A U-tube contains mercury, on which floats a rod supporting a scratching point, which makes a “tracing” on blackened paper wrapped round a revolving drum. Between the manometer and the cannula which is introduced into the central end of a cut artery is a three-way cock, which leads to a pressure-bottle containing a half saturated solution of sodic sulphate. This solution prevents blood from clotting. Before it is connected with the artery the apparatus is filled from the pressure-bottle. The cock is then turned into the second position, and the bottle raised until the mercury in the manometer stands at a level somewhat higher than that which it may be expected to attain under the influence of blood-pressure. The cannula being then inserted into an artery, the cock is turned into the third position, which places the manometer in connection with the blood, and excludes the pressure-bottle. As the mercury is a little higher than blood-pressure, some of the sodic sulphate solution enters the artery, but no blood enters the cannula. The scratching point, rising and falling with every variation in blood-pressure, makes a record on the soot-blackened paper, which is subsequently removed from the drum, and varnished.
When we consider the hydrostatics of the circulation, it becomes evident that changes in the force with which the heart beats, and changes in the calibre of the bloodvessels, work together in determining blood-pressure. Both vessels and heart contract automatically—the former continuously, the latter rhythmically. The heart of a frog, if it is enclosed in a moist chamber, beats for a long time after its removal from the animal. Even when cut in pieces, in certain ways, the separate pieces beat. A strip from the ventricle of a tortoise’s heart, kept gently stretched by the weight of a light lever attached to one of its ends, continued to contract rhythmically for forty-eight hours. When the heart has come to a pause, it cannot be started again by stimulating any nerve. It has in the most marked degree its own views as to the rapidity and force with which it ought to beat. But within certain limits it is under nervous control. The accelerators hasten it, to its own detriment. They belong to the division of katabolic nerves—a name given them to indicate that they waste the tissues, impoverishing their condition. The vagus nerve slows the heart. It protects it from itself. Its action is anabolic. The condition of the heart is improved under its influence. If it has been kept in check for a time by stimulation of the vagus, the heart beats more strongly when this nerve ceases to act than it did before it was induced to rest.
The arteries also are under the influence of two antagonistic sets of nerves. Those which increase their tonic contraction are almost universal in their distribution. It may be that those which actively check it are equally widespread, but the evidence is not altogether free from ambiguity. On certain organs—such as the salivary glands, already instanced—which require great variations in the amount of blood supplied to them, the influence of dilator nerves is very marked. The simplest hypothesis as to the mode of action of vaso-constrictor and vaso-dilator nerves leaves the initiative with the muscle-ibres of the vessel-wall. The distending internal pressure of blood is the stimulus which induces the muscle to contract. In some invertebrate animals—the snail, for example—if blood be prevented from entering the heart, so that there is no distending pressure, the heart stops. In higher animals the heart has acquired a habit of contracting, which keeps it going in the absence of its proper stimulus. The two classes of nerves exercise opposing influences on the muscle. Vaso-constrictor nerves increase the excitability of its fibres; vaso-dilator nerves diminish it. Only thus can we explain their action on a common basis. A good deal might be said as to the reasonableness of such an explanation. Our views as to the relation of nerve-influence and muscle-contraction are apt to go astray, owing to the fact that generations of physiologists have observed the phenomenon of a spasm of a muscle following on a sudden stimulus to a nerve. The two events are evidently related. The stimulus appears to set up a new condition in the nerve—to initiate a process which was not occurring before the electric current was passed through it. The muscular spasm equally appears to be an isolated event. As usual, we are misled by the analogy of human inventions. We compare the nerve-impulse to the fall of a hammer, the muscle-spasm to the explosion of gunpowder. We forget that nerve and muscle are in permanent connection; that the impulse is a sudden exaggeration of an influence which the nerve is continuously exerting, the contraction an exaggeration of metabolic changes which are constantly occurring in muscle. (See in this connection the explanation of muscle-tone, [p. 273].) In the case of plain muscle, nerve stimuli do not cause contraction; they merely increase the excitability of the muscle. It may be more difficult for us to figure to ourselves the way in which dilator nerves diminish excitability; but the existence of such an anabolic influence is beyond the reach of doubt. Heart and bloodvessels are part of the same system. The heart has its accelerator and inhibitory nerves, the bloodvessels their constrictor and dilator nerves. For both vessel-wall and heart the stimulus to contraction is the distending pressure of blood—although it is not altogether necessary that this stimulus should be acting at the time. Sympathetic and vagus nerves can to a certain extent control the beating of a bloodless heart. The heart-tissue has acquired the habit of beating, and the habit of listening to advice conveyed to it through these nerves.
