Arachnoid granulations

Arachnoid granulations are the aggregations of arachnoid villi. What are arachnoid villi, you may ask? Well, at specific points along the superior sagittal sinus, the arachnoid membrane extends through the inner layer of the dura mater, creating openings into the sinus. These projections of the arachnoid mater into the dural venous sinuses are known as arachnoid villi or arachnoid granulations. Each arachnoid villus functions as a one-way valve, enabling the flow of cerebrospinal fluid from the subarachnoid space into the dural venous sinus but not in the opposite direction.