The vestibule is a region of the inner ear which contains the saccule and the utricle, each of which contain a macula to detect linear acceleration.
A gelatinous cover called the otolithic membrane envelops the tips of the stereocilia and kinocilium.
The otolithic membrane is weighted with small densely packed protein-calcium carbonate granules called statoconica.
In vertical linear acceleration, the weighted otolithic membrane lags behind the stereocilia and kinocilium.
[1][2][3] This article incorporates text in the public domain from page 1052 of the 20th edition of Gray's Anatomy (1918)