Internally, the hypothalamus is composed of many small nuclei, which are divided by a parasagittal plane into a medial and lateral zone. Landmarks for this dividing point are the columns of the fornix and the mammillothalamic tract as they reside within this sagittal plane (eFig. 9.91). The hypothalamus is also divided by coronal planes into a preoptic area and periventricular zone. Note that the periventricular zone is not to be confused with the paraventricular nucleus, which is a thin region of cell bodies lying medial to the medial zone.