Cerebellar cortical efferent fibers in the North American opossum, Didelphis virginiana. II. The posterior vermis.

The projection pattern of corticonuclear and corticovestibular fibers from vermis lobules VI-X of the North American opossum, Didelphis virginiana, was studied using a modification of the Fink and Heimer ('67) technique. The evidence suggests that corticovestibular projections in opossum are ipsilateral and exit the cerebellum via the juxtarestiform body. Lobules VI and VII contribute few, if any, corticovestibular projections. Corticovestibular fibers from lobule VIII are sparse to moderate and those from lobules IX and X are numerous. The main targets of corticovestibular fibers are the superior and spinal vestibular nuclei with some input to the medial vestibular nucleus, particularly from lobules IX and X. Although degenerated axons course through the lateral vestibular nucleus, they do not appear to terminate therein. Corticonuclear fibers from lobules VI-X are ipsilateral and terminate in the caudal and caudoventral one-third of the medial cerebellar nucleus (NM). Fibers from lateral areas of some vermal lobules appear to enter contiguous parts of the immediately adjacent posterior interposed nucleus. Although each lobule projects into a specific area of caudal and caudoventral NM when the terminal fields for lobules VI-X are superimposed, it is apparent that they are largely coextensive. This is in contrast to the pattern seen in projections from the anterior vermis. The results further indicate that zones A and B are present in lobules VI-X of opossum.