The self-adjustment of the blood-tubes to the pressure to which they are exposed is exhibited in the adaptation of their degree of contraction to the position of the body—to the weight, that is to say, of the column of fluid which they have to support. Everyone has played the game of “right hand or left.” When the hand is held above the head the blood leaves it, and the hand becomes cold; but if there be need for adjustment, and time is given for the mechanism to come into play, it works to perfection. When we are standing erect, there is neither too much blood in the feet nor too little in the head. But after a fortnight in bed a convalescent finds, the first time that he stands upright, that his legs are quickly engorged—his slippers after a few minutes feel too tight for him—whereas the brain becomes so anæmic that he turns giddy, or even faints.
Numberless illustrations of vaso-motor action are met with in daily experience. It is a curious fact that the nerves which control the calibre of the bloodvessels tend to overact their part. When an organ demands more blood, it is supplied at the expense of the rest of the body, and especially of the parts most nearly adjacent. This is partly a mechanical effect. If all the houses in a terrace are supplied with water from a common main, the bursting of a water-pipe in one of them will reduce the supply of its neighbours more than it will reduce the supply of houses in distant parts of the town. But vaso-motor nerves, in their compensating adjustment, go farther than this. A thimbleful of blood removed by a leech produces an effect upon an underlying engorged organ altogether out of proportion to the hydrostatic requirements of the case. “Cupping” the loins diminishes the congestion of the kidneys. This is the explanation of the curative efficacy of various agents which, with improvements in surgery and the introduction of more reliable drugs, have almost disappeared from the surgeon’s armamentarium—scarification, blisters, setons, and the like. Such methods have been relegated to veterinary practice.
There is a marked tendency to see-saw between the skin and the mucous membrane of the alimentary canal. During active digestion, when the “splanchnic area” is full of blood, the skin is cold. Hot fomentations, by dilating the vessels of the skin, diminish congestion of the alimentary tract. An inflamed throat is relieved by a compress round the neck. Conversely, it must be admitted that, in certain persons, slight constriction of the vessels of the skin induces inflammation of the mucous membrane. This is one reason for the almost universal dread of draughts. A draught cools a limited area of the skin. Some of us cultivate a love of draughts. They are the sensible evidence of the entrance of fresh air. Yet we admit reluctantly that certain fragile mortals are not altogether fanciful in supposing that a draught may give them a catarrh or a toothache. If asked why they object to draughts, many persons answer that they “are afraid of catching a chill”—carrying us back to the time before the clinical thermometer was invented; to days when the shivering fit, or “rigor,” which first calls attention to the fact that the temperature is already two or three degrees above the normal, was supposed to be the commencement of the illness. The patient imagined that the “chill” caused him to shiver, and that if he had not “caught” it he would not have been ill. The substitution of the term “cold” for “rheum,” naming the malady after one of its prominent symptoms, has done much to perpetuate this superstition. “Chill” is a word we scarcely dare to mention. When doctors could no longer attribute to witchcraft the occurrence of disorders for which they had no other explanation, they invented the luminous theory that inflammatory diseases—especially those of the stomach, liver, and lungs—were produced by “a chill.” At one time all diseases which were not evidently infectious were caused by chill. The discovery of germs and the recognition of their maleficent activity has stripped this cloak of ignorance off almost every case of abnormal tissue-metabolism. It is recognized now that the germ is the disease, not the effects which the germ produces. Pneumonia is impossible in the absence of the pneumococcus, however severe the chill to which the patient was exposed when out in the cold and wet. Consumption is the effect produced by the tubercle bacillus. If there are no bacilli, there can be no consumption. Yet these two diseases illustrate the possibility of the use of the term “chill” without impropriety. The coccus of pneumonia may frequently be found in the mouth of a healthy person. If everyone with whom the tubercle bacillus has at some time come in contact were inevitably its victim, no human being would be free from phthisis, if any still survived. There are conditions of health, or rather of unhealth, in which the economy is less resistant than usual to the germs. Apparently the vaso-motor disturbances of internal organs caused by the cooling of the surface of the body, if it occur when health is otherwise depressed, contributes to the production of such a state.
