The various experimental results recently obtained on this subject (too numerous to be mentioned here), show that the vascular nerve mechanisms are very complex. The supposition of some such arrangements as the following may help the student.
1. The blood vessels have muscular elements which, though commonly controlled by nerves, are capable of automatic activity. A supply of arterial blood is sufficient stimulus for their moderate action, and mechanical or other local stimulus is capable of exciting increased constriction. We know that such automatic contractile elements exist in some of the lower animals (snail's heart, hydra, etc.), and we have no reason to doubt their existence in mammals. Moreover, such a view obviates the necessity of supposing that local nerve elements exist which cannot be recognized morphologically.
2. In the medulla oblongata there exist nerve cells which exert a constant influence over the activity of the vascular muscles. These groups of nerve cells which compose the vascular nerve centres may be divided into?notor and inhibitory. From these centres impulses of two distinct kinds emanate, the one increasing the action of the contractile elements, and the other diminishing it. They are intimately connected with the centres which preside over the functional activity of the various viscera, and are also closely related to the nerves coming from all parts of the circulatory apparatus.
3. Direct communication between these vasomotor and vaso-inhibitory centres and the blood vessels is kept up by means of efferent nerve channels, some bearing stimulating (vaso-constric-tor) others inhibitory (vaso-dilator) impulses.
4. The activity of the contractile elements of any given vascular area may be altered by influences from different sources, (a) Local influences are brought but little into play, but, if the part be cut off from the nervous centres, they are capable of controlling the local blood supply by changing the degree of local arterial constriction. (/B) Central influences from the medulla are habitually in action, affecting all the vessels and keeping up the vascular tone. These impulses are variously modified by changes occurring in distant parts of the,circulatory apparatus, and can be regarded as a general regulating mechanism. They pass through the sympathetic chain, (r) Special influences, which are associated with the functions of the different parts and organs, are only called into operation during the performance of the function, whatever it may be. These impulses are probably conveyed by the same nerves as excite the various forms of functional activity.
These three modes of regulation have different powers in different parts, and thus we find that section or stimulation of certain nerves gives vasomotor effects which appear contradictory.
Section of a sensory nerve causes temporary vasomotor paralysis, owing to the tonic constrictor influence being cut off. Stimulation of the peripheral stump causes vaso-constriction from excitation of the fibres bearing these impulses.
The stimulation of a motor nerve causes an increase in the flow of blood through the muscle, i. e., is associated with a vasodilator effect, probably dependent on the inhibitory influence of certain efferent fibres which check the local vascular agencies.
Thus we must suppose that there exist local agents under the control of the medullary centres, and that there are distinct sets of efferent, exciting and inhibitory fibres passing between the centre and periphery. One set of fibres lies in the ordinary functional nerve of the part, the other in the sympathetic, which to a great extent runs along the vessels themselves, and forms intricate networks.
As far as we know anatomically there are no local agents other than the muscles in the wall of the vessels. Since the impulses from the centres which can stimulate or inhibit the activity of the local agents travel by different fibres, all the observed phenomena may be explained without supposing local nerve centres to exist.