Fig. 35.1 Temporal bone
Left bone. The temporal bone consists of three major parts: squamous, petrous, and tympanic (see Fig. 35.2).
Fig. 35.2 Parts of the temporal bone
Structures in the temporal bone
The mastoid process contains mastoid air cells that communicate with the middle ear; the middle ear in turn communicates with the nasopharynx via the pharyngotympanic (auditory) tube. Bacteria may use this pathway to move from the nasopharynx into the middle ear. In severe cases, bacteria may pass from the mastoid air cells into the cranial cavity, causing meningitis.
The petrous portion of the temporal bone contains the middle and inner ear as well as the tympanic membrane. The bony semicircular canals are oriented at an approximately 45-degree angle from the coronal, transverse, and sagittal planes.
Irrigation of the auditory canal with warm (44°C) or cool (30°C) water can induce a thermal current in the endolymph of the semicircular canal, causing the patient to manifest vestibular nystagmus (jerky eye movements, vestibulo-ocular reflex). This caloric testing is important in the diagnosis of unexplained vertigo. The patient must be oriented so that the semicircular canal of interest lies in the vertical plane.
External Ear & Auditory Canal
The auditory apparatus is divided into three main parts: external, middle, and inner ear. The external and middle ear are part of the sound conduction apparatus, and the inner ear is the actual organ of hearing (see p. 619). The inner ear also contains the vestibular apparatus, the organ of balance (see p. 618).
Fig. 35.3 Ear: Overview
Coronal section through right ear, anterior view.
Curvature of the external auditory canal
The external auditory canal is most curved in its cartilaginous portion. When an otoscope is being inserted, the auricle should be pulled backward and upward so the speculum can Fig. 35.4 External auditory canal be introduced into a straightened canal.
Fig. 35.4 External auditory canal
Coronal section through right ear, anterior view. The tympanic membrane separates the external auditory canal from the tympanic cavity (middle ear). The outer third of the auditory canal is cartilaginous, and the inner two thirds are osseous (tympanic part of temporal bone).
Fig. 35.5 Structure of the auricle
The auricle of the ear encloses a cartilaginous framework that forms a funnel-shaped receptor for acoustic vibrations. The muscles of the auricle are considered muscles of facial expression, although they are vestigial in humans.
Fig. 35.6 Arteries of the auricle
Fig. 35.7 Innervation of the auricle
Middle Ear: Tympanic Cavity
Fig. 35.8 Middle ear
Right petrous bone, superior view. The tympanic cavity of the middle ear communicates anteriorly with the pharynx via the pharyngotympanic (auditory) tube and posteriorly with the mastoid air cells.
Fig. 35.9 Tympanic cavity and pharyngotympanic tube
Medial view of opened tympanic cavity.
Fig. 35.10 Tympanic cavity
Middle Ear: Ossicular Chain & Tympanic Membrane
Fig. 35.11 Auditory ossicles
Left ear. The ossicular chain consists of three small bones that establish an articular connection between the tympanic membrane and the oval window.
Fig. 35.12 Malleus (“hammer”)
Fig. 35.13 Incus (“anvil”)
Fig. 35.14 Stapes (“stirrup”)
Fig. 35.15 Tympanic membrane
Right tympanic membrane. The tympanic membrane is divided into four quadrants: anterosuperior (I), anteroinferior (II), posteroinferior (III), and posterosuperior (IV).
Fig. 35.16 Ossicular chain in the tympanic cavity
Lateral view of the right ear. Revealed: Ligaments of the ossicular chain and muscles of the middle ear (stapedius and tensor tympani).
Ossicular chain in hearing
Sound waves funneled into the external auditory canal set the tympanic membrane into vibration. The ossicular chain transmits the vibrations to the oval window, which communicates them to the fluid column of the inner ear. Sound waves in fluid meet with higher impedance; they must therefore be amplified in the middle ear. The difference in surface area between the tympanic membrane and the oval window increases the sound pressure 17-fold. A total amplification factor of 22 is achieved through the lever action of the ossicular chain. If the ossicular chain fails to transform the sound pressure between the tympanic membrane and the footplate of the stapes, the patient will experience conductive hearing loss of magnitude 20 dB. See p. 619 for hearing.
Arteries of the Middle Ear
Fig. 35.17 Arteries of the middle ear: Ossicular chain and tympanic membrane
Medial view of the right tympanic membrane. With inflammation, the arteries of the tympanic membrane may become so dilated that their course can be observed (as shown here).
Fig. 35.18 Arteries of the middle ear: Tympanic cavity
Right petrous bone, anterior view. Removed: Malleus, incus, portions of chorda tympani, and anterior tympanic artery.
The inner ear consists of the vestibular apparatus (for balance) and the auditory apparatus (for hearing). Both are formed by a membranous labyrinth filled with endolymph floating within bony labyrinth filled with perilymph and embedded in the petrous part of the temporal bone.
Fig. 35.19 Vestibular apparatus
Right lateral view.
Fig. 35.20 Auditory apparatus
The cochlear labyrinth and its bony shell form the cochlea, which contains the sensory epithelium of the auditory apparatus (organ of Corti).
Fig. 35.21 Innervation of the membranous labyrinth
Right ear, anterior view. The vestibulocochlear nerve (CN VIII; see p. 480) transmits afferent impulses from the inner ear to the brainstem through the internal acoustic meatus. The vestibulocochlear nerve is divided into the vestibular and cochlear nerves. Note: The sensory organs in the semicircular canals respond to angular acceleration, and the macular organs respond to horizontal and vertical linear acceleration.
Fig. 35.22 Blood vessels of the inner ear
Right anterior view. The labyrinth receives its blood supply from the internal auditory artery, a branch of the anteroinferior cerebellar artery (see p. 608).