Dorsal view of a white matter dissection of the diencephalomesencephalic junction. The stria medullaris travels along the dorsomedial surface of the thalamus and connects the septal and preopticohypothalamic region with the habenula. The habenular/posterior commissure interconnects the habenula and pretectal area of each side and is located cranial to the pineal recess and pineal gland. The subthalamic nucleus (STN) is a lens-shaped nucleus located ventral to the caudal thalamus that modulates basal ganglia output, therefore, it's used as a target for deep brain stimulation in Parkinson's disease. Injury to the STN causes a rare movement disorder called hemiballismus. The superior cerebellar peduncle (SCP) is the major efferent cerebellar fiber tract system containing mostly dentato-thalamic and cerebello-rubral fibers. The SCP crosses the midline at the level of the inferior colliculi in the caudal midbrain (this decussation resembles two crossed arms and is therefore also called brachium conjunctivum) and then forms the capsule of the red nucleus in the rostral midbrain. The dissection along the floor of the fourth ventricle reveals from medial to lateral: the medial longitudinal fasciculus, the central tegmental tract, and trigeminal mesencephalic tract. The trigeminal nerve is seen on the left side as it runs from Meckel's cave to enter the brainstem at mid-pons. The left facial-vestibulocochlear nerve complex is shown sectioned at the internal auditory canal. The right facial nerve is shown connecting with its geniculate ganglion. The glossopharyngeal nerve is seen exiting the skull base through pars nervosa (anteromedial) of the jugular foramen along with the inferior petrosal sinus. The vagus and accessory nerves exit the skull base through pars vascularis of the jugular foramen (posterolateral). (Image courtesy of AL Rhoton, Jr.)