The Ear

1. The Ear • Divided into: • External (outer) ear • Consists of : • Auricle or pinna • External auditory canal (external auditory meatus, external acoustic meatus) • Middle ear • Internal ear

2. Pinna External auditory meatus Helix • It is the outer expanded portion of the ear, projects from the side of the head • Serves to collect the vibrations of the air by which sound is produced • Consists of a thin plate of elastic cartilage covered by skin. • The skin is continuous with that lining the external acoustic meatus • Possesses both extrinsic and intrinsic muscles, which are supplied by the facial nerve Antihelix Tragus Concha Lobule Auricular muscles

3. Arteries : • Posterior auricular artery, branch of the external carotid artery • Anterior auricular artery, branch of the superficial temporal artery • Auricular branch of the occipital artery. • Veins accompany the corresponding arteries. • Sensory nerves: • Great auricular & Lesser occipital, from the cervical plexus • Auricular branch of the vagus nerve • Auriculotemporal branch of the mandibular nerve • Motor nerves: • Posterior auricular branch of facial nerve

4. External Auditory Meatus • Leads inward from the bottom of the auricle to the tympanic membrane (ear drum) • Conducts the vibrations to the tympanic cavity. • About 4 cm. in length if measured from the tragus • Forms an S-shaped curve, and is directed: • at first inward, forward, and slightly upward • it then passes inward and backward • lastly is carried inward, forward, and slightly downward.

5. The floor and the anterior wall of the meatus are longer than the roof and the posterior wall, because the tympanic membrane, which closes the inner end of the meatus, is placed obliquely The canal is formed partly by cartilage and partly by bone The outer ⅓ is formed of elastic cartilage, that is continuous with the cartilage of the auricule The medial ⅔is bony, formed by tympanic plate of the temporal bone. It is an oval cylindrical canal, narrower in the bony portion and presents two constrictions: one near the inner end of the cartilaginous portion another in the osseous (bony) portion

6. It is lined by thin skin that adheres closely to the underlying cartilage and the bone, and also covers the outer surface of the tympanic membrane Its outer third is provided with hairs and sebaceous and ceruminous (modified sweat) glands that secrete a yellowish brown wax. The hairs and the wax provide a sticky barrier that prevents the entrance of foreign bodies.

7. Arteries: Supplied by the branches of the: • Posterior auricular artery • Maxillary artery • Superficial temporal artery • Veins: Accompany the corresponding arteries • Nerves: Are chiefly derived from the: • Auriculotemporal branch of the mandibular nerve • Auricular branch of the vagus nerve • Lymphatics:Drain into the: • Superficial parotid lymph nodes • Mastoid lymph nodes • Superficial cervical lymph nodes

8. Tympanic Membrane (Ear Drum) • Separates the external acoustic meatus from the middle ear (tympanic) cavity. • It is a thin, semitransparent membrane, nearly oval in form, slightly broader above than below • It is directed obliquely downward, forward and laterally, & form an angle of about 55˚ with the floor of the meatus. • Its longest diameter measures from 9 to 10 mm & its shortest diameter measures from 8 to 9 mm.

9. The handle of the malleus is firmly attached to the medial surface of the membrane as far as its center, which it draws toward the tympanic cavity The lateral surface of the membrane is thus concave, and the most depressed part of this concavity is named the umbo (produced by the tip of the handle of malleus) When the tympanic membrane is illuminated through an otoscope, the concavity produces a cone of lightthat radiates anteriorly and inferiorly from the umbo

10. It is three-layered structure consisting of thin skin laterally, mucosa medially, and an intervening fibrous layer. The fibrous layer thickens along the circumference to form the annulus, an incomplete ring which is fixed into a groove, the tympanic sulcus in the bone at the inner end of the meatus The tympanic sulcus is deficient superiorly, which forms a notch From the sides of the notch, anterior & posterior malleolar folds pass to the lateral process of maleus Superior to the malleolar folds the small triangular area of the membrane is known as the pars flaccida. The remainder of the membrane forms the pars tensa

11. Arteries:derived from the branches of the: • Maxillary artery • Posterior auricular artery • Veins: • The superficial veins open into the external jugular vein • Those on the deep surface drain partly into the transverse sinus and veins of the dura mater, and partly into a plexus on the auditory tube. • Nerve supply:The tympanic membrane is extremely sensitive to pain and receives its nerve supply: • On the outer surface by the: • Auriculotemporal nerve • Auricular branch of the vagus nerve • On the mucosal surface by the: • Tympanic branch of the glossopharyngeal nerve

12. Middle Ear (Tympanic) Cavity • An irregular, slitlike, laterally compressed space within the petrous part of the temporal bone. • Long axis lies parallel to the plane of the tympanic membrane • Normally sealed laterally by the tympanic membrane • Lined with mucous membrane • Filled with air, which is conveyed to it from the pharynx through the auditory tube • Contains a chain of movable bones, the ossicles

