Chapter IV: Anatomy1 (3)

The _accessorius_ (XI.) is not in fishes separated as a distinct nerve from the vagus.

With increased development of the brain its hinder portion, giving rise to the vagus system, has apparently come to encroach on the anterior portion of the spinal cord, with the result that a number of spinal nerves have become reduced to a less or more vestigial condition. The dorsal roots of these nerves disappear entirely in the adult, but the ventral roots persist and are to be seen arising ventrally to the vagus roots. They supply certain muscles of the pectoral fins and of the visceral arches and are known as spino-occipital nerves.[54]

These nerves are divisible into an anterior more ancient set--the
occipital nerves--and a posterior set of more recent
origin--(occipito-spinal nerves). In Selachians 1-5 pairs of occipital
nerves alone are recognizable: in Dipnoans 2-3 pairs of occipital and
2-3 pairs of occipito-spinal: in Ganoids 1-2 pairs occipital and 1-5
pairs occipito-spinal; in Teleosts finally the occipital nerves have
entirely disappeared while there are 2 pairs of occipito-spinal. In
Cyclostomes no special spino-occipital nerves have been described.

The fibres corresponding with those of the _Hypoglossus_ (XII.) of higher vertebrates spring from the anterior spinal nerves, which are here, as indeed in Amphibia, still free from the cranium.

_Sympathetic._--The sympathetic portion of the nervous system does not in fishes attain the same degree of differentiation as in the higher groups. In Cyclostomes it is apparently represented by a fine plexus with small ganglia found in the neighbourhood of the dorsal aorta and on the surface of the heart and receiving branches from the spinal nerves. In Selachians also a plexus occurs in the neighbourhood of the cardinal veins and extends over the viscera: it receives visceral branches from the anterior spinal nerves. In Teleosts the plexus has become condensed to form a definite sympathetic trunk on each side, extending forwards into the head and communicating with the ganglia of certain of the cranial nerves. (J. G. K.)