On the occipital bone, the midpoint on the posterior margin of the foramen magnum is termed the opisthion. ==Additional images== ...
• (n.) The middle of the posterior, or dorsal, margin of the great foramen of the skull.
O·pis'thi·on noun [ New Latin , from Greek ... hinder.] (Anat.) The middle of the posterior, or dorsal, margin of the great foramen of the skull. 
Type: Term Pronunciation: ō-pis′thē-on Definitions: 1. The middle point on the posterior margin of the foramen magnum, opposite the basion.