The fibres begin in the hippocampus on each side of the brain as fimbria; the separate left and right sides are each called the crus of the fornix (plural crura). The bundles of fibres come together in the midline of the brain, forming the body of the fornix. The lower edge of the septum pellucidum (the membrane that separates the lateral ventricles) is attached to the upper face of the fornix body.

The posterior fibres (called the postcommissural fornix) of each side continue through the hypothalamus to the mammillary bodies; then to the anterior nuclei of thalamus, which project to the cingulate cortex. The anterior fibers (precommissural fornix) end at the septal nuclei and nucleus accumbens of each half of the brain.