The vaso-motor system is influenced by emotions. It is a little difficult to express accurately the relation between emotion and vaso-motor change. Some psychologists regard the vaso-motor change as the emotion. “All emotions,” says a prominent exponent of this view, “are wholly due to excitation of a particular kind of the vaso-motor centre.” The person about to be subject to an emotion of shame, anger, fear, disgust, recognizes a fact or circumstance, or conjunction of circumstances, which justifies the emotion. (We are assuming that emotions may be justified; that the intellectual appreciation of a situation and reasoned decision regarding the action which it demands is not sufficient.) This recognition as an intellectual act of the higher brain is accompanied by certain forms of enhanced activity or inhibition of activity of the vaso-motor centre in the medulla oblongata which cause changes in the degree of contraction of the bloodvessels of certain organs. The vascular changes produce an alteration in the state of the organ which is reflected in nerve-currents sent back to the brain, providing the background of feeling which constitutes emotional tone. We are not prepared to endorse this extreme view of the nature of an emotion. A maiden’s blush is not an emotion of embarrassment or shame. It is its harmony. Her mind plays the air. The sensations which originate in the flushed skin of the face sustain it with their accompaniment. The emotional tone keeps attention fixed on the fact or circumstance which led her to conclude, by the exercise of her reason, that she was placed in an awkward situation. This fixing of attention is frequently so pronounced as to inhibit all other intellectual action. The maiden is less quick than she would have been, had the emotion not glued her thoughts together, in recognizing the readiest means of extricating herself from embarrassment. All nerves found within the chest and abdomen were in very early times termed “sympathetic.” The cord in the neck was the “little sympathetic.” The name explains itself; but it will be understood that it implied much more in the days when the liver, spleen and heart were supposed to pour out emotions than it does now. The vagus nerve was termed the “middle sympathetic.” Shame inhibits the activity of the vaso-constrictor nerves of the face; dilation of the vessels which they supply is accompanied with constriction of other cutaneous nerves. Kipling must, we think, have embellished Nature when he represents the very unimpressionable hero of Lungtungpen as admitting “I niver blushed before or since; but I blushed all over my carkiss thin.” Usually the carmine of the face contrasts with the pallor and coldness of the hands. Still, we are not prepared to assert that it is impossible, under circumstances as trying as those in which Private Mulvaney and his companions were placed, for all the cutaneous constrictor nerves to let go their grip at the same time. Terror heightens the control of the vaso-motor centre over the vessels of the skin; it increases vagus inhibition of the heart. Even disgust evoked by a revolting sight or a foul smell may call the vagus so forcibly into action as to bring the heart to a standstill.
The Pulse.—The arterial system is always distended. The pressure in the largest arteries amounts to about 140 millimetres of mercury. The source of pressure is the beat of the heart pushing the blood forward against the resistance offered to its flow by the smallest vessels. At every stroke another 3 ounces is added to the already overfull vessels. In the aorta, therefore, the blood moves forward with jerks, but by the time it reaches the capillaries the intermittent accessions of force have been taken up by the elastic walls of the vessels and returned to the stream in the form of constant pressure. In the very smallest arteries the blood flows in a steady stream. If the corpuscles in a capillary vessel are watched under the microscope, they show no variations in rapidity synchronous with the beat of the heart. The “pulse” in the larger arteries is the push given to the column of blood by the sudden contraction of the left ventricle. Its propagation along the arteries will be understood if it is remembered that the blood is contained within elastic tubes. The first effect of the ejection into the aorta of an additional quantity of blood is the distension of its wall. The wave of distension travels down all the arteries of the body with gradually decreasing force.
