In 1851, Claude Bernard discovered that, when the sympathetic nerve is cut in the neck of a rabbit, the blood vessels in the ear of the same side become very much dilated. He and other observers afterward demonstrated that, if the peripheral (head) end of the severed nerve is stimulated electrically, the ear becomes blanched, owing to a constriction of the blood vessels. Bernard also discovered a second class of nerve fibers, which, when stimulated, caused a dilatation of the blood vessels in the area supplied.
The vasoconstrictor nerve fibers belong to the sympathetic autonomic system, consisting, therefore, of a preganglionic fiber arising in the central nervous system and a postganglionic fiber arising from the cell of some sympathetic ganglion. These nerves form plexuses in the media of arteries and terminate in contact with the muscle fibers. While the terminations of the nerves are in the media, the adventitia, or