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In 35 monkeys attempts were made to produce localized unilateral lesions i n individual vestibular nuclei in order to study vestibular projections to nuclei of the extraocular muscles. Portions of the medial, superior and inferior vestibular nuclei were destroyed selectively; lesions in Deiters' nucleus involved small portions of either the superior or inferior vestibular nuclei. Fiber degeneration was studied by the Nauta-Gygax technic.Exclusively ascending fibers from the superior vestibular nucleus project to ipsilateral extraocular nuclei. Ascending fibers from the inferior vestibular arise only from rostra1 portions of the nucleus, are not numerous and pass to all extraocular nuclei. The medial vestibular nucleus projects ascending fibers via the MLF bilaterally, asymmetrically and differentially to all extraocular nuclei. Prominent projections pass to: ( a ) the contralateral trochlear nucleus, and (b) the contralateral intermediate cell column and the ipsilateral ventral nucleus of the oculomotor complex. Ascending fibers from Deiters' nucleus, arising only from ventral portions of the nucleus, project primarily to: (a) the contralateral abducens and trochlear nuclei, and ( b ) specific asymmetrical portions of the oculomotor complex.Ascending vestibular fibers from the medial and lateral vestibular nuclei appear capable of mediating all patterned eye movements resulting from stimulation of ampullary nerves from individual semicircular canals. Vestibular projections to nuclei of the extraocular muscles arc most abundant to those nuclei innervating muscles whose primary functions concern horizontal and rotatory eye movements.The labyrinths, the vestibular nuclei and secondary vestibular pathways projecting to the nuclei of the extraocular muscles appear to play a major role in the control of conjugate eye movements. Anatomical evidence that the vestibular nuclei are concerned with conjugate eye movements appears to have a secure foundation, since secondary vestibular fibers ascending in the medial longitudinal fasciculus projecl to all the nuclei of the extraocular muscles (Szenthgothai, '43; Brodal and Pompeiano, '57; Carpenter, '60; Carpenter and McMasters, '63). Physiological studies (Szentagothai, '52; Fluur, '59; Cohen, SUzuki and Bender, '64) provide clear evidence of a precise functional correlation between particular semicircular canals and eye movements in specific directions. The investigations of Szenthgothai ('50) leave little doubt that functionally the most important impulses mediating ocular AM. J. ANAT., 118: 163-194. movements in response to stimulation of the semicircular canals ascend in the medial longitudinal fasciculus (hereafter abbreviated MLF). These results suggest that impulses from individual semicircular canals must ultimately be transmitted differentially to all the nuclei of the extraocular muscles, including specific subdivisions of the oculomotor nuclear complex. SzentAgothai's ('43) studies of the distribution of degenerated fibers in the nuclei of the extraocul...

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