Brain and vision. Coloured magnetic resonance imaging (MRI) scan of an axial section through the head. The eyes (black) are at top. The optic nerves (green) extend from the eyes towards the centre of the brain. At lower centre is the optic chiasma (X shape), where the optic nerves from the left visual field of both eyes go to the right hemisphere of the brain, and vice versa. Surrounding the optic chiasma is the circle of Willis (light green), where the basilar and 2 carotid arteries serving the brain terminate and give rise to the anterior, middle and posterior cerebral arteries. The loop structure means that if one of the serving arteries is damaged blood supply to the brain is not stopped.