Cross Section of Midbrain. The internal anatomy of the midbrain is revealed in this cross section at the level of the inferior colliculus.  The midbrain is divided anatomically into a dorsal tectum and two paired cerebral peduncles.  The tectum of the caudal midbrain consists of the paired oval-shaped inferior colliculi.  Each cerebral peduncle is divided, from ventral-to-dorsal, into the crus cerebri, substantia nigra, and midbrain tegmentum.   Within the tegmentum of the caudal midbrain can be seen the decussation of the SCP, reticular formation, medial lemniscus, spinothalamic tracts, medial longitudinal fasciculi, and trochlear nuclei.  The periaqueductal gray (PAG) surrounds the cerebral aqueduct.  The oculomotor nerves emerge from the anteromedial surface of the midbrain within the interpenduncular fossa. (Image courtesy of AL Rhoton, Jr.)