Middle Ear Cavity Flashcards
(5 cards)
Introduction,extent,parts,shape,dimensions of middle ear cavity.
Introduction: The middle ear located in the petrous portion of the temporal bone and is filled with air secondary to communication with the nasopharynx via the auditory (eustachian) tube.
Extent: The tympanic cavity (middle ear) extends from the tympanic membrane to the oval window and contains the bony conduction elements of the malleus, incus, and stapes.
Parts:
It is divided into two parts: Tympanic cavity proper which is opposite to tympanic membrane
and epitympanic recess which is above the level of tympanic membrane.
Shape: Biconcave
Dimensions:
Vertical and Anteroposterior: 15 mm
Transverse diameter:
Upper part: 6 mm
Middle part: 2 mm
Lower part: 4 mm
Boundaries of tympanic cavity
Roof: (Tegmental Wall): Formed by a thin plate of bone, the tegmen tympani, which separates
the cranial and tympanic cavities.
Floor: (Jugular Wall): Narrow, and consists of a thin plate of bone (fundus tympani) which separates the tympanic cavity from the jugular fossa and related to superior bulb of internal jugular vein.
Posterior wall (Mastoid wall): contains the mastoid antrum and communicates with the mastoid air cells
Anterior wall (Carotid wall): The anterior wall is also termed the carotid wall, because a thin plate of bone separates the carotid canal containing internal carotid artery and tympanic cavity.It has following features:
Opening for the canal of tensor tympani muscle
Opening of auditory opening
Both the openings are separated by a bony process known as Processus
cocholeariformis.
Medial Wall (Labyrnthic wall): is vertical in direction, and presents for examination the fenestræ vestibuli and cochleæ, the promontory, pyramid and the prominence of the facial canal.
Features in Medial wall:
The promontory is a rounded hollow prominence, formed by the projection
outward of the first turn of the cochlea; it is placed between the fenestræ
The fenestra vestibuli (fenestra ovalis-oval window) is a reniform opening leading from the tympanic cavity into the vestibule of the internal ear; located posterosuperior to promontory occupied by the base of the stapes.
The fenestra cochleæ (fenestra rotunda-round window) is located posteroinferior to promontory, leads into the cochlea of the internal ear; in the fresh state it is closed by a membrane, the secondary tympanic membrane.
Prominence for Facial canal is located at junction of posterior and medial wall and contains the facial nerve.
Pyramid: It is elevation between fenestrae behind the promontory and apex of it gives origin to stapedius mucle.
Lateral wall (Tympanic wall): The lateral wall contains the tympanic membrane and separates tympanic cavity from the external acoustic meatus
Contents of middle ear cavity
Multiple structures are contained within the confines of the tympanic cavity (middle ear). Muscles, nerves, and the auditory tube occupy space within the tympanic cavity. The cavity is covered in mucoperiosteum.
Ossicles
Incus (anvil)
Stapes (stirrup
These bony elements serve to transmit and amplify sound waves from the air to the perilymph of the internal ear. This bony conduction amplifies 10 times the sound wave from air.
Muscles
Tensor tympani: Tenses the tympanic membrane and supplied by mandibular nerve.
Stapedius: Prevents over-movement of spates and supplied by facial nerve.
Nerves:
Chorda tympani: Branch of facial nerve.
Tympanic branch of glossopharyngeal nerve
Tympanic plexus
Lesser petrosal nerve
Greater petrosal nerve.
Nerve supply:
Sensory information about the tympanic canal (middle ear) is carried by multiple nerves that contribute to the tympanic plexus located on the promontory of the medial wall.
Vascular supply
The arterial supply of the cavity includes the tympanic branch of the maxillary (tympanic membrane), stylomastoid branch of the posterior auricular (posterior cavity and mastoid), petrosal branch of the middle meningeal, branch of the ascending pharyngeal, tympanic branch of the internal carotid, and a branch from the artery of the pterygoid canal (follows the auditory tube). Venous drainage is to the pterygoid plexus and the superior petrosal sinus.
Applied Anatomy:
Infection of middle ear known as Otitis Media. It can occur due to blockage of auditory tube opening in the pharynx in upper respiratory infections in children.
Complications of Otitis media can lead to rupture of tympaninc membrane, spread of infection to brain and sigmoid sinus.
Vascular supply of middle ear cavity
- Tympanic branch of maxillary
- Stylomastoid branch of posterior auricular
- Petrosal branch of middle meningeal
- Branch of ascending pharyngeal
- Tympanic branch of internal carotid
- Branche from artery of pterygoid canal
Auditory tube
The auditory tube (Eustachian tube) is the channel through which the tympanic cavity
communicates with the nasal part of the pharynx.
Its length is about 36 mm., and its direction is downward, forward, and medialward, forming an angle of about 45 degrees with the sagittal plane and one of from 30 to 40 degrees with the horizontal plane.
Shape: Trumpet shaped.
It is divided in to two parts:
The osseous portion is about 12 mm. in length. It is formed partly of bone, partly of cartilage and fibrous tissue.
The cartilaginous portion, about 24 mm. in length, is formed of a triangular plate of elastic fibrocartilage, the apex of which is attached to the margin of the medial end of the osseous portion of the tube, while its base lies directly under the mucous membrane of the nasal part of the pharynx, where it forms an elevation, the tubal elevation.
Narrow region between osseous and cartilaginous part is known as Isthmus. Its function is to equalize pressure across the tympanic membrane.
Contraction of the tensor veli palatini and the salpingopharyngeus outside of the tympanic cavity (middle ear) dilate and open the auditory tube.
Applied anatomy: Upper respiratory tract infections in children can block the opening of auditory tube in the pharynx and lead to collection of fluid in middle ear and cause secondarily otitis media.
