No. 20

1. No. 20 • Vestibulocochlear Organ

2. Chapter 2 The Vestibulocochlear Organ • Division • The vestibulocochlear organ (ear) is divided into three parts: • The external ear, • The middle ear, • The internal ear.

4. Functions • The receptor for auditory sensation (spiral organ or cochlear organ) and organs of static balance (vestibular organ) are in the internal ear. The internal ear is such an organ that can receive the stimulation of both sound waves and changes of the position of the head. • The external and the middle ears are a sound collecting and transmitting apparatus.

5. Section 1 The External Ear • The external ear consists of: • the auricle, • the external acoustic meatus • the tympanic membrane.

6. Ⅰ. The Auricle • It projects from the side of the head. • It anterolateral surface shows irregularly concave, but its posteromedial surface presents convex. The orifice of the meatus, named the external acoustic pore, lies in the fossa of the anterolateral surface of the auricle.

8. Ⅱ. The External Acoustic Meatus • Morphological characteristics • It extends from the external acoustic pore to the tympanic membrane and is about 2.1～2.5 cm in length. Its lateral part is about one-third of the meatus, termed the cartilaginous part, and its medial part is about two-thirds of the meatus, termed the bony part.

10. The meatus passes medially, its lateral part runs forwards and upwards, and then backwards; and the medial part runs forwards and downwards. • In the clinical examination of the meatus, the auricle should be drawn upwards, backwards and slightly laterally to render the meatus as straight as possible, so that the tympanic membrane can be viewed.

11. Structural characteristics • In the subcutaneous tissue of the cartilaginous part of the meatus there are numerous sebaceous and ceruminous glands. The latter secrete the cerumen (ear wax).

12. Clinical points • The skin of the meatus is thin, and its subcutaneous tissue is scarce but rich in sensory nerve terminals, and is closely adherent to the cartilaginous and bony parts of the meatus, therefore under inflammatory condition, the meatus is extremely painful.

13. Ⅲ. The Tympanic Membrane • It is oval in form, thin and semi-transparent, separates the middle ear from the external acoustic meatus. It inclines greatly and forms an angle of 45 degrees with the floor of the meatus, hence the anteroinferior wall of the meatus is longer than the posterosuperior wall.

15. The thickened margin of the membrane, the fibrocartilaginous ring, is attached to the tympanic sulcus at the medial end of the external acoustic meatus. In the upper end of the tympanic membrane, two bands, the anteriorand posterior mallear folds, are prolonged to the lateral process of the malleus. The small part (1/4) of the membrane above these folds is lax and thin, called the flaccidpart, while the remainder is tightly stretched, that is the tense part of the membrane.

16. In the living body, it is pearly-grey in colour. The handle of the malleus is firmly attached to the inner surface of the tympanic membrane as far as its center, thus the outer surface of the membrane is concave with a central depression, the umbo, formed by the traction of the lower end of the handle of malleus. When the tympanic membrane is examined by otoscope, a bright area, the cone of light, anteroinferior to the umbo can be seen.

17. Section 2 The Middle Ear • The middle ear lies between the external and inner ears. • It includes three parts: • The tympanic cavity, • The auditory tube, • The mastoid cells.

18. Ⅰ. The Tympanic Cavity • It is an irregular air-filled space within the temporal bone, and lies between the tympanic membrane and the lateral wall of the inner ear. It is the principal part of the middle ear, its capacity is about 1～2 cm3. There are auditory ossicles, ligaments, muscles, vessels and nerves inside the tympanic cavity. The tympanic cavity communicates anteriorly with the nasopharyns through the auditory tube, and posteriorly with the mastoid cells through the mastoid antrum.

19. Ⅰ) The Walls of the Tympanic Cavity • It possesses six walls. 1. The tegmental wall (superior wall) • It is a thin plate of compact bone, the tegmen tympani, it separates the middle cranial fossa from the tympanic cavity. • In the first two years of childhood, the unossified suture of the superior wall may allow the infection to spread from the tympanic cavity into the cranial cavity directly.

