Which structure prevents spread of infection form middle ear to the brain?

Tegmen tympani (forming the roof of middle ear cavity) separates the tympanic cavity from the middle cranial fossa. Boundaries of Middle earRoof(Tegmental wall)Floor(Jugular wall)Posterior wall(Mastoid wall)oTegmen tympanic of the petrous pa of the temporal bone.oGreater superficial petrosal nerve.oLesser superficial petrosal nerveoSuperior bulb of internal jugular vein.oTympanic branch of glossopharyngeal nerve.oAditus to mastoid antrum.oPyramidal eminence and stapedius muscle.oFacial nerve (veical pa).oChorda tympani nerve (entry).Anterior wall(Carotid wall)Lateral wallMedial wall (Labyrinthine wall)oOpening of tensor tympani muscle.oOpening of auditory tube (Eustachian tube).oInternal carotid aery.oSympathetic plexus.oCanal of Huguier - Exit of chorda tympanic nerve oFormed by tympanic membrane and bony outer attic wall called scutum oPromontory oFenestra vestibule (Oval window)oFenestra cochlea (Round window - 2 o TM)oFallopian canal - Facial nerve canal prominence oProcess cochleariformis for tendon of tensor tympani Refer: PL Dhingra, Diseases of Ear, Nose & Throat, 7th edition, pg no. 6