Basics of Neuroanatomy Quiz

See Features of the Brain. Option C is correct. The optic chiasm is the X-shaped meeting point of the two optic nerves, seen on the underside of the brain just in front of the pituitary stalk. Here fibres from the nasal half of each retina (the half nearer the nose) cross to the opposite side, while fibres from the temporal half (nearer the temple) stay on the same side. The result is that everything seen in the right half of the visual field, which falls on the right nasal and left temporal retina, ends up in the left side of the brain, and vice versa. This arrangement allows each side of the visual cortex to compare the views from both eyes, which supports binocular vision. It is also clinically useful: a lesion that presses on the chiasm, classically a pituitary tumour, damages the crossing nasal fibres and causes bitemporal hemianopia, meaning loss of the outer half of the visual field in both eyes.

• Structure: The X-shaped meeting point of the two optic nerves, just in front of the pituitary stalk.
• Crossing: Fibres from the nasal half of each retina cross, while fibres from the temporal half stay on the same side.
• Lesion: Pressure on the chiasm, classically from a pituitary tumour, causes bitemporal hemianopia.