21. 2. The jugular wall (inferior wall) • It consists of the thin plate of bone which separates the tympanic cavity from the jugular fossa. The bone of this wall may be deficient, so the tympanic cavity is separated from the jugular vein by mucous membrane and fibrous tissue only. This may cause the beginning part of the jugular vein to project into the tympanic cavity.

22. 3. Thecarotid wall (anterior wall) • It is the posterolateral wall of the carotid canal, close to the internal carotid artery. At the superior part of the anterior wall, there are two parallel canals leading to the tympanic cavity. The upper is the smaller semicannal for tensor tympani, and the lower is a largersemicanal for auditory tube, the bony part of auditory tube.

23. 4. The mastoid wall (posterior wall) • It is pierced superiorly by the opening (aditus ) of mastoid antrum. The pyramidal eminence is situated below the opening of mastoid antrum. The cavity of the pyramidal eminence contains the stapedius.

25. 5. The membranous wall (lateral wall) • It is almost entirely formed by the tympanic membrane, only the superior part of this wall is formed by the lateral wall of the epitympanic recess.

26. 6. The labyrinthine wall (medial wall) • It is the lateral wall of the inner ear. A rounded elevation on the middle of this wall is named the promontory. Posterosuperior to the tympanic promontory is the fenestra vestibuli (oval window) being closed by the base of the stapes and annular ligament. The fenestra cochleae (round window) lies posteroinferior to the tympanic promontory, and is closed by the secondary tympanic membrane in vivo. The prominence of facial canal is an arcuate-like ridge formed by the bony canal for the facial nerve, and extends back and down to the posterior wall of the tympanic cavity. The facial canal is very thin, or incomplete. In inflammatory condition of the tympanic cavity, the facial nerve may be involved and leading to facial paralysis.

27. Ⅱ) The structures in the tympanic cavity • In the tympanic cavity there are three auditory ossicles, two muscles, one nerve and air equal to the atmosphere. • 1. The auditory ossicles and their joints • The tympanic cavity contains a chain of three ossicles: • The malleus, • TheIncus, • The stapes.

29. Laterally, the handle of malleus is attached to the tympanic membrane, and medially, the base of the stapes is fixed to the circumference of the fencestra vestibuli, while the incus is placed between the malleus and the stapes. • The three ossicles connencted by joints to form a jointed chain which connects the tympanic membrane with the fenestra vestibuli. • When the tympanic membrane is vibrated by the sound wave, the handle of the malleus is moved with it, then, the incus and the stapes transmit the vibrations to the inner ear.

30. 2. The muscles to motor the auditory ossicles • The muscles of the cavity are the tensor tympani and the stapedius. • The tensor tympani lies in the semicanal for tensor tympani, and ends to the handle of the malleus. It is supplied by the mandibular nerve of trigeminal nerve. • Thestapedius arises from the pyramidal eminence of posterior wall of the tympanic cavity and is inserted into the neck of the stapes. It is controlled by the facial nerve.

31. Under the normal conditions, the tnsor tympani and the stapedius contract simultaneously. When the tensor tympani contracts it pulls the handle of the malleus medially, tenses the tympanic membrane, and thus reduces the amplitude of vibration; whereas the contraction of stapedius renders base of stapes outwards, and thus reduces the pressure of sound wave for the inner ear.

32. Ⅱ. The Auditory Tube (Pharyngotympanic tube) 1. Divisions of the auditory tube • It is the channel through which the tympanic cavity communicates with the nasopharynx. It is approximately 3.5～4.0 cm long, and is divided into the cartilaginous and the bony parts. The junction of these two parts is narrowest, 1-2 mm long, termed theisthmus of auditory tube.

34. The bony part: • This part is the posterolateral part of the tube, about 1/3 of its total length. It begins in the anterior wall of the tympanic cavity and passes forward, downward and inward. • The cartilaginous part: • This part is the anteromedial part of the tube, about 2/3 of the tube, and opens into the nasopharynx.

