Optic chiasm

Where is the optic chiasm?

It lies at the base of the brain, under the hypothalamus, in the suprasellar cistern. (StatPearls, Optic Chiasm)

It forms part of the front of the floor of the third ventricle, and the anterior communicating and anterior cerebral arteries lie close to it. (StatPearls, Optic Chiasm)

Which fibers cross at the optic chiasm?

Axons from the medial, or nasal, half of each retina cross here; axons from the lateral half stay on their own side. (OpenStax A&P 2e, 14.2; StatPearls, Optic Chiasm)

Because of this crossing, each optic tract, and so each cerebral hemisphere, carries the opposite half of the visual field: the right tract carries the left visual field. (OpenStax A&P 2e, 14.2; StatPearls, Optic Chiasm and Visual Pathway)

The crossing fibers come from the medial retinae, so they carry the outer, peripheral part of the visual field. (OpenStax A&P 2e, 16.3; StatPearls, Optic Chiasm)

Inside the chiasm, crossing fibers from the upper nasal retina run toward the back and those from the lower nasal retina toward the front. (StatPearls, Optic Chiasm)

What connects to the optic chiasm?

The optic nerves enter the skull through the optic canals and meet here. Behind it, the regrouped fibers continue as the optic tracts. (OpenStax A&P 2e, 16.3 and 14.2; StatPearls, Optic Chiasm and Cranial Nerve 2 (Optic))

Most optic tract fibers end in the lateral geniculate nucleus of the thalamus, and others reach the superior colliculus of the midbrain. A small set of retinal axons, the retinohypothalamic tract, reaches the suprachiasmatic nucleus of the hypothalamus. (OpenStax A&P 2e, 14.2; StatPearls, Optic Chiasm, Visual Pathway and Nucleus Suprachiasmatic)

The retina began as part of the embryonic diencephalon: in the third week of gestation, optic grooves grow out from it and form the optic vesicles. (OpenStax A&P 2e, 13.1 and 14.2; StatPearls, Visual Pathway)

The pituitary gland lies about 10 mm below. A pituitary growth can press on the crossing fibers and take away the outer, temporal half of the visual field in both eyes. (OpenStax A&P 2e, 14.2 and 16.3; StatPearls, Optic Chiasm)

Where the sources differ

The sources disagree on where the pituitary sits. OpenStax A&P 2e puts it directly below the chiasm; StatPearls puts the chiasm in front of and above the gland, about 10 mm above the pituitary fossa. (OpenStax A&P 2e, 16.3; StatPearls, Pituitary Gland and Optic Chiasm)

The sources give the field loss from chiasm compression different names. OpenStax A&P 2e calls it bilateral hemianopia; StatPearls calls it bitemporal hemianopsia, loss of the temporal visual field in both eyes from compression of the crossing nasal fibers. (OpenStax A&P 2e, 14.2; StatPearls, Optic Chiasm)