One of the chief offices of the sympathetic nerve is to preserve tone of vessels. The nerves that insure tone in vessels issue from the sympathetic. They are always active and never in repose, - a characteristic of the sympathetic nerve. They pass to the muscular coat of the vessels and act as their permanent guardian, in preserving permanent vascular tone. Variation in this tone constitutes incipient disease.
Congestion is only the abolition of vascular tone.
Goltz's percussion experiment demonstrates the reflex nature of the action of vaso-motor nerves, as by tapping on the exposed viscera he could produce dilatation of their vessels. Hence in this case the centers for reflex action must lie in the walls of the vessels themselves. All these factors exist on and adjacent to vessels. For the reflex centers of vaso-motor movements we may look to the cardiac ganglia, the abdominal brain or, especially, to the ganglia around the vessels or in their walls. First, we must consider the abdominal brain, the semilunar ganglia or solar plexus, in the physiology of the sympathetic.