The vestibular portion of the membranous labyrinth consists of three pairs of semicircular canals, the horizontal, anterior, and posterior, and two pairs of otolith organs, the utricle and the saccule (Fig. 1). Like the cochlea, these structures are filled with endolymph, which communicates with the cochlea through the ductus reuniens. The horizontal, anterior, and posterior semicircular canals are oriented nearly at right angles to one another, and the result is the representation of the three rotational dimensions of space. When the head is tilted approx 25° forward, the horizontal canal is in the horizontal plane, and the anterior and posterior canals, also known as the vertical canals, are in vertical planes. As for the otolith organs, the utricle is oriented almost horizontally, and the saccule is oriented almost vertically in the sagittal plane. The receptors for the vestibular system are hair cells that are morphologically similar to those in the organ of Corti in the cochlea. These receptors are located in the cristae ampullares of the semicircular canals and in the maculae of the utricle and saccule.