The distribution and extent of the cellular system throughout the temporal bone vary greatly, both in infants and in adults. In discussing this problem, the bone as a whole must be considered and not merely the mastoid process.
Anatomically, as is well known, the temporal bone of the fetus consists of three parts, namely, the squamous, the petrous and the tympanic portions. This division still holds at birth or until the squamous and tympanic parts become fused into one. The tympanic ring in the new-born is a flat semicircle which is open above. As there is no meatus at this stage of development, the drum membrane, which fits into a groove in the tympanic ring, lies quite unprotected.In the new-born the mastoid process does not exist. The stylomastoid foramen lies on the lateral surface of what is later to be the prominence of the mastoid. The styloid process
TREMBLE GE. PNEUMATIZATION OF THE TEMPORAL BONE. Arch Otolaryngol. 1934;19(2):172–182. doi:10.1001/archotol.1934.03790020018002
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