fornix

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. An archlike anatomical structure or fold, such as the arched band of white matter located beneath the corpus callosum of the brain.

Century Dictionary and Cyclopedia

1. n. In anat.: A median symmetrical arched formation in the brain, beneath the corpus callosum and septum lucidum, vaulting over the optic thalami and the third ventricle, and running into the floor of each lateral ventricle. In the human brain it consists of two longitudinal bundles of fibers, one on each side, which rise from the corpora albicantia, pass up, as the anterior pillars of the fornix, in front of the foramina of Monro and behind the anterior commissure, then, somewhat flattened and in apposition to each other, arch backward beneath the corpus callosum and above the velum interpositum, forming the body of the fornix, and then diverge toward the back part of the corpus callosum, to turn down, as the posterior pillars of the fornix (crura Iornicis), into the floor of the descending cornua of the lateral ventricles, where their free edges form the fimbriæ. See cut under corpus.
2. n. Some other arched, vaulted, or fornicated formation: as, the fornix conjunctivœ, the vault of the conjunctiva.
3. n. In conch.: The vaulted or excavated part of a shell under the umbo.
4. n. The more concavo-convex one of the shells of an inequivulve bivalve, as an oyster.
5. n. In botany, a small arching crest or appendage in the throat or tube of a corolla.
6. n. In topical geom., a part of a surface which, in conjunction with another similar part which inseparably accompanies it, increases the cyclosis of the surface by two. If cut away, it would leave two holes in the surface. Viewed from one side of the surface, it appears as a tubular bar (however much distorted or knotted) bridging the space from the edge of one hole to that of the other and leaving a transverse tunnel beneath formed by the twin fornix; while viewed from the other side it appears as a tunnel joining one hole to the other with a transverse bridge above formed by the twin fornix.

Wiktionary

1. n. anatomy An archlike structure or fold.
2. n. anatomy Specifically, the arched bundle of fibres or axons at the base of the brain.

GNU Webster's 1913

1. n. An arch or fold
2. n. Esp., two longitudinal bands of white nervous tissue beneath the lateral ventricles of the brain.

WordNet 3.0

1. n. generally any arch shaped structure (but often it refers to the arched roof of an anatomical space)
2. n. 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