What does chiasm mean?
A chiasm is an anatomical structure shaped like the Greek letter chi (X), formed where two bundles of fibres cross over each other. By far its most familiar use is in the term optic chiasm — the X-shaped point beneath the brain where the optic nerves from each eye meet and partially swap sides, so that each half of the brain processes information from the opposite half of the visual field. Because of this arrangement, damage to the optic chiasm produces distinctive patterns of vision loss, making the term central to neurology and ophthalmology. More generally, any crossing of nervous pathways may be described as a chiasm. The word comes ultimately from Greek khiasmos, meaning a crosswise arrangement, and is closely related to chiasmus, the rhetorical figure built on the same criss-cross shape.
nounAn anatomical structure shaped like the Greek letter X, in which two parts cross each other; most commonly short for optic chiasm, the crossing point of the optic nerves at the base of the brain.
- Short for optic chiasm: the X-shaped structure on the underside of the brain where the optic nerves partially cross.
- Any anatomical structure resembling the letter X, formed by the crossing or decussation of parts.
"The tumour was pressing on the optic chiasm, causing progressive loss of vision in both eyes."
"Damage to the optic chiasm typically produces bitemporal hemianopia, loss of vision in the outer half of each visual field."
"In the spinal cord, sensory fibres form a chiasm as they decussate to the opposite side."
The plural is rare because the word almost always refers to a single anatomical structure; "chiasms" also occurs, and "chiasmata" is the plural of the variant chiasma.
"The dissection revealed several fibre crossings along the visual pathways."
Your left eye talks to your right brain and vice versa — all thanks to one X-shaped traffic interchange called the optic chiasm, named after the Greek letter chi because it looks exactly like an X.
Reviewed by Deb Chak, Editor. AI-assisted content curated by RJS Tech Solutions LLP.
Etymology of chiasm
Chiasm derives from the Greek word khiasmos, meaning a crosswise arrangement or marking with the letter chi (X), itself formed from the name of that letter. It entered scientific English in the early modern period alongside the Latinised form chiasma, which biologists adopted for chromosome crossover points in genetics. Its closest relatives are chiasmus and chiasma, both built on the same Greek root referring to the shape of the letter chi.
How chiasm is actually used
Chiasm is primarily a technical term used in anatomy, neurology, ophthalmology, and related medical sciences; in general prose it appears almost exclusively in the phrase "optic chiasm." The fuller Latinised form "chiasma" (plural chiasmata) is common in biology, especially genetics, for the X-shaped contact points between paired chromosomes. Both spellings are acceptable in anatomy, though "optic chiasm" is the more frequent clinical usage.
Easily confused with chiasm
A chiasm is a physical X-shaped anatomical structure where fibres cross, while a chiasmus is a rhetorical device that reverses the order of words in successive clauses.
A chasm is a deep fissure or gaping void, whereas a chiasm is an X-shaped crossing of structures such as nerve fibres.