VAN 121: Veterinary Gross Anatomy-II (2+2)

MEMBRANOUS LABYRINTH

• This consists of a series of interconnected sacs and ducts lying within the osseous labyrinth. The membranous labyrinth has nearly the same general form as the osseous part but does not fill it completely, being considerably smaller in diameter. It is separated from the wall of bony labyrinth and is surrounded by perilymph but is anchored by connective tissue trabeculae extending from the membranous part to the bony wall at certain points. The various parts of the membranous labyrinth are interconnected and form a closed system containing a fluid known as endolymph.
• The membranous labyrinth consists of four parts utricle, saccule, three semicircular ducts and cochlear duct. .
• 1. The utricle is the larger of the two sacs and lies in the superoposterior part of the vestibule, in contact with the elliptical recess. It receives the openings of the semicircular ducts and opens anteromedially into a small ductus utriculosaccularis. The lateral wall of the utricle shows a thickened area the macula utriculi, which is lined by neuroepithelium.
• 2. The saccule is the smaller spherical sac located in front of the utricle, occupying the spherical recess of the vestibule. The anterior wall of the saccule shows a thickening, the macula sacculi, which is lined by neuroepithelium. View animation... 
• 3. The semicircular ducts correspond to the osseous semicircular canals and fill up only one fourth of the lumen of the bony canals except at the ampullae, where they fill up the entire canal. In each of the ampulla the wall is thickened and projects as transverse elevation, the crista ampullaris containing neuroepithelium.
• 4. Cochlear duct (Scala media): The bony cochlea is incompletely divided by osseous spiral lamina into an upper scala vestibuli and lower scala tympani. A membrana basilaris extends from the osseous spiral lamina to the lateral wall of bony cochlea to complete this division. The lateral wall is lined by thick fibrous tissue called the spiral ligament. Another membrane, the vestibular or Resissner’s membrane extends from above the osseous spiral lamina, outwards and upwards to the lateral wall of the cochlea. A canal is thus formed which is triangular in section between the vestibular membrane above; spiral ligament laterally and the membrana basilaris below. This is termed as the cochlear duct. Its origin in the basal coil of the cochlea is blind but immediately after its origin it is connected to the saccule by the ductus reunions. The cochlear duct traverses through the bony cochlea and terminates blindly at the cupola, to which it is attached. The cochlear duct contains endolymph.

• The scala vestibuli contains perilymph and opens into the vestibuli and is in close relation to the oval window or fenestra ovalis.
• The scala tympani is in communication with scala vestibule at the apex of the cochlea through a small canal, the helicotremma but at the base, it ends against the round window or fenestra rotundum, which is closed by the secondary tympanic membrane. Extending from the scala tympani is the perilymphatic duct that passes through the aquaeductus cochleae and opens at the ventral margin of petrous temporal bone into the subarachnoid space in the cranial cavity.
• The auditory or eighth cranial nerve divides at the bottom of the internal auditory meatus into cochlear and vestibular divisions. The vestibular nerve close to the meatus presents the vestibular ganglion. The peripheral processes of the bipolar cells in the ganglion are distributed to the macula of the utricle and crista ampullaris of the semicircular ducts. A separate branch ends on the macula sacculi. The cochlear nerve passes though the axial canal in the modiolus and fibres end in a series of bipolar cells in the spiral canal in the osseous spiral lamina, which form the spiral ganglion. The peripheral processes of the cells in the spiral ganglion pass through openings on the tympanic lip of osseous spiral lamina and end on the hair cells of the organ of corti on the membrana basilaris.