Brainstem organization and branchiomeric nerves.

The branchiomeric portion of the vertebrate head represents a region of specialized function and, at the same time, a transition between head and body. Recent experimental work has called into question the 'visceral' nature of the branchiomeres, interpreting them instead within the framework of the segmentation of the body, as is seen clearly in rhombomeres. The vagus nerve is critical to our understanding of this region. It is usually viewed as a serial homologue of dorsal roots of spinal nerves, and the hypoglossal, with its several roots, has been seen as arising from associated ventral roots. The accessory is often considered to have been derived directly from the vagus, having become individuated in tetrapods in general and especially in amniotes. Work on amphibians is examined with respect to these issues, and within structuralist, functionalist and phylogenetic frameworks. The accessory in mammals and birds is a composite; the spinal motor nucleus, which can be traced at least to elasmobranchs, is distinctly different in origin from the more recently added bulbar portion, which is derived from the vagus. The spinal portion appears to have evolved independently of the system associated with branchially derived musculature. The hypoglossal is derived from ventral-column material, but it is not clearly associated with the vagus phylogenetically.