Every motion of the oval base of the stapes causes a wave to pass along the liquid in the labyrinth. The bony case of the internal ear being firm, the wave travels through all parts of the internal ear. Through the cochlea it arrives at the inner tympanic membrane which closes the fenestra rotunda, and separates the cavity of the tympanum from the scala tympani of the cochlea. The waves have a very complex route in passing from the fenestra ova/is closed by the stapes to the membrane closing the cochlea. By means of the liquid lying around the membranous labyrinth - perilymph - the waves pass up the vestibular spiral of the cochlea, and arriving at its summit, they descend by the tympanic spiral to the fenestra rotunda. In this course they pass over and under the fluid - endolymph - contained in the membranous canal of the cochlea in which the special nerve terminations are placed.
Fig.238. Diagram of the membranous labyrinth, all of which is filled with endolymph and surrounded by perilymph, a, b, c, semicircular canals opening into the ventricle d; e, the saccule from which the uniting canal,f, leads into the membranous canal of the cochlea, g. (Cleland).
For the construction of the labyrinth the student is referred to the text-books of anatomy, as space only admits of a brief account of the special arrangements of the nerve ending.