Midbrain

b. Mid brain : The midbrain lies between the diencephalon and the pons varolii and contains the cerebral aqueduct (iter), the narrow canal linking the third and fourth ventricles. On its dorsal surface sit the corpora quadrigemina, four rounded elevations: the upper pair, the superior colliculi, mediate visual reflexes, while the lower pair, the inferior colliculi, act as relay centres for auditory reflexes -- for instance, turning the head to hear a sound more clearly. On its inferior surface, two thick fibrous tracts called the cerebral peduncles (crura cerebri) carry ascending and descending nerve fibres of the reticular activating system, linking the cerebrum to the midbrain. Near the midbrain's centre, a mass of grey matter scattered within the white matter forms …