ABSTRACT
This chapter presents an overview of the functional anatomy of the peripheral autonomic nervous system of each of the vertebrate classes. When combined with developmental studies, there are several clues as to the likely course of evolution of the different divisions of the autonomic nervous system. The enteric nervous system probably is the most primitive division, with apparently homologous neurons occurring in cephalochordates. An oculomotor pathway to ocular tissues is present in all gnathostome classes. Cranial autonomic innervation of the upper jaw via the palatine branch of the facial nerve first appears in amphibians. The cranial autonomic innervation of the lower jaw is primitively derived from the post-trematic branches of the facial and glossopharyngeal nerves, as seen in elasmobranchs. In amphibians, the glossopharyngeal pathway predominates, but in amniotes, the facial pathway is dominant. The vagus provides autonomic pathways to the foregut and its derivatives, and to the heart, via pathways that seem to be highly conserved. The sympathetic division is likely to have evolved from aggregations of chromaffin tissue primarily associated with the origins of the segmental blood vessels and the posterior cardinal veins. The pelvic plexuses are a complex mixture of pathways containing sympathetic and sacral parasympathetic components, mainly involved with the control of the hindgut and its .derivatives. Overall, there is a clear evolutionary trend for an increase in the level of organisation of autonomic pathways. This trend presumably is related to a corresponding increase in the ability of vertebrates to flourish independently of environmental constraints.
