Nasal Cavity, Sinuses and Ear - LO 1
1. What are the three divisions of the ear? Describe the boundaries, contents, and functions of each part.
The ear is comprised of three parts: the external, middle, and internal ears. Most of the ear (with the exception of most of the external ear) occupy space within the temporal bone.
External ear: The external ear may be subdivided into the auricle (pinna) and the external acoustic meatus (EAM). The external ear gathers and modifies sounds, and then transmits sounds to the tympanic membrane (eardrum) via the external acoustic meatus. The external ear is largely a cartilaginous sound receptacle, capable of modifying sound by means of its shape, which can be altered by a series of extrinsic and intrinsic muscles controlled by the facial n. The EAM extends approximately 2.5 cm from the concha of the auricle to the tympanic membrane. The proximal third of the EAM is cartilaginous, and the remaining two-thirds is osseous. The tympanic membrane separates the external meatus from the tympanic cavity. Its fibrous basis is attached to the tympanic plate of the temporal bone and is covered laterally by epidermis and medially by the mucous membrane of the middle ear. The larger portion of the membrane is its tense part; the anterosuperior corner, or flaccid part, is bounded by anterior and posterior mallear folds. The membrane can be examined easily in vivo by an otoscope speculum.
Middle ear: The middle ear is mucosa-lined space within the petrous part of the temporal bone. The middle ear conducts sound energy from the tympanic membrane via three ossicles (malleus, incus, & stapes) to the cochlea of the inner ear. The tympanic cavity of the middle ear is conceptualized to have a roof & floor, medial & lateral walls, and anterior & posterior walls. Each wall and surface is associated with particular openings, ridges, prominences, muscle attachments, etc.
Roof (tegmen tympani):
canal for tensor tympani m.
atticus -space that contains portions of the malleus and incus
Floor (jugular wall):
tympanic canaliculus
accessory mastoid air cells
Medial wall (superior-to-inferior):
prominence of lateral semicircular canal (shared with posterior wall)
facial prominence (shared with posterior wall) - bone outer border for facial canal
oval window (fenestra vestibuli) - communication to vestibule of inner ear; receives footplate of stapes
promontory & tympanic plexus
round window (fenestra rotundum) - communication to cochlea; covered by a secondary tympanic membrane, which allows for cochlear fluid expansion
Posterior wall:
entrance (aditus) to mastoid antrum - communication with the mastoid air cells
prominence of lateral semicircular canal (shared with medial wall)
facial prominence (shared with medial wall) - bone outer border for facial canal
pyramidal eminence - contains the stapedius m. & transmits the tendon of stapedius m. anteriorly to the neck of the stapes
opening for chorda tympani - from facial canal
Lateral wall:
tympanic membrane with manubrium of malleus attached - tensor tympani m. attached to manubrium of malleus
chorda tympani - surrounded by malleus (superolaterally), incus (superomedially), and tensor tympani m. (inferiorly)
Anterior wall:
canal for tensor tympani m. - contains tensor tympani m.; tendon transmitted to manubrium of malleus
opening to pharyngotympanic (auditory) tube - communication with nasopharynx
also the posterior wall of carotid canal
Inner ear: The inner ear consists of a bony apparatus lined by a series of membrane-lined bony passages which facilitate hearing and balance. The osseous tissue of the bony apparatus is more dense than that of the surrounding petrous portion of the temporal bone.
Vestibule - The central portion of the bony apparatus is the vestibule, which communicates with the middle ear via the oval window. Contains vestibular sacs (utricle, saccule, etc.), which detect linear acceleration of the head to promote balance
Cochlea - the anteromedial portion of the bony apparatus; shaped like a snail shell (cochlos = snail in Greek); facilitates hearing
Semicircular canals (anterior, posterior, lateral) - detect angular acceleration of the head to promote balance
