Fornix (brain)

Last revised by Yvette Mellam on 18 May 2024

The fornix (plural: fornices) is the main efferent system of the hippocampus and an important part of the limbic system. It is one of the commissural fibers connecting the cerebral hemispheres.

Roughly C-shaped, the fornix extends from the hippocampus to the mammillary bodies of the hypothalamus and the anterior nuclei of the thalamus.  It is a curvilinear bundle of white matter fibers that begins as a group of myelinated fibers called the alveus. The alveus joins to form the fimbria of the hippocampus. The fimbria of each hippocampus thickens and then splits off from the hippocampus to form the crus (leg) of the fornix.

The fornix is composed of four parts: 

  • bilateral crura: ascending anterior to the splenium of the corpus callosum at the level of the superior colliculus (best seen on coronal images)

  • commissure: connecting the crura

  • body: from the merging of the crura, provides one of two major paths through which the hippocampi communicate with each other

  • bilateral columns (anterior pillars): curve anteriorly and dives inferiorly into the hypothalamus (mammillary bodies)

At the level of the anterior commissure, the fornix divides into anterior and posterior fibers. The anterior fibers (called precommissural fibers) end in the septal region and nucleus accumbens 1,2. The posterior fibers (called the postcommissural fornix) end through the mammillary body of the hypothalamus.

The fornix connects:

  • the hippocampus to the mammillary bodies

  • the hippocampus to the septal nuclei and the nuclei accumbens

  • the mammillary bodies to the anterior nuclei of the thalamus

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