The posterior semicircular canal is a part of the vestibular system

Vestibular system

The vestibular system, which contributes to balance in most mammals and to the sense of spatial orientation, is the sensory system that provides the leading contribution about movement and sense of balance. Together with the cochlea, a part of the auditory system, it constitutes the labyrinth of...

 and detects rotations of the head in the sagittal plane.

Structure

It is directed superiorly, as per its nomenclature, and posteriorly, nearly parallel to the posterior surface of the petrous bone. The vestibular canaliculus is immediately medial to it.

It is part of the bony labyrinth

Vestibule of the ear

-Definition:The vestibule is the central part of the osseous labyrinth, and is situated medial to the tympanic cavity, behind the cochlea, and in front of the semicircular canals.The etymology comes from the Latin vestibulum, literally an entrance hall....

 and is used by the vestibular system

Vestibular system

The vestibular system, which contributes to balance in most mammals and to the sense of spatial orientation, is the sensory system that provides the leading contribution about movement and sense of balance. Together with the cochlea, a part of the auditory system, it constitutes the labyrinth of...

 to detect rotations of the head in the sagittal plane.

It is the longest of the three canals, measuring from 18 to 22 mm.

Its lower or ampullated end opens into the lower and back part of the vestibule, its upper into the crus commune.