Within the head are small cavities or spaces referred to as sinuses, and they primarily serve as drainage for venous blood. Some are found within the cranial bones and others, in soft tissues of the head. Several of such sinuses are located within the dura mater (the dura mater is bilaminar membrane which is also called pachymeninx and it is one of the three coverings of the brain that constitute the cranial meninges), and includes superior sagittal sinus, inferior sagittal sinus, straight sinus, occipital sinus, transverse sinuses, cavernous sinus, basilar sinus, superior petrosal sinuses, inferior petrosal sinuses, and the sigmoid sinuses.
The sigmoid sinuses are a pair of endothelium-lined spaces located between the periosteal and the meningeal layers of the dura mater. They follow an S-shaped course in the posterior cranial fossa, forming deep grooves in the temporal and occipital bones. Each sigmoid sinus turns anteriorly and then continues inferiorly as the internal jugular vein (IJV) after traversing the jugular foramen, thus expanding into the jugular bulb which occupies the posterior and largest compartment in the jugular foramen. The sigmoid sinus commences as the termination of the transverse sinus, deeply grooving the inner surface of the mastoid part of the petrous bone, as well as the temporal and occipital bones. It is connected with the exterior in its upper part by the mastoid emissary vein which joins the posterior auricular vein and in its lower part by a vein, which passes through the posterior condylar foramen to join the sub-occipital plexus of veins. It receives the superior petrosal sinus at its upper end and the occipital sinus at its lower end. Cerebellar veins drain into it, and it receives veins also from the mastoid air cells. Thrombophlebitis in these veins may lead to cerebellar abscess (pus) from mastoid infection.