• (a.) Having a medium orbital index; having orbits neither broad nor narrow; between megaseme and microseme.
Mes'o·seme adjective [ Meso- + Greek ... sign, mark; confer French mésosème .] (Anat.) Having a medium orbital index; having orbits neither broad nor narrow; between megaseme and microseme. 
Type: Term Pronunciation: mez′ō-sēm Definitions: 1. Denoting an orbital aperture with an index of 84-89; characteristic of the white race.