The mastoid antrum
This is quite distinct from the mastoid cells. It is an air-chamber communicating with the attic of the tympanum. It is separated from the middle cranial fossa by the posterior portion of the tegmen tympani; the floor is formed by the mastoid portion of the petrosal; it communicates with the mastoid cells. The outer wall is formed by the squamosal below the supra-mastoid crest. In children the outer wall is exceedingly thin, but in the adult it is of considerable thickness. The external semicircular canal projects into the antrum on its inner wall, and is very conspicuous in the fetus.
A canal occasionally leads from the mastoid antrmii through the petrous bone to open in the recess which indicates the position of the floccular fossa; it is termed the petro- mastoid canal. (Gruber.)