Cochlea and Organ of Corti.

The shape of the cochlea resembles that of a snail shell.

It is a passageway in the bone filled with fluid and having the form of a spiral with two and one-half turns.

    Inside the cochlea, a bony shelf projects from one wall in such a way as to divide the cochlea lengthwise into three parallel compartments.

      The outer two, called the vestibular canal and the tympanic canal, connect with each other in the final turn, but the central one, called the cochlear canal, is closed at one end and connects with the sacculus at the other.

      The cochlear canal is filled with endolymph, the vestibular and the tympanic canals with the perilymph.

     The vestibular canal arises from a rounded cavity, the vestibule, at the base of the snail-like coil. It touches at one end against the inner wall of the air-filled middle-ear cavity at an opening known as the oval window (fenestra vestibuli).

      The tympanic canal communicates with the middle ear by an opening known as the round window (fenestra cochleae).

These windows do not allow free passage of fluid.

      The footplate of the stapes closes the oval window, and the round window is closed from the middle ear by a thin membrane.

     The membrane separating the cochlear canal from the tympanic canal is called the basilar membrane and resembles a miniature stringed instrument.

      It contains a row of parallel filaments that are stretched across the spiral canal and arranged progressively according to the thickness and length like the strings of a tiny harp.

      On this membrane are rows of cells, known collectively as the organ of Corti, which appear to have the highly specialized function of converting the vibrations of the basilar membrane to nerve impulses.