13. Communicates: Posteriorly: with the mastoid air cells through the mastoid antrum Anteriorly: with the nasopharynx through the auditory tube Consists of two parts: Tympanic cavity proper, lies opposite the tympanic membrane Attic or Epitympanic recess, lies above the level of the membrane; contains the upper half of the malleus and the greater part of the incus & communicates with the mastoid antrum via the aditus

14. Tympanic Cavity cont’d • It is a six-sided cavity and has a: • Roof • Floor • Lateral wall • Medial wall • Anterior wall • Posterior wall

15. Roof (Tegmental Wall): Formed by a thin plate of bone, the tegmen tympani, part of the petrous temporal bone. Separates the tympanic cavity from the cranial cavity. Floor(Jugular Wall): Is narrow, formed by a thin plate of bone (sometimes just fibrous tissue) Separates the tympanic cavity from the superior bulb of the internal jugular vein in the jugular fossa

16. Lateral (Membranous) Wall: Formed mainly by the tympanic membrane Partly by the ring of bone into which this membrane is inserted. Medial (Labyrinthic)Wall: Vertical in direction, formed by the lateral wall of the inner ear Presents the following features: Promontory Fenestræ vestibuli Fenestra cochleæ Prominence of the facial canal.

17. Promontory Rounded hollow prominence Formed by the projection outward of the first turn of the cochlea Furrowed on its surface by small grooves, for the lodgment of branches of the tympanic plexus. Fenestra vestibuli (oval window) An oval opening located above and behind the promontory Closed by the base of stapes Deep to the window lies the perilymph in the scala vestibuli of the internal ear

18. Fenestra cochleae (round window) Situated below and a little behind the promontory Leads into the blind end of the scala tympani of the internal ear In the fresh state, closed by the secondary tympanic membrane Prominence of the facial canal Lies horizontallyabove the fenestra vestibuli, and behind that opening curves nearly vertically downward along the posterior wall Indicates the position of the bony canal containing the facial nerve

19. Posterior (Mastoid) wall Wider above where it shows a large irregular aperture, the aditus, which leads backward from the epitympanic recess into the tympanic or mastoid antrum Below it is a small, hollow, conical projection called the pyramid, that contains the stapedius muscle; its summit projects forward toward the fenestra vestibuli, and is pierced by a small aperture which transmits the tendon of the muscle.

20. Anterior (Carotid) Wall: Wider above than below In the lower part, formed by a thin plate of bone that separates the cavity from the internal carotid artery The upper part shows two orifices: Upper small orifice is the opening of the canal for the tensor tympani muscle Lower larger opening is the tympanic orifice of the auditory canal.

21. The two canals are separated from each other by a thin horizontal plate of bone, whichpasses backward on the medial wall above the promontory and fenestra vestibuli. It supports the tensor tympani muscle, and terminates there by curving upward to form a pulley, the processus cochleariformis, over which the tendon of the tensor tympani muscle bends laterally to reach its insertion on the handle of the malleus

22. Ossicles • The smallest bones in the body, three in number, linked to each other • Lie in the tympanic cavity • Convert the sound waves striking the eardrum into mechanical vibrations and then convey the vibrations across the cavity to the internal ear. • From lateral to medial, these are the: • Malleus • Incus • Stapes

23. Malleus • The largest ossicle, possesses a head, a neck, a long process or handle, an anterior process, and a lateral process. • Head: rounded, articulates posteriorly with the incus. • Neck : constricted part below the head. • Handle: passes downward and backward and is firmly attached to the medial surface of the tympanic membrane. It can be seen through the tympanic membrane on otoscopic examination. • Anterior process: a spicule of bone, connected to the anterior wall of the tympanic cavity by a ligament. • Lateral process: projects laterally, is attached to the anterior and posterior malleolar folds of the tympanic membrane.

24. Incus • The incus possesses a large body and two processes • Body: rounded and articulates anteriorly with the head of the malleus • Long process descends behind and parallel to the handle of the malleus. Its lower end bends medially and articulates with the head of the stapes. Its shadow on the tympanic membrane can sometimes be recognized on otoscopic examination • Short process projects backward and is attached to the posterior wall of the tympanic cavity by a ligament

25. Stapes • The stapes has a head, a neck, two limbs, and a base • The head is small and articulates with the long process of the incus • The neck is narrow and receives the insertion of the stapedius muscle • The two limbs (crura) diverge from the neck and are attached to the oval base. • The edge of the base is attached to the margin of the fenestra vestibuli by a ring of fibrous tissue, the annular ligament