35. 2. Function of the auditory tube • The function of auditory tube is to maintain the balance of pressure on both sides of the tympanic membrane. • In the normal condition, the pharyngeal orifice and cartilaginous part is closed; during the deglutition they are opened and allow the air to center or leave the tympanic cavity, this balances the pressure on both sides of the tympanic membranes and censures the tympanic membrane to vibrate freely.

36. 3. Clinical points • The tube is easily blocked by swelling of its mucous membrane. When it is blocked, the residual air in the tympanic cavity is absorbed, resulting in the retraction of the tympanic memrane and interference with its free movement.

37. 4. Characteristics of children’s auditory tube • In childhood, the auditory tube is shorter and wider than in adult. Its direction is more horizontal, therefore, the inflammation of the pharynx may along the auditory tube spread into the tympanic cavity and causes the otitis media.

38. Ⅲ. The Mastoid Antrum and Mastoid Cells • They are air-filled spaces in the mastoid process of the temporal bone. They are a series of intercommunicated cavities, and anteriorly, through the mastoid antrum they communicate with the tympanic cavity. Since the mucous membrane of the mastoid air cells is continuous with that of the mastoid antrum and tympanic cavity, the otitis media may spread to the mastoid antrum and the mastoid cells.

40. Section 3 The Internal Ear • The inner (internal) ear lies in the petrous part of the temporal bone. • It consits of two parts: the bony labyrinth and the membranous labyrinth. • The former is composed of the compact bone, and the latter, a series of communicating membranous sacs and ducts, is contained within the bony labyrinth. • The membranous labyrinth is filled with endolymph, and the space between the membranous and bony labyrinth is filled with perilymph. The endolymph does not communicates with the perilymph.

42. Ⅰ. The Bony Labyrinth • From before backwards, the bony labyrinth is divided into three parts: • The cochlea, • The vestibule, • The bony semicircular canals. • They are various with each other in shape, but communicate with each other.

44. Ⅰ)The Vestibule • It is the central part of the bony labyrinth, and is situated medial to the tympanic cavity. It is a somewhat ovoid space. There are three foremen on the posterior part of the vestibule communicating with three bony semicircular canals. It appears four walls:

45. 1. Lateral wall • It is the medial wall of the tympanic cavity. • Fenestra vestibuli and fenestra cochleae: • On its lateral wall there are two openings: the fenestra vestibuli communicates with the tympanic cavity and is closed by the base of the stapes with its annular ligament in vivo; the fenestra cochleae is closed by the secondary tympanic membrane.

46. 2. Medial wall • It is the posterior part of the fundus of the internal acoustic meatus, through which the peripheral branches of the vestibulocochlear nerve pass into the membranous labyrinth. 3. Anterior wall • On this wall there is the inlet of cochlear spiral canal communicating with the scala vestibuli of the cochlea. 4. Posterior wall • On the posterior wall of the vestibule there are the five openings of the semicircular canals.

48. Ⅱ) The Cochlea • The cochlea is placed anterior to the vestibule, resembling the shell of a snail. It is composed of modiolus andcochlear spiral canal winding spirally for 2.5 turns around the central modiolus. • Its apex or cupula of cochlea points anterolaterally, its base is directed posteromedially towards the bottom of the internal acoustic meatus. • The modiolus is the conical osseous central pillar of the cochlea, through which the vessels and nerves pass.

49. The osseous spiral lamina of modiolus projects from the modiolus into the spiral canal, and divides the cochlear canal into scala vestibuli and the scala tympani with the basilar membrane of the cochlear duct. The scala vestibuli and the scala tympani pass to the fenestra vestibuli and the fenestra cochleae respectively and are filled with perilymph.

50. The width of lamina of modiolus gradually decreases from the basal to the apical coil of the cochlea, and near the summit of the cochlea the lamina ends in a hook-shaped process, the hamulus of spiral lamina. • The hamulus and the modiolus form the helicotrema, through which the scala vestibuli and the scala tympani communicate with each other.