11. Conclusions
Perhaps the most striking feature of the comparative anatomy of
the vertebrate autonomic nervous system is the great similarity in its
arrangement — all the way from fish to tetrapods. The one exception is
the cyclostomes, where deviations from the vertebral template are
plentiful. Cranial pathways in the facial (VII) and glossopharyngeal
(XI) cranial nerves must have co-evolved with the appearance of
lachrymal and salivary glands in tetrapods. Oculomotor innervation
(III) of the iris is a general feature in vertebrates possessing eyes,
although the functional pattern lacks an obvious phylogenetic trend.
Similarly, the vagus (X) innervates abdominal viscera when present:
heart, lungs/swimbladder, bronchi/air ducts and the gut.
A spinal autonomic system as such does not exist in cyclostomes,
but is increasingly well developed from elasmobranchs and teleosts to
the tetrapods. It is not a prominent feature in dipnoans, and in the
lungfish it may in some ways be functionally related to the chromaffin
systems of the heart, the azygos vein and the intercostal arteries.
Longitudinally connected sympathetic chains are found in teleosts
and the tetrapods, with the additional anatomical feature in birds,
where the chains run enclosed in the vertebrae.