Although the anatomy of the posterior longitudinal fasciculus has been studied in much detail (see Marburg, Ziehen, Spitzer, Muskens and Grey1), knowledge of its function is uncertain on many points. It is known that one-sided injuries of this bundle cause a paralysis of the gaze to this side, which seems to show that each posterior longitudinal fasciculus carries the impulses for conjugate movements of both eyes to its own side (Spitzer2). The importance of the posterior longitudinal fasciculus for the conduction of vestibular impulses to the ocular muscles has been doubted by Lorente de Nó,3 who stated recently that reactions of the ocular muscles supplied by the third and fourth nerves could be produced by stimulation of the labyrinth even if the posterior longitudinal fasciculus was severed on both sides. This result was confirmed by one of us (Spiegel4), who furthermore injured the brain stem