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Pronunciation: for'niks, -ni-sis, -ni-sez


  1. In general, an arch-shaped structure; often the arch-shaped roof (or roof portion) of an anatomic space.
  2. The compact, white fiber bundle by which the hippocampus of each cerebral hemisphere projects to the contralateral hippocampus and to the septum, anterior nucleus of the thalamus, and mammillary body. Arising from pyramidal cells of the Ammon horn, the fibers of the fornix form the alveus hippocampi and the fimbria hippocampi, and in their further course compose, sequentially, the commissure of the fornix [TA], also called the hippocampal commissure [TA] (commissura hippocampi [TA]), the crus of fornix [TA] (crus fornicis [TA]), the body of fornix [TA] (corpus fornicis [TA]), and the column of fornix [TA] (columna fornicis [TA]), which divides into a smaller portion of precommissural fibers [TA] that pass anterior to the anterior commissure to the septal area and a larger portion of postcommissural fibers [TA] that pass posterior to the anterior commissure to end mainly in the mammillary nuclei and to a lesser extent in the anterior thalamic nucleus.

Synonym(s): trigonum cerebrale

[L. arch, vault]

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© Copyright 2018 Wolters Kluwer. All Rights Reserved. Review date: Sep 19, 2016.