fornix of brain

The fornix is a C-shaped white matter tract (bundle of myelinated axons) that forms the main output pathway from the hippocampus. It carries signals from the hippocampus primarily to the mammillary bodies of the hypothalamus and the septal nuclei. This pathway is a key component of the limbic system's Papez circuit, which is involved in memory formation, emotional processing, and spatial navigation.

The fornix is divided into several segments: the fimbriae (finger-like projections emerging from the hippocampus), the crura (the two curved pillars that rise from each hippocampus), the body (where the two crura merge under the corpus callosum), the commissure (connecting the two sides, also called the hippocampal commissure), and the columns (the forniceal pillars that descend to reach the mammillary bodies). Pre-commissural and post-commissural fibers project to different targets.

Bilateral fornix damage causes significant anterograde amnesia — impaired ability to form new memories — because the hippocampus can no longer relay information to memory-consolidation targets in the mammillary bodies and cingulate cortex. The classic Papez circuit disruption can produce amnesia disproportionate to other cognitive deficits. Unilateral fornix damage is typically less severe due to bilateral hippocampal representation.