Figure 246

Photograph of a cast of the bony labyrinth of the internal ear. The cochlear portion of the bony labyrinth appears blue-green; the vestibule and semicircular canals appear orange-red. (Courtesy of Dr. Merle Lawrence.)

The vestibule is the central space that contains the utricle and saccule of the membranous labyrinth

The vestibule is the small oval chamber located in the center of the bony labyrinth. The utricle and saccule of the membranous labyrinth lie in elliptical and spherical recesses, respectively. The semicircular canals extend from the vestibule posteriorly, and the cochlea extends from the vestibule anteriorly. The oval window into which the footplate of the stapes inserts lies in the lateral wall of the vestibule.

The semicircular canals are tubes within the temporal bone that lie at right angles to each other

Three semicircular canals, each forming about three quarters of a circle, extend from the wall of the vestibule and return to it. The semicircular canals are identified as anterior, posterior, and lateral and lie within the temporal bone at approximately right angles to each other. They occupy three planes in space, sagittal, frontal, and horizontal. The end of each semicircular canal closest to the vestibule is expanded to form the ampulla (Fig. 24.7a). The three canals open into the vestibule through five orifices; the anterior and posterior semicircular canals join at one end to form the common bony limb (Fig. 24.7a).

The cochlea is a cone-shaped helix connected to the vestibule

The lumen of the cochlea, like that of the semicircular canals, is continuous with that of the vestibule. It connects to the vestibule on the side opposite the semicircular canals. Between its base and the apex, the cochlea elliptical recess for utricle spherical recess for saccule cochlea

anterior l posterior I lateral canals semi-posterior \ circular common bony limb g vestibule round window oval window anterior l posterior I lateral canals semi-posterior \ circular vestibule round window oval window common bony limb g cochlea elliptical recess for utricle spherical recess for saccule endolymphatic duct

semi-

posterior circular lateral j ducts endolymphatic sac k ductus reuniens endolymphatic duct cochlear duct ductus reuniens anterior } ___;

semi-

posterior circular lateral j ducts endolymphatic sac k makes about 23/4 turns around a central core of spongy bone called the modiolus. A sensory ganglion, the spiral ganglion, lies in the modiolus. One opening of the canal, the round window on its inferior surface near the base, is covered by a thin membrane (the secondaiy tympanic membrane).

macula of saccule

macula of saccule

crista ampullaris of posterior semicircular duct crista ampullaris of anterior semicircular duct crista ampullaris of lateral semicircular duct organ of ¡ Corti ¡

cochlear !

crista ampullaris of posterior semicircular duct vestibular .

labyrinth labyrinth FIGURE 24.7

Diagrams of the human internal ear. a. This lateral view of the left bony labyrinth shows its divisions: the vestibule, cochlea, and three semicircular canals. The openings of the oval window and the round window can be observed, b. Diagram of the membranous labyrinth of the internal ear lying within the bony labyrinth. The cochlear duct can be seen spiraling within the bony cochlea. The saccule and utricle are positioned within the vestibule, and the three semicircular ducts are lying within their respective canals. This view of the left membranous labyrinth allows the endolymphatic duct and sac to be observed, c. This view of the left membranous labyrinth shows the sensory regions of the internal ear for equilibrium and hearing. These regions are the macula of the saccule and macula of the utricle, the cristae ampullaris of the three semicircular ducts, and the spiral organ of Corti of the cochlear duct.

The membranous labyrinth contains the endolymph and is suspended within the bony labyrinth

The membranous labyrinth consists of series of communicating sacs and ducts containing endolymph. It is suspended within the bony labyrinth (Fig. 24.7b), and the remaining space is filled with perilymph. The membranous labyrinth is composed of two divisions: the cochlear labyrinth and the vestibular labyrinth (Fig. 24.7c). The vestibular labyrinth contains

• Three semicircular ducts, which lie within the semicircular canals and are continuous with the utricle

• Utricle and saccule, which are contained in recesses in the vestibule and are connected by the membranous utriculosaccular duct

The cochlear labyrinth contains the cochlear duct, which is contained within the cochlea and is continuous with the saccule (see Fig. 24.7b and c).

Specialized sensory cells are located in six regions in the membranous labyrinth

Six regions of sensory receptors project from the wall of the membranous labyrinth into the endolymphatic space in each internal ear (see Fig. 24.7c):

• Three cristae ampullaris (ampullary crest) located in the membranous ampullae of the semicircular ducts. They are sensitive to angular acceleration of the head (i.e., turning the head).

• Two maculae, one in the utricle (macula of utricle) and the other in the saccule (macula of saccule). They sense the position of the head and its linear movement.

• The spiral organ of Corti, which projects into the endolymph of the cochlear duct. It functions as the sound receptor.

Hair cells are epithelial mechanoreceptors of the vestibular and cochlear labyrinth

The hair cells of the vestibular and cochlear labyrinths function as transducers; i.e., they convert mechanical energy into electrical energy that is then transmitted via the vestibulocochlear nerve to the brain. They possess numerous stereocilia, actually modified microvilli, called sensoiy hairs (Fig. 24.8). In the vestibular system, each hair cell possesses a single true cilium called a kinocil-

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