Out of 30 adult human craniums and with a portable tomograph (vestibulograph) allowing 5 degrees of liberty (2 rotations and 3 translocations), a "medio-labyrinthic" plane was determined from the two "vestibions" put in the main X rays axis. Under those requirements, the lateral ductus becomes constantly visible on the cranian teleradiographies and the founded horizontal directions are identical either for the left than for the right (Student t's test always shows non significant differences).