Transverse anastomoses of the veins at the base of the brain.

Venous anastomoses at the base of the brain are represented by the anterior and posterior communicating veins. The anterior communicating vein anastomoses with the anterior communicating vein anastomoses with the anterior cerebral veins. The posterior communicating veins join the basal veins through the interpenduncular veins. The functional value of these venous anastomoses is less important than that of the arterial polygon of Willis. This anastomotic function depends on anatomical constancy and on the calibre of these transverse veins. Under certain pathological conditions the variations of intracranial pressure result in contralateral venous drainage through these anterior and posterior anastomotic veins.