Part of the diencephalon, of which the major part is the subthalamic nucleus, and which connects to the globus pallidus, part of the telencephalon.
Source:
PAC, 2016, 88, 713. 'Glossary of terms used in developmental and reproductive toxicology (IUPAC Recommendations 2016)' on page 809 (https://doi.org/10.1515/pac-2015-1202)