Ventricles of brain. Coloured composite 3-D magnetic resonance imaging (MRI) scan of a human brain, showing the ventricles (orange). The front of the brain is at top. These four cavities circulate the cushioning cerebrospinal fluid (CSF). The two large lateral ventricles lie either side of the mid-line of the brain, one in each cerebral hemisphere. They communicate with the third ventricle (yellow, centre), which lies between and below them. This leads to the fourth ventricle (orange, lower centre) via a narrow channel (vertical line), the cerebral aqueduct. Beneath the lateral ventricles lie the sensory- processing thalami (brown, centre).