The midbrain is the most caudal part of the brainstem connecting the pons and cerebellum with the cerebrum.

Structures and connections of the midbrain are associated with such functions as hearing, vision, motor control, alertness, temperature sensation, sleep, and wakefulness.

The midbrain is the shortest part of the brainstem and it lies in the posterior cranial fossa.

Through the midbrain passes the cerebral aqueduct, a passageway that connects the third ventricle and fourth ventricle.

The midbrain has a ventral surface and a dorsal surface.

Ventral surface

The main characteristics of the ventral surface are the cerebral peduncles. An area called the interpeduncular fossa is formed between the peduncles near the pons.

The floor of the fossa is made by a region known as the posterior perforated substance, which is perforated by branches of the posterior cerebral artery.

The ventral surface presents two paired grooves or sulci:

Sulcus of the oculomotor nerve - on the medial surface of the cerebral peduncles; it serves as the exit point for the third cranial nerve - the oculomotor nerve (CN III);
Lateral mesencephalic sulcus - on the lateral surface of the cerebral peduncles; the trochlear nerve (CN IV) crosses it horizontally.

Dorsal surface

The dorsal surface of the midbrain is also called the mesencephalic tectum. Below the tectum a pair of peduncles - the superior cerebellar peduncles - are located, which connect the cerebellum to the midbrain.

The dorsal surface also contains four tubercles known as colliculi (singular: colliculus) which are separated by the cruciform sulcus, and these colliculi are:

Lateral from each colliculus extends an eminence consisting of white matter called brachium - it extends in a forward and upward direction. The superior brachium and inferior brachium connect vision and hearing centres of the midbrain to centres located in the diencephalon.

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Midbrain