Fornix

At a glance

English

Fornix

Latin (TA2)

Fornix

System

Brain and spinal cord

Region

Head

Side

Paired, left and right

About the fornix

The fornix (Latin for an arch or vault) is the chief efferent pathway of the hippocampal formation and part of the limbic system. Each side starts in the temporal lobe and sweeps upwards, forwards and then down again, so that the paired bundle draws a C around the thalamus. Along its course it also brings some fibres the other way, into the hippocampus from the septal area, hypothalamus and brainstem.

Its fibres collect on the ventricular surface of the hippocampus as a white coat, the alveus, and gather along its medial edge as the fimbria, a flat band in the floor of the temporal horn of the lateral ventricle. Leaving the back of the hippocampus, each fimbria becomes a crus, which curves up behind the pulvinar of the thalamus and runs forward against the underside of the corpus callosum. Under the middle of the callosum the two crura meet and travel side by side as the body of the fornix, roofing the third ventricle; the septum pellucidum is fastened to its upper surface. Between the diverging crura stretches a thin triangular sheet, the commissure of the fornix, which crosses fibres between the two hippocampi.

Above the interventricular foramina the body parts again into two columns. Each column bends down in front of the foramen, passing just behind the anterior commissure, and sinks into the side wall of the third ventricle. Fibres that pass behind the commissure (the postcommissural fornix) run through the hypothalamus to end mainly in the mammillary body; from there the mammillothalamic tract continues to the anterior nuclei of the thalamus. Fibres that pass in front of it (the precommissural fornix) end in the septal nuclei, the nucleus accumbens and nearby basal forebrain.

These connections make the fornix a link in the Papez circuit and in the system that lays down new memories. Damage to it, from surgery through the roof of the third ventricle, a colloid cyst or other tumour, or injury, can leave a person unable to recall recent events and new facts, while recognition of familiar things is relatively spared. Shrinkage of the fornix is seen early in Alzheimer disease, and it has been a target for deep brain stimulation trials in memory disorders.

Function and supply

Function

Carries hippocampal output to the mammillary bodies, anterior thalamus and septal area, forming a key link in memory circuits; also carries some input back to the hippocampus.

Blood supply

Branches of the anterior cerebral and anterior communicating arteries supply the columns and body; posterior choroidal branches of the posterior cerebral artery reach the crura.