While neuropathological studies have indicated that the substantia nigra is one of the principal neural structures affected with a high degree of consistency in paralysis agitans (Brissaud, 1895; Trktiakoff, '19; Foix, '21; Spatz, '27; Hassler, '39; Timmer, '40; Heath, '47), relatively few experimental studies have contributed information concerning the physiological function of this large mesencephalic nucleus. Data concerning the anatomical connections of this nucleus appear incomplete even though numerous descriptions of pathways to and from the substantia nigra have been reported in the literature. The efferent fibers of the substantia nigra are the least understood because it has been technically difficult to produce localized lesions in this nucleus in experimental animals. Studies of nigral efferent fibers based upon lesions in this nucleus have utilized the Marchi method (Ranson and Ranson, '42; Mettler, '42, '43; Kimmel, '42; Rosegay, '44) which because of inherent limitations cannot be considered conclusive (Bowsher, Brodal and Walberg, '60). The objects of the current study of the substantia nigra in the monkey were to determine: ( 1 ) the course and distribution of nigral efferent fibers, and ( 2 ) what physiological disturbances, if any, are associated with discrete lesions in this nucleus. MATERIAL AND METHODSTwenty-six rhesus monkeys were used in this study. In these animals attempts were made to produce discrete unilateral and bilateral lesions in the substantia nigra by stereotaxic techniques. Coordinate S~S L tems for this procedure were developed from dissections of formalin fixed heads with brains in situ (Carpenter and Whittier, '52). Two different stereotaxic approaches were used in producing these lesions: ( 1 ) a frontal approach, in which electrodes were introduced into the brain in parasagittal planes in nearly the rostrocaudal axis of the substantia nigra, and (2) a lateral approach, in which electrodes were inserted in the lateromedial axis of the substantia nigra. These different coordinate systems were used because it was realized that the passage of electrodes through adjacent neural structures and concomitant destruction of parts of neighboring nuclei would make it difficult to evaluate the fiber degeneration resulting from nigral lesions.Animals were anesthetized with Nembutal intravenously and surgery was performed aseptically. Following the insertion of the electrode according to the adopted coordinate system, stimulating square wave currents of low voltage were applied in attempts to confirm the position of the electrode tip. Physiological effects produced by these stimulations were recorded. At what appeared to be optimal locations, lesions were produced by a direct current of 3 ma and 400 volts applied for intervals of 15 seconds. When attempts were made to produce lesions in the rostrocaudal axis of the substantia nigra, four separate lesions were made serially at 1 mm intervals along a single electrode track. Two electrode placements usually were made 2 mm apart, at the s...

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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