Meson \Mes"on\, n. [NL., fr. Gr. me`son middle, neut. of me`sos, a., middle.]

(Anat.) The mesial plane dividing the body of an animal into similar right and left halves. The line in which it meets the dorsal surface has been called the dorsimeson, and the corresponding ventral edge the ventrimeson. --B. G. Wilder.

Source: Webster's Revised Unabridged Dictionary (1913)