A group of widely spaced, medium-size neurons in the dorsomedial region of the upper mesencephalic tegmentum, immediately lateral to the nucleus of Darkschewitsch; together with the latter, the interstitial nucleus is closely associated with the medial longitudinal fasciculus, through which it receives fibers from the vestibular nuclei and projects crossed fibers through the posterior commissure to the oculomotor nucleus; also projects fibers to all spinal levels. It is believed to be involved in the integration of head and eye movements, particularly eye movements of a vertical or oblique nature.