[Position of semicircular canals in humans and cryptogenetic cupulolithiasis]

Semicircular canals in humans are three membranous tubes each forming two-thirds of a circle, arranged in a coordinate system. The horizontal semicircular canal makes a 30 degree angle with the horizontal plane, a phenomenon neither easily understood nor explained in Literature. This paper reports a personal opinion based on the theory of evolution. Before man assumed erect posture (Australopithecus Afarensis), more than 3 million years ago, the head was bent forward (as in other primates) so that the temporal bone (structures inside included) laid on the horizontal plane. In the Author's opinion the present alignment might have come about after the gradual change in posture in human evolution. The ampulla of the posterior canal and the non ampullary branch of the horizontal semicircular canal became the lowest part of the utricule thereby allowing otoconial debris to settle. On the basis of this hypothesis the Authors attempt to explain the high prevalence of idiopathic cupulolithiasis.