The vestibular complex of the American opossum, Didelphis virginiana. I. Conformation, cytoarchitecture and primary vestibular input.

Degeneration experiments reveal that the vestibular nerve of the adult opossum distributes to an extensive and diverse area of the brainstem and, as in placental mammals, the traditionally named nuclei do not receive a uniform distribution of primary afferent fibers. Nevertheless, such nuclei as well as other nearby cell groups have been considered as vestibular since they share other mutual connections. Except for subgroups "l", "g" and the nucleus supravestibularis all of the vestibular subdivisions generally recognized in more specialized placentals (e.g., the cat) have been identified in the marsupial opossum. Each of the vestibular nuclei and "extra-nuclear" cell stations are described as to their boundaries, cytoarchitecture and dendritic domain. Whenever possible we have employed a terminology consistent with that used for the cat. In several instances we have used results from experimental degeneration material to identify nuclear boundaries and divisions which are not apparent in routine Nissl, Golgi or silver preparations.