Relationship of Meyer’s Loop to the Mediobasal Temporal Structures. A, Inferior view of the right temporal lobe. The parahippocampal gyrus and hippocampal head and body have been removed to expose the roof of the temporal horn. The uncus has an anterior segment that faces, anteromedially, a posterior segment that faces the cerebral peduncle and an apex that extends medially below the optic tract. The amygdala, outlined in green, is positioned predominantly in the anterior segment in the anterior wall of the temporal horn. The blue lines show the different levels behind the limen at which the angles of entry in the floor of the sylvian fissure that avoid the optic radiations were measured. B, Tapetal fibers, caudate tail, and striae terminalis in the roof of the temporal horn have been removed to expose the optic radiations. Meyer’s loop extends forward to the anterior tip of the temporal horn. The fibers forming Meyer’s loop leave the lateral geniculate body, pursue a curved anterior course to the tip of the temporal horn, and turn backward along the roof of the temporal horn. The cortex and white matter around the uncus have been removed to expose the amygdala, which forms the anterior wall of the temporal horn. The green line outlines the position of the edge of the limen insula, and the red line shows the position of the inferior insular sulcus in the floor of the sylvian fissure. The blue line outlines the margins of the temporal horn. The yellow lines outline a triangular safe area below the floor of the sylvian fissure through which the mediotemporal structures can be approached from above without damaging the optic radiations and optic tract. The apex of this angle is at the lateral geniculate body. The amygdala sits in the widest part of the safe area near the base of the triangle. The black line marks the level 15 mm behind the limen. The safe area behind the 15 mm line is very narrow. (Images courtesy of AL Rhoton, Jr.)