Crus commune is a pa of:

Semicircular CanalsThere are 3 semicircular canals - the lateral, posterior and superior which lie in a plane of right angles to one anotherEach canal has an ampullated end which opens independently into the vestibule and a non ampullated endThe non-ampullated ends of posterior and superior canals unite to form a common channel called the crus commune.So the three canals open into the vestibule by 5 openings.