Dictionary.net

• z[=i]'gon, n. a connecting bar: an [H shape]-shaped fissure of the brain. [Gr., a yoke.]

1908 - Chambers's Twentieth Century Dictionary of the English Language
By Thomas Davidson

• The bar or stem connecting the two branches of a zygal fissure.

1898 - American pocket medical dictionary
By Willam Alexander Newman Dorland