Semicircular canals

Three bony semicircular canals (anterior or superior, posterior and lateral). They lie posterosuperior to the vestibule, and are set at right angles to each other. Each canal describes two-thirds of a circle, and is dilated at one end to form the ampulla. The anterior semicircular canal (canalis semicircularis anterior) is directed vertically at right angles to the axis of the pyramid.

The posterior semicircular canal (canalis semicircularis posterior) is situated nearly parallel to the posterior surface of the pyramid.

The lateral canal (canalis semicircularis lateralis) lies horisontali, protruding toward the tympanic cavity.

Three canals open into the vestibule by 5 openings, because the ends of the anterior and posterior canals join to form a common limb termed the crus commune.

2. The membranous labyrinth (labyrinth membranaceus) lies inside the bony labyrinth and repeats its configurations more or less exactly.

-Its walls are formed of a thin semitransparent connective-tissue membrane.

-Parts of the epithelium of the membranous labyrinth are specialized to form receptors for sound (organ of Corti), for static balance (maculae), and for linetic balance (cristae).

-This labyrinth is filled with a transparent fluid called the endolymph.

-The perilymphatic space (spatium perilymphaticum) is the space between the walls of two labyrinths. Its filled with perilymph.

Like the bony labyrinth, the membranous labyrinth also consists of 3 parts:

(1). – the spiral duct of the cochlea (organ of hearing);

(2). – the utricle and saccule (organs of static balance), within the vestibule;

(3). – the semicircular ducts (organs of kinetic balance), posteriorly.

Saccule and utricle

The saccule lies in the anteroinferior part of the vestibule, and is connected to the basal turn of the cochlear duct by the ductus reunions.

The utricle lies in the posterosuperior part of the vestibule. It has a shape of a closed tube and occupies the elliptic recess of the vestibule and communicates posteriorly with 3 membranous semicircular ducts (ductus semicircularis) which lie in the same kind of bony canals exactly repeating their shape.

The duct of the saccule unites with the duct of the utricle to form the ductus endolymphaticus (ductus endolymphaticus).

The ductus endolymphaticus ends in a dilatation the endolymphatic sac (saccus endolymphalicus).

The ductus and saccus occupy the aquaeduct of the vestibule.

The medial walls of the saccule and utricle are thickened to form a macula in each chamber. The maculae are end organs that dive information about the position of the head. They are static balance receptors. They are supplied by peripheral processes of neurons in the vestibular ganglion.

Duct of the cochlea

The duct of the cochlea (ductus cochlearis) occupies the middle part of the cochlear canal between the scala vestibule and the scala tympani.

It is triangular in cross section. The floor is formed by the basilar membrane; the roof, by the vestibular (Reissner’s) membrane, and the outer wall by the bony wall of the cochlea.

The basilar membrane supports the spiral organ of Corti which is the end organ for hearing.

The organ of Corti is innervated by peripheral processes of bipolar cells located in the spiral ganglion which is located in the spiral canal. The central processes of the ganglion cells form the cochlear nerve.

The sound waves reaching the endolymph through the vestibular membrane make appropriate parts of the basilar membrane vibrate, so that different parts of the organ of Corti are stimulated by different frequencies of sound. The loudness of the sound depens on the the amplitude of the vibration.