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 fibres connecting the cerebral hemispheres.

Gross anatomy

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 fibres that begins as a group of myelinated fibres 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 ^1-5: 

• 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 anteroinferiorly and divide anterior and posterior to the anterior commissure into postcommissural and precommissural fibre tracts ⁵.

Fibre tracts

At the level of the anterior commissure, the columns divide into anterior and posterior fibres. The anterior fibres (called precommissural fibres) end in the septal region and nucleus accumbens, medial prefrontal cortex and septal nuclei ^1,2,5. The posterior fibres (called the postcommissural fornix) project to the mammillary bodies. Small fibres are also sent to the anterior thalamus ⁵.

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

Relations

• superiorly: septum pellucidum anteriorly, splenium of corpus callosum posteriorly

• inferiorly: thalamus anteriorly

• medially: third ventricle (lies inferior to the body of the fornix)

• laterally: medial aspects of the cerebral hemispheres

• anteriorly: anterior commissure

• posteriorly: posterior commissure, splenium of corpus callosum

Arterial supply

• mainly from the short medial central arteries (derived from the proximal anterior cerebral artery) and/or the subcallosal artery (branch of the anterior communicating artery) ^3,4

Related pathology

• amnestic syndrome of the subcallosal artery ⁴

• transection of the fornix: at the level of its body can lead to memory loss ^1,2