[Developmental studies on the petrous part of the human temporal bone--special references to the morphogenesis of the facial nerve canal].

Development and formation of the petrous bone was examined in total of 343 Japanese skulls. The materials used consisted of 310 skulls of Japanese fetuses ranging from the fourth to tenth month, 19 skulls of Japanese juveniles from the third month to 7 years of age, and 20 temporal bones obtained from 14 adult cadavers. A total of six group of ossification centers appear in the petrous part during 5th fetal month, and they form the petrous bone at 6th fetal month. The firstly-appeared ossification center is just above the round window, and the second is on the ampulla of anterior semicircular canal. Other ossifications are observed between the cochlea and semicircular canals, on the brim of internal acoustic porus, on the superior surface of the petrous apex, and on the summit of posterior semicircular canal. The ossification of the facial canal starts at 6th fetal month, though the geniculate part and tympanic part do not complete until one year old after birth. Even in adults, the facial canal dehiscence are found at more than 10% of cases, mainly locating in the tympanic part. On the basis of these results, formation of the petrous bone including facial canal and other bony structures was discussed from the viewpoints of the ossification and pre- and postnatal middle ear development.