One of the mysteries of the English language finally explained.
Originally: †the opening of the cervix into the vagina (obsolete). In later use: the constriction of the cervical canal at the junction with the body of the uterus.
Mid 18th century; earliest use found in William Smellie (1697–1763), man-midwife. From post-classical Latin os internum from classical Latin ōs + internum, neuter of internus inner.
os internum/ˌɒs ɪnˈtəːnəm/
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