The fibres begin in the hippocampus on each side of the brain (where they are also known as the fimbria); the separate left and right sides 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 two lateral ventricles) is attached to the upper face of the fornix body.
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.