Cross-sections of the pons at rostral (A), mid (B), and caudal (C) levels.

(A) The rostral pons at the isthmus. Fibres of the trochlear nerve (IV) are crossing as they leave dorsally. The medial lemniscus is moving laterally and beginning to rotate to a dorsoventral position. The superior cerebellar peduncle is moving towards the midline. The rostral-most edge of the middle cerebellar peduncle is present. The corticospinal, corticobulbar, and corticopontine fibres, which constitute the cerebellar peduncle, are separating as they plunge into the basilar pontine grey matter.

(B) The midpons at the level of the trigeminal nerve (V). Fibres of the trigeminal nerve separate the main sensory trigeminal and motor trigeminal nuclei. The cell bodies of proprioceptive trigeminal afferents constitute the mesencephalic nucleus. The trigeminal nerve leaves through the middle cerebellar peduncle. The medial lemniscus has begun to move laterally towards the spinothalamic tract. The superior cerebellar peduncle forms the lateral wall of the fourth ventricle as it descends from the cerebellum towards to midbrain tegmentum. Pontocerebellar fibres which receive input from the corticopontine fibres are streaming across the midline to form the middle cerebellar peduncle. The corticospinal, corticobulbar, and corticopontine fibres are scattered throughout the basilar pontine grey matter.

(C) The caudal pons at the level of the abducens (VI) and facial (VII) nerves. The abducens nerve leaves ventrally through the basal pons near the midline; the facial nerve loops medially around the abducens nucleus and then courses laterally to emerge at the caudal edge of the middle cerebellar peduncle. The pontine grey matter is sending pontocerebellar fibres across the midline to form the middle cerebellar peduncle. The superior cerebellar peduncle is projecting towards the midbrain. The medial lemniscus has rotated to a mediolateral position and is obscured by trapezoid fibres of the auditory system that cross the midline; the trapezoid fibres will turn rostrally to ascend in the lateral lemniscus. Primary afferents from the trigeminal nerve have formed the spinal trigeminal tract. (Modified from Patton et al. (1976).)