The membranous labyrinth is a collection of fluid-filled tubes and chambers that contain receptors for the senses of balance and hearing. It is located in the bony labyrinth of the inner ear and has the same general shape; However, it is much smaller and partially separated from the bony walls by a lot of fluid, the perilymph. Since the membranous labyrinth is smaller than the bony labyrinth, the two are separated by the perilymph, which does not communicate with the endolymph contained in the membranous labyrinth. The membranous labyrinth or endolymphatic labyrinth is the part of the inner ear that is housed in the bony labyrinth. It consists of 3 different but connected sensory sacs and channels fed by the vestibulocochlear nerve (CN VIII): However, this hypothetical venous network of mammalian stems has a structural difference from the venous network of recent therians. In recent therians, the collecting venules of the scala tympani and the capillaries of the scala media are embedded in the soft tissues of the membranous labyrinth of the bone cochlear duct. But the mesh-shaped venous network in mammals is partially intramural and the small veins are located in the bone of the pars cochlearis. In addition, there is a phylogenetic difference – this network of mesh stem mammals, as seen in docodonts and morganucodons, is best developed in the central part of the promontory; from there, the network is connected to a hypocochlear venous plexus via the cochlear pars to the inferior petrosus sinus (Panciroli et al., 2018). In comparison, the net-like part of this mammalian stem network is reduced or completely lost in eutriconodonts and stemmed mammals, while the hypocochlear plexus is still present in Mesozoic crowned mammals. This difference suggests that parts of the vascular network to feed the cochlea in mammals are modified, its drainage network is shifted backwards to be associated with the canal for Aqueductus cochleae, all of which are traits derived from therians (Harper and Rougier, 2019). The membranous vestibular labyrinth of each inner ear consists of three sacs inside the vestibule: the utricle, sacculus, endolymphatic canal, and sac, and three semicircular canals contained in the bony ducts (Figure 35.1).
The utricle has an elongated shape and is located in the posterior upper part of the vestibule, near the ampullae of the upper and horizontal canal. The saccule is more circular and is located deeper and anteriorly in the bony vestibule, closer to the cochlea. The saccule is connected to the membranous duct of the cochlea by a thin ductus called ductus reuniens. From the utricle and saccule arise small lines, the utricular or sac-shaped canal, which merge into the endolymphatic canal. This duct ends in a blind endolymphatic sac located between the periosteum of the petrous bone and the dura mater. The endolymphatic duct and sac are extremely important for a variety of regulatory, homeostatic and protective functions related to endolymph. In humans, the diameter of the membranous semicircular canals is about 25% of the diameter of the bone canals, but otherwise the membranes take on the shape and configuration of these bone canals, including ampulla, and long, thin canals. The ampullary wall of each of the canals is thickened and protrudes like a transverse ridge or ridge in the canal called crista ampullaris. Each crista ampullaris consists of a sensory epithelium – sensory hair cells and supporting cells – which are supported by the connective tissue comb.
A strong gelling mass called a cup fills the bulb, sits immediately above the Crista and extends to the roof of the bulb. The hair bundles of the receptor cells protrude into the cup. The crescent-shaped side walls of the bulb on both sides of the crista are called plana semilunata. The areas on the canalicular and utricular side of the Crista and the homologous regions of the otolithic organs are lined with dark cells; specialized epithelial cells that are important for endolymphatosis and otoconial metabolism. The space inside the membranous labyrinth is filled with endolymphatic fluid. In contrast, the outside of the membranous labyrinth is filled with perilymph, which separates the membranous labyrinth from the walls of the bony labyrinth. This space, which separates the two labyrinths, is called the perilymphatic space. In some places, the structures of the membranous labyrinth are attached to the walls of the bone capsule. An accidental injury in or through the bony and membranous labyrinth of the inner ear during wart surgery can lead to hearing loss and vestibular symptoms. However, in translabyrinthine and transcochlear approaches, the labyrinth is intentionally erased to access the cerebellious angle and internal auditory canal, with the expected sacrifice of auditory function and balance.
In an approach such as the infratemporal fossa approach, a subtotal petrosectomy is performed, which involves a complete mastoidectomy with labyrinth preservation. In other approaches to the petrous tip by the mastoid or transchannel route, care is taken not to violate the basal rotation of the cochlea. The bony labyrinth is a series of bony cavities in the petrose part of the temporal bone. It consists of three parts – the cochlea, the vestibule and the three semicircular canals. Since the membranous labyrinth is a closed system, issues of endolymph flow and ablation are also important. Endolymph is thought to be absorbed through the endolymphatic sac, although this seems to be only part of the story. Other cochlear and vestibular tissues may. Accidental penetration into the labyrinth should be repaired by immediately placing a fascia on the defect.
Bone wax or bone dust block can be placed on the fascia to secure its position. Care is taken not to suck directly into the fistula site to preserve labyrinthine fluids. The bony labyrinth is located in the middle of the oil wall and can be divided into anterior and posterior labyrinth. The anterior labyrinth is the organ of hearing, which consists of the cochlea and cochlear canal.