View of midbrain tegmentum and thalamoperforating arteries traveling toward the mesencephalic periaqueductal gray substance. 1, P1 segment; 2, thalamoperforating arteries; 3, oculomotor nerve; 4, mesencephalic tegmentum; 5, periaqueductal gray substance; 6, quadrigeminal plate; 7, optic nerve; 8, anterior commissure; 9, fornix column; 10, foramen of Monro; 11, lamina terminalis; 12, chiasmatic recess; 13, optic chiasm; 14, infundibular recess; 15, mamillary body; 16, third ventricle; 17, hypothalamic sulcus; 18, velum interpositum cistern. (Image courtesy of E de Oliveira)