The emissary veins are valveless veins which normally drain the intracranial venous sinuses to veins on the outside of the skull. However, because they are valveless, blood can flow into the skull through them as well, making them a possible route for transmission of extracranial infection to get into the skull.
One important emissary vein communicates from outside the skull through the foramen of Vesalius 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 nerves III, IV, V1, V2, and VI and the internal carotid pass through the cavernous 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.