The fibres begin in the hippocampus on each side of the brain; the separate left and right side are each called the crus of the fornix. The bundles of fibres come together in the midline of the brain, forming the body of the fornix. The inferior edge of the septum pellucidum (a membrane that separates the left and right ventricles) is attached to the upper face of the fornix body.
When the body is formed from the two crura (sing. crus), there is a small number of fibres that cross over to the other side at what is called the hippocampal commissure. Most fibres stay on their original side.
The body of the fornix travels anteriorly and divides again near the anterior commissure. The left and right parts reseparate, but there is also an anterior/posterior divergence. The posterior fibres (called the postcommissural fornix) of each side continue through the hypothalamus to the mammillary bodies. The anterior fibers (precommissural fornix) end at the septal nuclei of each half of the brain.