The term medial mammillary nucleus refers to a well defined group of cells located ventromedially in the posterior hypothalamic region of the human ( Saper-2004 ), macaque ( Paxinos-2009a ), rat ( Paxinos-2009b ) and mouse ( Franklin-2008 ). Defined on the basis of Nissl stain it is the most prominent cell group in the mammillary body. It has at least two parts in each of the species: the medial part of the medial mammillary nucleus and the lateral part of the medial mammillary nucleus. They are partially separated by fibers of the fornix. In the rat and the mouse the nucleus has a third component: the median part of the medial mammillary nucleus. Some authors do not distinguish medial and lateral parts of the nucleus in the rat ( Swanson-2004 ) and the mouse ( Hof-2000 ). It is composed of two parts: the body of the medial mammillary nucleus and the median part of the medial mammillary nucleus. Functionally the nucleus is part of the behavior control column of the subcortical motor system ( Swanson-2004 ).