Midsagittal Perspective of Hypothalamus and Third Ventricle. The midbrain is sectioned transversely and is divided anatomically into a dorsal tectum and a ventral cerebral peduncle.  The latter is divided further into a midbrain tegmentum, substantia nigra, and crus cerebri.  The hypothalamus forms the floor and anterolateral wall of the midline III ventricle and is separated from the thalamus by a shallow depression, the hypothalamic sulcus.  Blood vessels that contribute to the Circle of Willis are prominently displayed in the center of the field.  Further laterally, the C-shaped fornix can be seen, starting from the fimbria attached to the dorsomedial surface of the hippocampus, and continuing superoposteriorly as the crus.  The body of the fornix is out of view in this image, but then sweeps anteriorly and inferiorly as the column of the fornix before dividing into its pre- and post-commissural divisions, eventually terminating in the anterior septal nuclei and mammillary body, respectively. (Image courtesy of AL Rhoton, Jr.)