Balance and equilibrium, I: The vestibule and semicircular canals.

In this, our second temporal bone installment, we will emphasize the vestibular portion of the labyrinth, that relating to balance and equilibrium. Before proceeding, we must again remind the reader of the basic structure of the labyrinth: an inner membranous labyrinth (endolymphatic) surrounded by an outer osseous labyrinth with an interposed supportive perilymphatic labyrinth. We recommend perusal of the first installment before continuing if there are any uncertainties in this regard. The vestibule, the largest labyrinthine cavity, measures 4 to 6 mm maximal diameter (1–3) (Figs 1–3). The medial wall of the vestibule is unique in that it contains two distinct depressions (Fig 4). Posterosuperiorly lies the elliptical recess, where the utricle is anchored. The smaller spherical recess lies anteroinferiorly and contains the saccule. Between these two concavities lies the vestibular crest, a ridge that divides posteriorly into two limbs bounding an additional small depression, the cochlear recess, which is the most proximal portion of the cochlear apparatus and leads to the scala vestibuli. This area will be discussed in greater detail in subsequent installments. The utricle is firmly attached within the elliptical recess by connective tissue and filaments of the utricular branch of the superior vestibular nerve (4) (Fig 5). The saccule is ovoid and smaller than the utricle. It is adherent to the spherical recess by fibrous tissue and the saccular branch of the inferior vestibular nerve. The utricle and saccule communicate with each other via the utriculosaccular duct. The utricle also communicates with the semicircular canals and endolymphatic sinus (via the utricular duct). Additional saccular communications include the endolymphatic sinus (via the saccular duct) and cochlea (via the ductus reuniens).