fornix

**Anatomy / course:** Fibers arise from the hippocampus and subiculum and form the alveus, then the fimbria, then the crus of the fornix, then the body, then the columns of the fornix. The columns arch over the interventricular foramen of Monro and pass through the hypothalamus to end mainly in the mammillary bodies, with some fibers going to the anterior thalamic nuclei and septal nuclei. The body lies along the roof of the third ventricle, beneath the corpus callosum and attached to it by the septum pellucidum. Fibers cross at the hippocampal commissure.

**Papez circuit (memory):** Hippocampus → fornix → mammillary bodies → mammillothalamic tract → anterior nucleus of thalamus → cingulate gyrus → entorhinal cortex → hippocampus.

**Clinical significance:** - **Anterograde amnesia:** Lesions of the fornix, mammillary bodies, or anterior thalamus impair formation of new declarative memories. Causes include colloid cyst of the third ventricle (near the foramen of Monro), surgical injury (e.g., transcallosal approaches), tumors, and trauma. - **Wernicke-Korsakoff syndrome:** Thiamine (B1) deficiency, classically in alcohol use disorder, damages the mammillary bodies and medial dorsal thalamus. This causes confusion, ophthalmoplegia, and ataxia (Wernicke), then anterograde and retrograde amnesia with confabulation (Korsakoff). - **Alzheimer disease:** Early hippocampal atrophy and fornix degeneration contribute to memory loss. Fornix deep brain stimulation has been investigated as a therapy. - **Temporal lobe epilepsy:** The hippocampus and its connections are the typical epileptogenic focus (hippocampal sclerosis). - **Imaging:** Colloid cyst on CT/MRI appears as a hyperdense, round mass at the foramen of Monro. It can cause obstructive hydrocephalus (headache, positional symptoms, rarely sudden death) and may compress the fornices.

**High-yield associations:** Fornix = main output of the hippocampus. Mammillary body lesion = Wernicke-Korsakoff. Hippocampus = declarative memory. The fornix is part of the limbic system.

**Other uses of the word:** In anatomy, "fornix" means an arch- or vault-shaped structure. Examples are the **vaginal fornix**, where the posterior fornix is a site for culdocentesis and is next to the rectouterine pouch (pouch of Douglas), and the **conjunctival fornix**, the pocket where the palpebral and bulbar conjunctiva meet. A foreign body may lodge in the superior conjunctival fornix.