The term caudal linear nucleus of the raphe refers to one of two parts of the linear nucleus of the raphe in the ventral midbrain tegmentum at the level of the oculomotor nuclear complex. It is located in the midline of the human ( Hornung-2012 ), the macaque ( Paxinos-2009a ), the rat ( Swanson-2004 ), and the mouse ( Paxinos-2001 ); it is dorsal, variably, to the interfascicular nucleus, the interpeduncular nucleus, the decussation of the superior cerebellar peduncle and the ventral tegmental decussation, It contains a large number of serotonergic neurons ( Hornung-2012 ). The other component is the rostral linear nucleus of the raphe. Functionally it is part of the behavioral state system ( Swanson-2004 ).