Crus commune is a part of:
Cochlea
Middle ear
Semicircular canal
Vestibule
(c) Semicircular canal(Ref Shambaugh, 6th ed., page 42)The posterior and superior semicircular canals fuse together to form a common crus called as "crus commune" which opens into the utricle. Because of this the three semicircular canals open into utricle by five openings instead of six.
Ear