That the arteries possessed elastic resiliency and vital contractility which regulated the amount of blood flowing to any given part was observed by John Hunter in studying inflammation.
The muscle cells have long since been clearly demonstrated in the middle coats of the arteries, but nothing was known of the nervous channels which bore the stimulus to the vessels, or the nerve centres which regulated their contraction, until comparatively recent times.
The first definite knowledge concerning special nerves for the control of the muscular wall of the vessels is due to Claude Bernard. He showed that cutting the sympathetic nerve in the neck was followed by an increase in temperature of that side of the head, and a great dilatation of the arteries.
It was further observed that stimulation of the superior ganglion of the sympathetic brought about an opposite result, namely, a fall in temperature and contraction of the vessels on the side at which the stimulus was applied. If the stimulus was increased, the vessels contracted more than the normal, but on cessation of the stimulus they became dilated above the normal and the temperature again rose; the effects of the stimulus gradually passed off. From this it was concluded that the sympathetic in the neck contained constrictor fibres which conveyed impulses causing habitual tonic contraction of the vessel wall, corresponding to what was already recognized as arterial tone. On section of the nerve the tonic contraction disappeared, but on gentle stimulation it reappeared, and if more strongly stimulated an excessive contraction set in causing occlusion of many of the vessels.
Subsequent experiments have shown that all the vessels are supplied with similar vasomotor (constrictor) nerves, section of which causes dilatation, while stimulation causes contraction of the vessels in the territory presided over by the stimulated nerves.
It has also been shown that the vaso-constrictor nerves for all parts of the body come from the cerebro-spinal axis, passing out from the spinal cord as extremely fine medullated fibres (white rami communicantes) by the anterior roots of all the spinal nerves between the 2d thoracic and 2d lumbar. They join the sympathetic, which may be regarded as a chain of vasomotor ganglia, and are distributed to the vessels either as special nerves, branches of the sympathetic, as the splanchnics, or with the general peripheral nerve trunks.
Although stimulation of almost any nerve causes vascular contraction, it has been shown that in some parts stimulation gives rise to an opposite result, viz., vascular dilatation. Thus, stimulation of the chorda tympani or nervi erigentes is followed by dilatation of the arterioles of the submaxillary gland and penis respectively. This dilatation is caused by the nerve impulses checking the normal contraction by inhibiting the activity of the vascular muscles. It is believed that all arterioles may be influenced by such fibres, but the greater power of the constrictor fibres in most nerves prevents their demonstration.
These vaso-dilator fibres also come from the central nervous system, but leave it by routes quite different from those traversed by the vaso-constrictor fibres, and are not connected with the sympathetic ganglia. They pass out above by the vagus and glosso-pharyngeal nerves and below with the lower sacral nerves. No vaso-inhibitory fibres have been found to pass by the other spinal roots.