All parts of the brain › Optic chiasm

Where is the optic chiasm?

The optic chiasm is a small X-shaped crossing on the underside of the brain, on the midline just below the hypothalamus and in front of the pituitary stalk, where the left and right optic nerves meet. Here the fibres from the inner (nasal) half of each retina cross to the other side, so each half of the brain receives the opposite half of what both eyes see. [1]

The 3D brain needs JavaScript. The text below still says where the optic chiasm is and what it does.

Slice

Opens cut by a top (axial) slice through the optic chiasm. Drag to turn, pinch or scroll to zoom, tap any part to name it; .

Next to it, each lit on its own page: optic nerve, optic tract, hypothalamus, pituitary gland, third ventricle.

Takes part in: vision — see every part of each lit together.

Where it is

On the underside (base) of the brain, on the midline, immediately below the hypothalamus; the two optic nerves join it from the front and the two optic tracts leave it at the back. [1]
It lies just above the pituitary gland, which sits below it in the sella turcica of the skull, and in front of the infundibulum (pituitary stalk) that hangs the gland from the hypothalamus. [1][2]
It forms part of the floor of the third ventricle; a small pocket of the ventricle, the optic recess, reaches down just above and in front of it. [3]

What it does

Fibres from the nasal half of each retina (the half nearer the nose) cross to the opposite side here, while fibres from the temporal half stay on their own side. [1][4]
The result: the right side of the brain receives the left half of the visual field from both eyes, and the left side the right half. Bringing the matching halves from both eyes together is what lets the visual cortex combine them for binocular and depth (stereoscopic) vision. [1][4]
Just above it sits the suprachiasmatic nucleus of the hypothalamus (“supra-chiasmatic” means above the chiasm), the body’s master clock, which takes a direct feed of light signals from the retina to keep daily rhythms in step with day and night. [5]

Connections

In, from the front: the left and right optic nerves, carrying the axons of the retina’s ganglion cells. [1][6]
Out, at the back: the left and right optic tracts, each now holding the fibres for one half of the visual field from both eyes, running to the lateral geniculate body of the thalamus. [7]

Other names

Optic chiasma, chiasma opticum (Latin). “Chiasm” comes from the Greek letter chi (χ), for its X shape. [1]

Size

On this model the optic chiasm measures about 10 mm front to back, 8 mm top to bottom and 15 mm side to side (the box it fits in, not its volume). [8]
Centre of the optic chiasm compared with the centre of the pituitary gland: 11 mm higher; 11 mm apart in a straight line. [8]
Centre of the optic chiasm compared with the centre of the hypothalamus: 5 mm further forward; 5 mm apart in a straight line. [8]

Sources

Facts describe anatomy and normal function only. The 3D model is the adult male BodyParts3D brain (credits and licence); real brains vary in size and shape, so its measurements describe one model, not anyone’s brain.