Arachnoid cyst. Coloured magnetic resonance imaging (MRI) scan of an axial (horizontal) section through the brain showing an arachnoid cyst (green, centre right). The eyes (circles) are at upper left and right. The brain's cerebellum (pink, lower centre) and brainstem (purple, centre) are also visible. The cyst is an abnormal swelling filled with fluid or semi-solid material in the arachnoid mater, the central of the three membranes (meninges) that cover and protect the brain. Although non-malignant, this massive cyst has displaced part of the brain. Symptoms include headaches, vomiting, visual disturbances and drowsiness. The cyst must be removed surgically.