26. Muscles of the Ossicles • Tensor tympani • Origin: Wall of auditory tube and wall of its own canal • Insertion: Handle of malleus • Nerve Supply: Mandibular division of trigeminal nerve • Action: Dampens down vibrations of tympanic membrane • Stapedius • Origin: Pyramid (bony projection on posterior wall of middle ear) • Insertion: Neck of stapes • Nerve Supply: Facial nerve • Action: Dampens down vibrations of stapes

27. Nerves in the Tympanic Cavity • Facial nerve (intrapetrous part) • Chorda tympani, branch of facial nerve • Greater petrosal nerve, branch of facial nerve • Tympanic nerve, branch of glossopharyngeal nerve • Tympanic plexus • Lesser petrosal nerve (parasympathetic) • Deep petrosal nerve (sympathetic)

28. Facial Nerve • Leaves cranial cavity through the internal acoustic meatus • Enters the facial canal within the petrous part of temporal bone • Runs laterally above the vestibule of internal ear • Reaches the medial wall of the middle ear and bends sharply backward above the promontory. At the bend possesses the geniculate ganglion • On reaching the posterior wall of the middle ear cavity it bends downward on the medial side of the aditus to mastoid antrum and then descends behind the pyramid to exit from the stylomastoid foramen

29. Branches of the Intrapetrous Part of the Facial Nerve • Greater petrosal nerve arises from the facial nerve at the geniculate ganglion..The nerve emerges on the superior surface of the petrous part of the temporal bone and is eventually joined by the deep petrosal nerve & forms the nerve of the pterygoid canal, which ends in the pterygopalatine ganglion. • Nerve to the stapediusarises from the facial nerve as it descends in the facial canal behind the pyramid. It supplies the muscle within the pyramid. • Chorda tympani arises from the facial nerve just above the stylomastoid foramen. It enters the middle ear close to the posterior border of the tympanic membrane. It then runs forward over the tympanic membrane and crosses the root of the handle of the malleus. The nerve leaves the middle ear through the petrotympanic fissure and enters the infratemporalfossa, where it joins the lingual nerve

30. Auditory Tube (Eustachian Tube) • The channel through which the tympanic cavity communicates with the nasopharynx. • Formed partly of bone (posterior 1/3), partly of cartilage and fibrous tissue (anterior 2/3) • The mucous membrane of the tube is continuous with that of the nasopharynx, and the tympanic cavity • The Eustachian tube functions to equalize air pressure on both sides of the eardrum.

31. Mastoid Antrum • The Mastoid Antrum lies behind the middle ear in the petrous part of the temporal bone. It communicates with the middle ear by the aditus • Relations: • Anterior wall is related to the middle ear and contains the aditus to the mastoid antrum • Posterior wall separates the antrum from the sigmoid venous sinus and the cerebellum • Lateral wall is (1.5 cm) thick 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, the tegmen tympani, which is related to the meninges of the middle cranial fossa and the temporal lobe of the brain • Inferior wall is perforated with holes, through which the antrum communicates with the mastoid air cells • Mastoid Air Cells • The mastoid air cells are a series of communicating cavities within the process that are continuous above with the antrum and the middle ear.They are lined with mucous membrane.

32. Inner Ear • The inner ear is made up of both hearing (auditory) and balance (vestibular) components & functions to convert mechanical energy into neural impulses. • Situated in the petrous part of the temporal bone, medial to the middle ear • Consists of a delicate membranous inner ear, the membranous labyrinth enclosed and protected by a bony chamber that is referred to as the bony labyrinth

33. There are three main divisions of the bony labyrinth. Vestibule : Lies medial to the oval window Contains the utricle and the saccule, two organs of balance Leads to both the cochlea and the semicircular canal Cochlea: A snail-shaped chamber anterior to the vestibule Bulges into the middle ear as promontory on its medial wall Communicates with the middle ear via the round window. Converts sound waves into neural impulses Semicircular canals vestibule cochlea

34. Semicircular Canals: Three in number Project posteriorly from the vestibule Consist of a superior, posterior and lateral (horizontal) canals These organs detect angular acceleration There are two compartments of fluid in the inner ear The space within the bony labyrinth surrounding the membranous labyrinth contains perilymph The space within the membranous labyrinth contains endolymph

35. The nerve fibers from the labyrinth make up the auditory nerve which consists of a: Cochlear nerve and Vestibular nerve with both afferent and efferent fibers from the respective sensory end organs. The auditory nerve enters the cranial cavity via the internal auditory meatus

36. Clinical Notes • The otoscopic exam is performed by gently pulling the auricle upward and backward. In children, the auricle should be pulled downward and backward. This process will move the acoustic meatus in line with the canal. • Too much cerumen can block sound transmission. • This ear-throat connection makes the ear susceptible to infection (otitis media). • Infection of mastoid air cells