Figure 17. Schematic representation of the human hypothalamus in coronal orientation (A-D: rostral to caudal), demonstrating the location of major nuclear groups. Drawings correspond to MRI images in Fig. 26. Using the fornix (fx) as an anatomic landmark as it passes through the mid-portion of the hypothalamus on each side of the third ventricle, it is convenient to divide the hypothalamus into medial and periventricular zones (that lie largely medial to the fornix) and a lateral zone (that lies lateral to the fornix). The medial and periventricular zones contain most of the hypothalamic cell groups, and the lateral zone contains relatively fewer neurons. This is because the lateral zone is largely composed of a massive bidirectional fiber pathway – the medial forebrain bundle – that extends through the hypothalamus and interconnects it with the limbic system and brainstem autonomic centers.