Ventral surface of brain showing the optic chiasm, hypothalamus and Circle of Willis.  At the base of the frontal lobes, the optic nerves fuse at midline to form the optic chiasm.  Within the chiasm, the fibers undergo a partial decussation and continue caudally into the optic tracts.  The optic tracts run along the lateral margin of the hypothalamus and the crus cerebri (the latter structures not visible in this dissection) to terminate in the lateral geniculate nucleus of the thalamus.  The ventral surface of the hypothalamus has been incised at the midline and retracted in order to open the midline III ventricle.  The hypothalamus forms the floor and inferolateral walls of the III ventricle. Caudally, the III ventricle continues into the midbrain as the tunnel-like cerebral aqueduct.  The pituitary stalk attaches to the ventral surface of the hypothalamus immediately caudal to the optic chiasm; however, in this dissection the pituitary stalk has been retracted rostrally and is seen just anterior to the optic chiasm.  The cerebral blood vessels that make up the polygonal Circle of Willis surround the optic chiasm and the hypothalamus.  In this brain, the posterior communicating arteries are especially large and suggests that much of the blood flow to the posterior cerebral arteries (PCAs) derives from the internal carotid artery and not from the basilar artery.  This is one of several relatively common anatomical variations in the Circle of Willis. (Image courtesy of AL Rhoton, Jr.)