If the spinal cord of an animal be cut across near the medulla oblongata, respiration being maintained by pumping air into and out of the lungs, the heart continues to beat with undiminished force, but the pressure in the large arteries falls to one-third of its normal height. Constricting impulses no longer pass down the spinal cord from the vaso-motor centre. This experiment also illustrates the truth of the statement that models of the vascular system—arrangements of pumps and indiarubber tubes—are more likely to mislead than to inform. In an artificial schema the relaxation of the constriction of the small tubes on the proximal side of the capillary vessels would reduce friction. Fluid would reach the capillaries in larger quantity, and pass through them more quickly. The pressure in the tubes which represented veins would consequently approach more nearly to that on the arterial side. But when the spinal cord is divided the pressure falls in the veins, as well as in the arteries. This is due to another factor, and one of very great importance in the regulation of the circulation. The blood from the digestive organs is collected by the “portal system” of veins. These do not join the inferior vena cava; they go to the liver, where they again break up into capillaries. It is not until after this second distribution through minute vessels that the blood is re-collected by the hepatic veins and forwarded to the heart. As in the case of the arteries, the portal system of vessels is controlled by the nervous system. When the spinal cord is divided they also dilate. The whole vascular system becoming more capacious, blood-pressure falls in veins as well as in arteries.
When the digestive organs are active, other parts of the body are kept short of blood. It chanced to the writer, in his student days, to spend the early summer in Paris, with a big healthy Yorkshireman as companion. We dined together each night at one of the restaurants of the Palais Royal à prix fixe. After dinner, with British regularity, my friend called for the Times. Then followed a short period of placid reading, interrupted by the remark: “How cold it is!” Half an hour later, giving himself a shake: “Suppose we go and dine somewhere else?” His well-ordered digestive organs had made short work of the two-franc dinner. They had been ably supported by the vaso-motor system of nerves which provided them with the bulk of the blood, while limbs and skin ran short.
Vaso-constrictor nerves leave the spinal cord by the roots (called “rami communicantes”) of sympathetic ganglia. Beyond the ganglia they apply themselves to the large arteries whose course they follow. The constrictor nerves for the face and neck leave the spinal cord within the chest by the roots of the first four thoracic nerves. They do not at once apply themselves to the great artery of the head. Until the upper part of the neck is reached, they traverse the ganglionated sympathetic cord, which lies behind the carotid artery and internal jugular vein. If in a rabbit this cord be cut, the vessels of its ear dilate, as evidenced by the rosy blush which is observed when a light is held behind it. If the upper part of the sympathetic cord be stimulated, the ear grows pale. The redness of the ear remains for many days after section of the nerve; but gradually the engorgement diminishes, and the vessels acquire the power of automatically regulating the flow.
The classical experiment with the rabbit’s ear suffices to show the relation of bloodvessels and nerves which holds good for all areas of the skin. The condition of the skin is the chief factor in regulating the temperature of the body. In a cold atmosphere its vessels are severely constricted to limit loss of heat. When one passes into a warm room the constriction is relaxed. The skin is flushed; heat is thrown off by radiation. The sweat-glands secrete water, which is evaporated by the heat of the skin. Constriction and remission of constriction are the processes which diminish or increase loss of heat.
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.
A sensory nerve of the heart, termed the “depressor,” is the chief agent in lowering blood-pressure. Its course is not the same in all animals, but it runs more or less in conjunction with the vagus. Usually it joins its superior laryngeal branch. Impulses which ascend this nerve inhibit the constriction of the splanchnic vessels. They open a floodgate which brings down the general pressure. The severe pain and extreme distress of angina pectoris are the cry of the heart when blood-pressure is too high—when it feels unable to work against it. This was recognized by physiologists long before a remedy was known. A systematic search was instituted for a drug which could be used with safety to lower blood-pressure. The discovery that the inhalation of amyl nitrite answers this purpose and fulfils this condition was the result.
Fig. 13.—Manometer for measuring Blood-Pressure.