Dictionary.net

The middle segment of the brain; the midbrain. Sometimes abbreviated to mesen. See Brain.

The middle segment of the brain; the midbrain. Sometimes abbreviated to mesen. See Brain.

The mid-brain, the second of the three primitive cerebral vesicles from which are developed the corpora quadrigemina, the crura cerebri, and the aqueduct of Sylvius.

The corpora quadrigemina and crura cerebri together.

The second of the three primitive cerebral cesicles from which are developed the middle cerebral ganglia, including the corpora quadrigemina and crura cerebri. [Gr.]