Emissary veins passing through the foramen ovale, jugular foramen, foramen lacerum, and hypoglossal canal. Because the emissary veins are valveless, they are an important part in selective brain cooling through bidirectional flow of cooler blood from the evaporating surface of the head. In general, blood flow is from external to internal but the flow can be altered by increased intracranial pressure. A type of emissary vein goes 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
Diploic veins Diploic veins The diploic veins are large, thin-walled valveless veins that channel in the diploë between the inner and outer layers of the cortical bone in the skull. They are lined by a single layer of endothelium supported by elastic tissue. They develop fully by the age of two years. The diploic veins drain this area into the dural venous sinuses. The four major types of diploic veins found on each side of the head are frontal, anterior temporal, posterior temporal, and occipital diploic veins. The "frontal", which opens into the supraorbital vein and the superior sagittal sinus. The "anterior