The emissary veins are valveless veins which normally drain from the dural venous sinuses into veins that lie outside the skull (external veins). However, because they are valveless, pus can flow into the skull through them as well, making them a possible route for transmission of extracranial infection. It is through these emissary veins that infections of the scalp can give rise to encephalitis.
One important emissary vein communicates from outside the skull through the sphenoidal emissary foramen inferior to the zygomatic arch with the cavernous sinus on the inside of the skull. This is an important route for spread of infection because cranial nerve VI and the internal carotid pass through the cavernous sinus, with cranial nerves III, IV, V1, and V2 passing alongside the lateral wall of the sinus. Subsequent infection or inflammation in the cavernous sinus can result in damage to any of the cranial nerves that pass through it or meningitis. Also, rupturing the emissary veins will result in a subdural hematoma which will compress the brain.