| operculum | A lid or cover becoming detached at maturity by abscission, in Eucalyptus (for example), a cap covering the bud and formed by fusion or cohesion of perianth parts. (09 Oct 1997) |
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| operculum ilei | A thickening of circular musculature at the free margin of the ileal papilla. Synonym: ileocaecocolic sphincter, marginal sphincter, operculum ilei, Varolius' sphincter. (05 Mar 2000) |
| occipital operculum | A portion of the occipital lobe of the brain demarcated by the simian fissure (sulcus lunatus cerebri) when present in humans. (05 Mar 2000) |
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| trophoblastic operculum | The mushroom-shaped plug of fibrin that fills the aperture in the endometrium made by the implanting ovum. (05 Mar 2000) |