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July 1999
FIG. 2. Right: schematic diagram of a dorsal view of hindbrain of dogfish, Scyliorhinus canicula, to show
distribution of motonuclei (indicated by hatched areas) supplying efferent axons to cranial nerves innervating respiratory muscles and heart. These are adductor mandibulae nucleus of cranial nerve V, supplying closing muscles in jaw;
facial motor nucleus of VII, supplying muscles of jaw and spiracle; glossopharyngeal nucleus of IX, supplying first gill
arch; dorsal motor nucleus of vagus Xm, which is divided rostrocaudally into separate subnuclei innervating branchial
arches 2– 4 (Xm 1–3), branchial arch 5 plus branchial cardiac nerve (Xm4), and heart plus anterior regions of gut (X vis).
Lateral nucleus of vagus (Xl) supplies axons solely to branchial cardiac nerve. Hypobranchial nucleus (hy) supplies
axons to feeding muscles via occipital nerve XI and anterior spinal nerves (not shown). Other labels indicate cerebellum
that overhangs medulla and is outlined by heavy line, a cerebellar auricle (aur) octavus nerve (VIII) and spinal canal
(spc). Obex marks position where roof of 4th ventricle is devoid of nervous tissue rostrally and covered by choroid
plexus. Left: a diagrammatic transverse section (T.S.) taken at obex; through medulla of dogfish (indicated by divided
line on right panel) to show vertical and horizontal disposition of vagal and hypobranchial nuclei. Labels indicate a vagal
rootlet (X), dorsal motor nucleus of vagus (Xm), lateral vagal nucleus (Xl), vagal sensory nucleus (Xs), hypobranchial
nucleus (hy) and sulci in 4th ventricle (4th V), namely, sulcus medianus inferior (smi), sulcus intermedius ventralis (siv),
and sulcus limitans of His (slH). [From Taylor (608).]
and Xth cranial nerves fire sequentially in the order of the
sequential rostrocaudal distribution of their motonuclei in
the brain stem and rostral spinal cord. The resultant
coordinated contractions of the appropriate respiratory
muscles may relate to their original segmental arrangement before cephalization, an arrangement which is retained in the hindbrain of the fish in the sequential topographical arrangement of the motor nuclei, including the
subdivisions of the vagal motonucleus (Fig. 2). This traditional view of the origin of the jaws and visceral arches
and their innervation (161) has recently been questioned
on the basis of developmental studies of the role of neural
crest cells (215). These suggest a separate origin for the
jaws as feeding structures, independent of the visceral
arches, which combined ventilation with filter-feeding, a
view supported by study of marker genes (586). A possible evolutionary antecedent of the jaws may be the velum
of filter feeding protochordates or larval cyclostomes
(M. A. Smith, personal communication).
Both elasmobranchs and teleosts can recruit an additional group of muscles into the respiratory cycle to
provide active jaw occlusion. These are derived phylogenetically from the forward migration of four anterior myotomes (the hypaxial muscles) to form a complex ventral
sheet of muscle, inserted between the pectoral girdle, the
lower jaw, and the ventral processes of the hyoid and
branchial skeleton. They are associated primarily with
suction feeding and ingestion in water-breathing fishes