Source : Webster's Revised Unabridged Dictionary (1913)

Fornix \For"nix\, n.; pl. {Fornices}. [L., an arch.] (Anat.)
   (a) An arch or fold; as, the fornix, or vault, of the
       cranium; the fornix, or reflection, of the conjuctiva.
   (b) Esp., two longitudinal bands of white nervous tissue
       beneath the lateral ventricles of the brain.

Source : WordNet®

fornix
     n 1: generally any arch shaped structure (but often it refers to
          the arched roof of an anatomical space)
     2: an arched bundle of white fibers at the base of the brain by
        which the hippocampus of each hemisphere projects to the
        contralateral hippocampus and to the thalamus and
        mamillary bodies [syn: {trigonum cerebrale}]
     [also: {fornices} (pl)]

fornices
     See {fornix}