sacculus

Sacculus is a term used in biology to denote a small sac or pouch. In anatomy, the best-known use is the saccule of the inner ear, a vestibular organ involved in sensing body position and movement.

The saccule is a membranous sac located in the vestibule of the inner ear, near the utricle. It is connected to the cochlear duct by the ductus reuniens and forms part of the vestibular apparatus, which includes the utricle and the semicircular canals. The inner surface of the saccule bears the macula sacculi, a sensory epithelium containing hair cells supported by supporting cells.

The macula sacculi bears otoliths embedded in a gelatinous layer. When the head undergoes linear acceleration or changes in gravity, the otoliths shift and bend the stereocilia and kinocilium of the hair cells. This mechanical deflection generates neural signals that travel via the vestibular nerve to brain centers, contributing to balance, spatial orientation, and gaze stabilization.

Beyond the inner-ear context, sacculus is a generic term used for other small sacs in various organisms, including invertebrates, where it may refer to analogous fluid-filled chambers. The name derives from the Latin sacculus, meaning “little bag.”

See also: saccule, utricle, vestibular system, otolith.