With regard to the brain, the fimbria is a prominent band of white matter along the medial edge of the hippocampus. It is a complex fibre bundle that runs from the lateral surface of the Hippocampal formation to other regions of the forebrain
The fimbria is an accumulation of myelinated axons (mostly efferent) that first collect on the ventricular surface of the hippocampus as the alveus (a thin layer resembling an inverted trough).
Near the splenium the fimbria separates from the hippocampus as the crus of the fornix. So that section which runs rostrally from the hippocampus forms the fornix. This is divided in turn into: