Fornix of forebrain

The fornix is a C-shaped fiber tract that connects the hippocampus with the hypothalamus and with the septal and preoptic nuclei. (Neuroscience Online, Limbic System: Hippocampus and Hypothalamus: Structural Organization; OpenStax Behavioral Neuroscience, 1.4)

Latin fornix (Wikipedia, Fornix (neuroanatomy), CC BY-SA 4.0) · FMA61965 · Nervous system

The brain model seen from the right with the cortex faded, showing the fornix highlighted as an arch deep in the brain. — The fornix highlighted in the brain model, seen from the right through the faded cortex. Render: ScienceQuest, CC BY 4.0. Meshes: BodyParts3D, © The Database Center for Life Science, CC BY 4.0

The right and left fornix and the commissure of the fornix shown alone and seen from the right, a C-shaped arch. — The fornix on its own, seen from the right. Render: ScienceQuest, CC BY 4.0. Meshes: BodyParts3D, © The Database Center for Life Science, CC BY 4.0

Open it in the Brain Explorer See it alone in 3D

How does the fornix form?

It begins as the alveus, white matter made of myelinated fibers running to and from the hippocampus. As the fibers run back, they gather toward the midline into the fimbria, which thickens as it goes. (Neuroscience Online, Limbic System: Hippocampus)

Each fimbria then leaves its hippocampus as a crus, and the two crura join to form the hippocampal commissure. (Neuroscience Online, Limbic System: Hippocampus)

Where does the fornix run?

Past the hippocampal commissure the single bundle is the fornix proper, which arcs forward to the anterior commissure. (Neuroscience Online, Limbic System: Hippocampus)

At the anterior commissure it splits three ways: the precommissural fornix curves down in front of the commissure, the postcommissural fornix passes behind it, and some fibers cross in the commissure to the hippocampus of the other side. (Neuroscience Online, Limbic System: Hippocampus; Uberon, precommissural fornix and postcommissural fornix)

What does the fornix do?

It is one of the two main routes into and out of the hippocampus, and it carries incoming and outgoing fibers together. (Neuroscience Online, Limbic System: Hippocampus)

The precommissural fornix goes to the septal nuclei, the ventral striatum and the cingulate cortex. The postcommissural fornix goes to the mammillary bodies and the anterior nuclei of the thalamus. (Neuroscience Online, Limbic System: Hippocampus)

How does the fornix fit into the Papez circuit?

The postcommissural fornix carries hippocampal output to the mammillary bodies. The mammillothalamic tract continues to the anterior thalamic nucleus, which connects to the cingulate cortex. (Neuroscience Online, Limbic System: Hippocampus)

The stria terminalis of the amygdala follows the same plan, with precommissural and postcommissural branches. (Neuroscience Online, Limbic System: Amygdala)

Touches in this model

Measured on the BodyParts3D meshes: surfaces within 2 mm of the fornix of forebrain, nearest first, 12 of 33.

Anterior commissure in contact
Choroid plexus of left cerebral hemisphere in contact
Choroid plexus of right cerebral hemisphere in contact
Hypothalamus in contact
Interventricular foramen in contact
Left amygdala in contact
Left caudate nucleus in contact
Left hippocampus in contact
Left lateral ventricle in contact
Left stria medullaris of thalamus in contact
Left stria terminalis in contact
Left thalamus in contact

Sources

Neuroscience Online, McGovern Medical School, Section 4, Chapter 5, Limbic System: Hippocampus. Anthony Wright, Ph.D.. Last update Revised 07 July 2026. Open access; no reuse licence stated. Read 2026-10-25.
OpenStax Introduction to Behavioral Neuroscience, section 1.4, The Brain: Structure and Function. CC BY-NC-SA 4.0. Read 2026-10-25.
Neuroscience Online, McGovern Medical School, Section 4, Chapter 1, Hypothalamus: Structural Organization. Patrick Dougherty, Ph.D.. Last update Last Review 20 July 2026. Open access; no reuse licence stated. Read 2026-10-25.
Uberon anatomy ontology, UBERON:0014539, precommissural fornix of forebrain (read on EBI OLS). CC BY 3.0. Read 2026-10-25.
Uberon anatomy ontology, UBERON:0003016, postcommissural fornix of brain (read on EBI OLS). CC BY 3.0. Read 2026-10-25.
Neuroscience Online, McGovern Medical School, Section 4, Chapter 6, Limbic System: Amygdala. Anthony Wright, Ph.D.. Last update Revised 20 July 2026. Open access; no reuse licence stated. Read 2026-10-25.