A portion of the lateral venous sinus, bulging prominently into the mastoid cavity, that serves as a principal conduit by which blood leaves the cranium[EPIC]. The sigmoid is drained by the internal jugular vein [ http://en.wikipedia.org/wiki/Sigmoid_sinus http://palaeos.com/vertebrates/glossary/glossaryS.html ]
The sigmoid sinuses, within a human head, are 2 areas beneath the brain, which allow blood veins to span the area, from the center of the head downward. They drain from the transverse sinuses (under the back of the brain) to the internal jugular vein. Each sigmoid sinus begins beneath the temporal bone and follows a tortuous course to the jugular foramen, at which point the sinus becomes continuous with the internal jugular vein[Wikipedia:Sigmoid_sinus].