A Sympathetic Nervous System has
been observed in all the Vertebrata
. It consists, essentially, of two longitudinal
cords, placed one upon each side of the inferior face
of the cranio - spinal axis. Each cord receives communicating
fibres from the spinal nerves of its own side, and, when complete,
from all the cranial nerves except those of the special
senses of hearing, sight, and smell - the Vidian nerves constituting
the anterior terminations of the sympathetic cords. At
the points of communication ganglia are developed, and the
nerves which emerge from these ganglia are distributed to the
muscles of the heart and vessels, and to those of the viscera.
These peripheral nerves of the sympathetic system frequently
present small ganglionic enlargements.
In the Marsipobranchii
, the place of the sympathetic appears
to be taken, to a great extent, by the pneumogastric;
and, in Myxine
, the two pneutnogastrics unite upon the intestine,
and follow it, as a single trunk, to the anus.