The internal ear is that portion of the organ of hearing which perceives the impression of sound, and transmits it directly to the brain. It is hollowed out in the petrous bone, and is divided naturally into three distinct compartments, named the vestibule, the semicircular canals, and the cochlea or snail-shell. These divisions form together one of the most complex and delicate pieces of mechanism in the human body.

The labyrinth is composed of a bony cavity which incloses a membranous cavity in a portion of its space, and from this circumstance arises the distinction made by anatomists between the osseous and membranous labyrinths. We shall first consider the osseous labyrinth.

The vestibule is an ovoid cavity placed in the centre of the internal ear, between the semicircular canals and the cochlea. It communicates with the drum by the fenestra ovalis, which is closed by the base of the stapes. In it are seen the openings of the five semicircular canals, of the vestibular stair of the cochlea, and of the vestibular canal. This latter is the opening of a vascular canal which traverses the petrous bone.

Semicircular Canals

This is the name given to three curved tubes forming axes of circles, one which is horizontal and is placed between the two others, which are vertical. They are each enlarged into a bulbous cavity (ampulla) at one extremity, and communicate with the vestibule by five orifices.