midbrain Organ component of neuraxis that has as its parts the tectum, cerebral peduncle, midbrain tegmentum and cerebral aqueduct[FMA]. The brain region between the forebrain anteriorly and the hindbrain posteriorly, including the tectum dorsally and the midbrain tegmentum ventrally[ZFA]. The midbrain is the middle division of the three primary divisions of the developing chordate brain or the corresponding part of the adult brain (in vertebrates, includes a ventral part containing the cerebral peduncles and a dorsal tectum containing the corpora quadrigemina and that surrounds the aqueduct of Sylvius connecting the third and fourth ventricles)[GO]. midbrain regional part of brain structure with developmental contribution from neural crest