Water on the brain. Coloured magnetic resonance imaging (MRI) scan of a sagittal section through the head of a 22 year old woman showing water on the brain (yellow/white), or hydrocephalus. This condition is caused by an overproduction of cerebrospinal fluid (CSF), the liquid that surrounds and protects the brain and spinal cord. Here the accumulation is due to a congenital narrowing of the aqueduct of Sylvius, a channel that links the third and fourth brain ventricles. The ventricles normally contain CSF, but an increase in its volume can put pressure on the brain, leading to impaired mental function. The fluid must be drained.