The hypothalamus, a continuation of the central periaqueductal gray matter, comprises the center of the neuroendocrine system; it surrounds the third ventricle with groups of neurons anteriorly and central gray matter posteriorly. The periventricular fibers, the medial forebrain bundle, and the dorsal longitudinal fasciculus are important fiber tracts that pass through the region. Relevant to the third ventricle because of its location, the posterior hypothalamus lies between the midcommissural point and the posterior commissure, below the anterior commissure-posterior commissure (AC — PC) line and includes the area 5–6 mm from the midline.