The motor nuclei and primary projections of the IXth, Xth, XIth and XIIth cranial nerves in the monitor lizard, Varanus exanthematicus.

The motor nuclei and sensory connections of the IXth, Xth, XIth, and XIIth cranial nerves of the reptile Varanus exanthematicus were studied with the methods of anterograde degeneration and anterograde and retrograde axonal transport. The motor nuclei of nerve IX are located ventrally in the rhombencephalon and are constituted medially by the large-celled glossopharyngeal part of the nucleus ambiguus and laterally by the small-celled nucleus salivatorius inferior. The motor nuclei of nerve X consist of the dorsomedially located dorsal motor nucleus of the vagus and the laterally located vagal part of the nucleus ambiguus. The rostral portion of the latter cell group contains smaller cells than its caudal portion and is rostrally continuous with the nucleus salivatorius inferior of nerve IX. The efferent axons of nerves IX and X arising from the ventrolateral medulla first course dorsomedially, form genua beneath the IVth ventricle, and then exit the brainstem. All primary afferent fibers of nerve IX and the majority of those of nerve X enter the solitary tract. Terminations of vagal fibers were observed in the postvagal portion of the nucleus of the solitary tract, the dorsal motor nucleus of the vagus, and the nucleus of the commissura infima. A small contingent of vagal fibers courses caudally just dorsolateral to the descending trigeminal tract. A separate spinal component of nerve XI could not be found. The bulbar component of this nerve forms part of nerve X and takes its main origin from a detached caudal element of the nucleus ambiguus. The motor nuclear complex of nerve XII consists of a large dorsal nucleus and a small ventral nucleus that extend from the medulla oblongata into the first segment of the cervical spinal cord.