Exercise 25.1

As you read and using your textbook as a guide, label Figure 25.1.

The ear is divided into three parts: the external ear, the middle ear, and the inner ear.

The external ear consists of two parts: the auricle (or pinna) and the external auditory canal. The auricle (or pinna) is a triangular shaped appendage consisting of skin and elastic cartilage attached to the lateral aspect of the head. The external auditory canal is a tube about 2.5 centimeters long that extends from the auricle to the tympanic membrane (eardrum).

The tympanic membrane serves to separate the external ear from the middle ear. The middle ear contains three tiny bones. The tympanic membrane is attached to the malleus (a hammer shaped bone), which is connected to the incus (an anvil shaped bone), which is connected to the stapes (a stirrup shaped bone). Beneath the stapes is the oval window. Covering the oval window is a thin membrane which serves to separate the perilymph of the inner ear from the middle ear. The perilymph is fluid that serves to conduct sound vibrations within the inner ear. The middle ear connects to the nasopharynx by way of the eustachian tube. One function of the eustachian tube is to equalize the pressure on the two sides of the tympanic membrane.

The inner ear consists of two parts: the bony labyrinth and the membranous labyrinth. The bony labyrinth is so named because it is within the temporal bone of the skull. The membranous labyrinth is so named because it is made up of membranous tissues; it fits within the bony labyrinth. Within the membranous labyrinth is a fluid called endolymph. The perilymph is located between the membranous labyrinth and the bony labyrinth. Both of these fluids are involved in sound vibration transmission and the maintenance of equilibrium.

The bony labyrinth is made up of three parts: the vestibule, the cochlea, and the semicircular canals. The vestibule is an enlarged bulb shaped structure that has the stapes attached to it. The cochlea is shaped like a snail's shell and is attached to the vestibule. There are three semicircular canals which connect to the vestibule. The semicircular canals lie at right angles to one another. The superior semicircular canal is oriented vertically and projects upward. The posterior semicircular canal is oriented vertically and projects toward the back of the head. The lateral semicircular canal is oriented horizontally and projects toward the lateral aspect of the head. Each semicircular canal has an enlargement called an ampulla. An ampulla connects the semicircular canal to the vestibule and contains an equilibrium sensory structure called the crista ampullaris, which functions in dynamic equilibrium (equilibrium during movement).

There are two nerve branches that attach to the bony labyrinth. The vestibular branch of the vestibulocochlear nerve Vm has sensory branches leading away from the ampulla and vestibule. The cochlear branch of the vestibulocochlear nerve Vm has sensory branches leading away from the cochlea.

The cochlea is divided into three chambers. The uppermost is called the scala vestibuli. The scala vestibuli is continuous with the vestibule and contains perilymph. The middle chamber is called the cochlear duct and it contains endolymph. The lower chamber is called the scala tympani. It, like the scala vestibuli, contains perilymph. At the stapedial end of the scala tympani is a round, membrane-covered opening called the round window.

The scala vestibuli is separated from the cochlear duct by a thin membrane called the vestibular membrane. The cochlear duct is separated from the scala tympani by the basilar membrane. Resting on the basilar membrane is the organ of Corti. The organ of Corti is the organ of hearing since it contains receptor cells. These receptor cells are called hair cells. The axons of these sensory hair cells form the cochlear nerve fibers. Covering the organ of Corti is a gelatinous like structure called the tectorial membrane.

Ear Round Window Covering
Figure 25.1 Anatomy of the Ear

Ampullae of semicircular canals Auricle (Pinna) Cochlea Cochlear branch of vestibulocochlear nerve VIII Eustachian tube External auditory canal External ear Incus Inner ear Lateral semicircular canal Malleus Middle ear Oval window Posterior semicircular canal Round window Stapes Superior semicircular canal Tympanic membrane Vestibular branch of vestibulocochlear nerve VIII Vestibule

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