a. Tympanic Membrane. At the inner end of the external auditory meatus is the tympanic membrane. The tympanic membrane (eardrum) is a circular membrane separating the external auditory meatus from the middle ear cavity. The tympanic membrane vibrates (mechanically oscillates) in response to airborne sound waves.
b. Middle Ear Cavity. On the medial side of the tympanic membrane is the middle ear cavity. The middle ear cavity is a space within the temporal bone.
c. Auditory Ossicles. The auditory ossicles (OSSICLE = small bone) are three very small bones which form a chain across the middle ear cavity. They join the tympanic membrane with the medial wall of the middle ear cavity. In order, the ossicles are named as follows: malleus, incus, and stapes. The malleus is attached to the tympanic membrane. A sound stimulus is transmitted from the tympanic membrane to the medial wall of the middle ear cavity by way of the ossicles. The ossicles vibrate (mechanically oscillate) in response to the sound stimulus.
d. Auditory (Eustachian) Tube. The auditory tube is a passage connecting the middle ear cavity with the nasopharynx. The auditory tube maintains equal air pressure on the two sides of the tympanic membrane.
e. Association With Other Spaces. The middle ear cavity is associated with other spaces in the skull. The thin roof of the middle ear cavity is the floor of part of the cranial cavity. The middle ear cavity is continuous posteriorly with the mastoid air cells via the antrum (an upper posterior recess of the middle ear cavity).
11-33. THE INTERNAL EAR
a. Labyrinths (Figure 11-13).
a. Labyrinths (Figure 11-13).
(1) Bony labyrinth. The bony labyrinth (LABYRINTH = a maze) is a complex cavity within the temporal bone. It has three semicircular canals, a vestibule (hallway), and a snail-shaped cochlear portion.
(2) Membranous labyrinth. The membranous labyrinth is a hollow tubular structure suspended within the bony labyrinth.
b. Fluids of the Internal Ear. The endolymph is a fluid filling the space within the membranous labyrinth. The perilymph is a fluid filling the space between the membranous labyrinth and the bony labyrinth. These fluids are continuously formed and drained away.
ENDO = within
PERI = around c. The Cochlea. The cochlea is a spiral structure associated with hearing. It has 2-1/2 turns. Its outer boundaries are formed by the snail- shaped portion of the bony labyrinth.
(1) The central column or axis of the cochlea is called the modiolus. Extending from this central column is a spiral shelf of bone called the spiral lamina. A fibrous membrane called the basilar membrane (or basilar lamina) connects the spiral lamina with the outer bony wall of the cochlea. The basilar membrane forms the floor of the cochlear duct, the spiral portion of the membranous labyrinth. Within the cochlear duct, there is a structure on the basilar membrane called the organ of Corti. The organ of Corti has hairs which are the sensory receptors for the special sense of hearing.
LAMINA = thin plate
(2) Within the bony cochlea, the space above the cochlear duct is known as the scala vestibuli and the space below is known as the scala tympani. (See figure 1114.) Since the scalae are joined at their apex, they form a continuous channel and the connection between them is called the helicotrema.
(3) Between the scalae and the middle ear cavity are two windows.
(a) Fenestra vestibuli (oval window). Between the middle ear cavity and the scala vestibuli is an oval window called the fenestra vestibuli. It is filled with the foot plate of the stapes.
(b) Fenestra cochleae (round window). Between the middle ear cavity and the scala tympani is a round window called the fenestra cochleae. It is covered or closed by a membrane.
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