1.  fornix(n = noun.body) - generally any arch shaped structure (but often it refers to the arched roof of an anatomical space);
is a kind of body part

2.  fornix(n = noun.body) trigonum cerebrale - 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;
is a kind of fascicle, fasciculus, fiber bundle, fibre bundle
is a part of limbic brain, limbic system, visceral brain
has parts: carina fornicis

   An arch or fold; as, the fornix, or vault, of the cranium; the fornix, or reflection, of the conjuctiva. [1913 Webster]