The limbic system and deep cortical structures. The corpus callosum is a semicircular band of commissural fibers that forms the anterior and superior borders of the body of the lateral ventricles. Connecting the corpus callosum with the fornix is a double membrane called the septum pellucidum. The C-shaped fornix lies within the curve of the lateral ventricle and rostrally emanates from the mamillary bodies. The fornix continues anteriorly as the columns of the fornix, followed by the body of the fornix, the crus of the fornix, and transitions into the fimbria of the fornix adjacent to the hippocampus. Between the fornix and the thalamus is a cleft, the choroidal fissure, along the medial edge of the lateral ventricles which serves as an attachment site for the choroid plexus and through which the choroidal arteries penetrate. The inferior choroidal point is the entry point for the anterior choroidal artery into the temporal horn and lies just medial to the fimbria of the fornix and just superior to the head of the hippocampus along the floor of the temporal horn. (Image courtesy of AL Rhoton, Jr.)