The cerebrospinal fluid circulates between the ventricles and into the spinal canal. It also enters the subarachnoid and subdural spaces through apertures near the fourth ventricle (see Figure 12.24). It therefore also bathes the brain and spinal cord providing buoyancy. Eventually it must be reabsorbed into the bloodstream and this occurs at pockets of arachnoid tissue which invaginate into a large vein called the superior sagittal sinus. These pockets are called the arachnoid granulations or arachnoid villi and allow the fluid to move into the veins by osmosis and a pressure gradient produced by lower pressure in the vein. The absorption must occur at the same rate as production of CSF in order to prevent an imbalance. If absorption is insufficient it causes a condition known as hydrocephaly, "water on the brain". This condition usually shows up in early childhood and can damage the brain and lead to abnormal development. It is usually corrected surgically with a shunt, a small tube which drains fluid from the meninges and short circuits it into a nearby vein.