Cingulum. A, Medial view of the medial left hemispheric surface. The cortex and short fibers of the cingulate gyrus have been removed from the paraterminal gyrus to the isthmus of the cingulate gyrus, to expose the cingulum running in a longitudinal direction, above the corpus callosum. An important contingent of fibers from the precuneus is observed becoming incorporated into the cingulum. The mammillothalamic tract courses between the mammillary body and the anterior nucleus of the thalamus. B, Tractography of the cingulum coursing between the paraterminal gyrus and the isthmus. Some commissural fibers of the corpus callosum crossing below the cingulum are shown. The ROI (orange) is selected in the color-coded DTI axial map at the most medial fasciculus with an anteroposterior orientation (green) and at the deep level of the cingulate gyrus (inset). C, Further fiber dissection of the left medial hemispheric surface. The cingulum narrows at the level of the isthmus of the cingulate gyrus where the commissural fibers of the forceps major cross in front of the fibers of the cingulum. Below this level, the cingulum courses near the optic radiations covering the inferior lip of the anterior part of the calcarine sulcus. The gray matter of the parahippocampal gyrus has been removed to expose the continuation of the cingulum toward the anterior parahippocampal region adjacent to the hippocampus. Multiple fibers from the precuneus are observed becoming incorporated into the cingulum. D, Complete tractographic reconstruction of the cingulum, which travels from the medial temporal lobe to the medial parietal and frontal regions, and forms the so-called external limbic ring. E, Superior view of fiber dissection of the cerebral hemispheres. The left and right cingulum course above the corpus callosum in an anteroposterior direction, and form the white matter of the cingulate gyrus. The fibers of the body of the corpus callosum cross to the opposite hemisphere and under the cingulum. The corona radiata is situated lateral to the cingulum and is intermingled with the callosal radiations. F, Tractographic reconstruction of the left and right cingulum to be correlated with E. (Images courtesy of E de Oliveira)