Efferent axons leaving the hippocampal formation primarily exit from the subiculum and form the fornix (Latin for “arch”), a white matter structure that arches over the ventricular system (eFig. 9.95). The fornix begins with axons exiting the hippocampus to form the alveus along the ventricular surface of the hippocampus. As the axons come together medially, they form a bundle referred to as the fimbria of the fornix. The fornix then emerges from the hippocampal formation and curves under the corpus callosum before coursing medially to run adjacent to the midline (eFig. 9.105). At the anterior commissure, the fornix divides into a precommissural fornix and postcommissural fornix to reach the nucleus accumbens, septal nuclei, medial frontal cortex, mammillary nucleus, ventromedial nucleus of the hypothalamus, and anterior nucleus of the dorsal thalamus (eFig. 9.95).