Foramen magnum

Foramen magnum (lat. «stot hull») er det anatomiske begrepet på den åpningen i hodeskallenn der ryggnerven går ut fra hjernekassen. Hos menneske er åpningen oval med en diameter på rundt 3 cm hos voksne.

Anatomi

Hos primater er denne åpningen omgitt av en enkel knokkel, bakhodebeinet, mens det hos andre pattedyr og i andre virveldyrgrupper kan være omsluttet av flere bein. På nedsiden av åpningen sitter det en leddflate som er feste mot atlasvirvelen (den øverste virvelen i ryggsøylen. Hos de fleste virveldyr er dette et enkelt konvekst kuleledd, mens hos pattedyr og enkelte krypdyr er dette to leddflater, en på hver side av åpningen.^[1]

Størrelse og plassering

Størelsen på foramen magnum er avhengig av en rekke faktorer, først og fremst størrelsen på hjernen og kroppen, men er også avhengig av hvor manghe nerver som trengs for å kontrollere kroppens forskjellige funksjoner. Dyr med særdeles følsome sanseorganer knyttet til lemmene, eller med behov for svært nøyaktig styring av enkelte kroppsdeler, vil måtte ha en større foramen magnum enn like store arter uten slike behov.^[2] Plasseringen av foramen magnum og leddflatene mot ryggsøylen på et fossil gjør det mulig å fastslå hvordan ryggsøylen har vært vinklet mot hodeskallen, og derved hvorvidt en art har gått på to eller fire bein.^[3]

Referanser

^ Wake, M.H., red. (1992). Hyman's comparative vertebrate anatomy (3. (pocket-) utg.). Chicago: Univ. of Chicago Press. s. 228. ISBN 0226870138.
^ MacLarnon, A. (1996). «The evolution of the spinal cord in primates: evidence from the foramen magnum and the vertebral canal». Journal of Human Evolution. 30 (2): 121–138. doi:10.1006/jhev.1996.0009.
^ Switek, B. (2013). «The Way You Walk is Tied to a Hole in Your Skull». Phenomena. National Geographic. Besøkt 18. juli 2017.

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