is suspended. It consists of three parts: A. Vestibule: is central part of BL that lies posterior to cochlea & anterior to semicircular canals. In its lateral wall are fenestra vestibule & fenestra cochleae. Vestibule contains Saccule & Utricle of ML in its anterior & posterior parts respectively. B. Three semicircular canals (superior, posterior & lateral): open into posterior part of vestibule by five orifices. Each canal has a swelling at one end (ampulla). Lodged within canals are semicircular ducts. C. Cochlea: (Snail shell Like) It opens Into anterior part of vestibule & consists of a central pillar (Modiolus) around which a hollow bony tube makes 2 & 1/2 spiral turns. The first basal turn of cochlea causes the promontory seen on medial wall of middle ear.
Cochlea Modiolus has a broad base perforated by branches of cochlear nerve. A spiral ledge (Spiral Lamina) winds around modiolus & projects into interior of canal & partially divides it. Basilar membrane stretches from free edge of spiral lamina to outer bony wall, thus dividing cochlear canal into Scala vestibuli above & Scala tympani below. Perilymph within scala vestibuli is separated from TC by base of stapes & anular ligament at fenestra vestibuli. Perilymph in scala tympani is separated from TC by secondary tympanic membrane at fenestra cochleae.
Membranous Labyrinth (ML) Membranous labyrinth is lodged (suspended) within bony labyrinth. It is filled with endolymph & surrounded by perilymph. It consists of freely communicating parts: • Utricle & saccule: lodged in central bony vestibule. • Three semicircular ducts: lodged within bony semicircular canals. • Duct of Cochlea: lodged within the bony cochlea.
End of the Lecture GOOD LUCK