The medial longitudinal fasciculus (MLF) is a prominent bundle of nerve fibres which pass within the ventral/anterior portion of periaqueductal gray of the mesencephalon (midbrain).

[1] It contains the interstitial nucleus of Cajal, responsible for oculomotor control, head posture, and vertical eye movement.

It extends across the dorsal tegmentum of all three parts of the brainstem, as well as reaching caudally into the upper cervical spinal cord levels.

The paramedian pontine reticular formation (PMPRF) is involved in coordinating horizontal conjugate eye movements and saccades.

It projects to the cervical and upper thoracic spinal cord to mediate reflex turning of the head and trunk in the direction of startling sensations.

[3] A lesion of the medial longitudinal fasciculus produces slowed or absent adduction of the ipsilateral eye upon contralateral gaze.

[9] This name stuck despite other authors attempting further renaming (Ramon y Cajal's periependymal in 1904, Theodor Ziehen's nubecula dorsalis in 1913).