This mechanism is different in the case of glands and some other structures which, when active, require an abundant supply of blood. Such organs are provided with vaso-dilator in addition to vaso-constrictor nerves. The most conspicuous example of this is to be seen in the case of the submaxillary gland. The nerve to this gland runs for some distance as an isolated thread—the chorda tympani. Stimulation of the chorda tympani has the double effect of dilating the arteries of the gland and of causing it to secrete. But the administration of atropin prevents secretion. Vaso-dilation is then the only visible effect. Stimulation may increase sixfold the outflow of blood from the veins of the gland. It rushes through with such rapidity that it retains its bright arterial hue. The gland also receives a twig from the sympathetic cord in the neck, which, as already stated, controls the vessels of the face. By stimulating the one nerve or the other the physiologist can at will increase or diminish the amount of blood flowing through the submaxillary gland. Stimulating any sensory nerve causes in a reflex manner an increased outflow of constrictor impulses from the centre in the medulla oblongata to all parts of the body, with the exception of the part to which the sensory nerve appertains. Its own constituency receives an increased supply of blood. It is not difficult to appreciate the importance of this double action. A part is injured. The restrictions placed upon its supply of blood are suspended. Lest its increased consumption should lead to a general fall in pressure, all other parts have their supply curtailed. The effect is even more pronounced than this. The whole blood-pressure is raised above its ordinary level. The flow of blood to the injured part is therefore greater than it would be were relaxation of its arteries the only change.
The most important of all constrictor nerves are the splanchnics which control the supply to the stomach and intestines. When these nerves are cut, the digestive organs become engorged to such an extent that a pronounced fall of the general blood-pressure is the result. Their stimulation renders the digestive organs anæmic. We have already shown that the relaxation of vaso-constriction occurs in a reflex manner. The reflex relaxation of the splanchnic area is a matter of great importance, because it can be brought about by stimulation of one of the sensory nerves of the heart. The higher the blood-pressure, the harder the heart would work if left to itself. It is an impetuous organ, always trying to quicken its pace and to increase the force of its beat. Excessive zeal would get it into trouble if severe precautions were not taken to hold it in check. True, it is encouraged by certain “accelerator nerves”—sympathetic filaments which leave the spinal cord by the anterior roots of the second and third thoracic nerves; but the influence which the accelerators exert under normal conditions is not, it would seem, very pronounced. The nerves which restrain the heart are much more in evidence than those which urge it on. The arrangements for diminishing the work of the heart are of two kinds. In the first place, branches derived from the vagus act as a continuous check. From a certain spot in the medulla oblongata, the cardio-inhibitory centre, impulses are always descending to slow the heart. They are of reflex origin, but a high blood-pressure in the centre increases the facility with which they are transmitted. Some of these stimuli originate in the heart itself, ascending and descending the vagus nerve. The remainder come from various sources. A severe injury to any part of the body slows the heart. Injury to the intestines, such as occurs in peritonitis, is particularly effective in increasing vagus inhibition. Slowing of the heart lowers blood-pressure. When both vagi are cut, the heart begins to gallop whatever may be the pressure against which it has to work.