This ossifies from four or more centres that appear in the fifth month and are indistinguishable as separate centres in the sixth month. These are :-
(1) Opisthotic : the first to appear, forms that part of the petrous that hes below the level of the internal meatus : this includes the lower part of the inner wall of the tympanum and, in front of this, the carotid canal.
(2) Pro-otic, the main centre, forming the part of the petrous that lies above the internal meatus, as far forward as the apex and back to the mastoid region : the cerftre appears near the eminentia arcuata.
(3) Pterotic : a small centre responsib'e for the roof of the tympanum and tegmen tympani; there may, however, be a separate centre for the latter.
(4) Epiotic: one or more from which the mass of the mastoid region is ossified.
The tympano-hyal centre makes its appearance late in foetal life : the stylo-hyal centre appears shortly after birth. The process remains attached by cartilage until nearly middle life, when it unites with the tympano-hyal.
At birth only the central part of the carotid canal is covered in : subsequent growth of the opisthotic and of the tympanic plate completes the process. The growth of the plate outwards is brought about largely by the spread of ossification from two main points of the ring, which by their ultimate fusion form the floor of the external meatus : when they first join a small hiatus may exist between them and is normally present (foramen of Huschkc) up to the age of five or six, and may persist beyond this. Other points of interest in the young temporal bone are mentioned in the section on the " Fcetal Skull".