Both the quickness of the heart's beat and the contraction of the arteries are regulated by the nervous system; and it is generally by their action on it that drugs alter the blood-pressure, though it must be constantly borne in mind that they may also do so by acting directly on the muscular walls of the heart and arteries themselves. The parts of the nervous system chiefly concerned in regulating the circulation are :

I. The motor cardiac ganglia which lie in the walls of the heart, and are under ordinary circumstances the cause of its rhythmical action.

II. Inhibitory nerves, which render the heart's action slow, and, if irritated very strongly, may stop its beating altogether, and produce quiescence in diastole. The inhibitory fibres have their origin or roots in the medulla, and proceed in the vagi to the heart. In probably all the higher animals they are normally in more or less constant action. In men and dogs they maintain a well-marked action; and, after they are cut or paralysed, the heart beats in the dog three or four times as quickly, and in man twice as quickly, as before. In rabbits and cats they act less, and their division only makes the heart go one-half or one-fourth faster. In frogs they are not in constant action, so that their section does not usually quicken the beats of the heart in these animals.

A drug may irritate them, and render the heart's action slow 1. By acting directly on (a) their roots in the medulla, (b) their ends in the heart;

2. Indirectly, through its action on other parts, producing (a) increased blood-pressure, or (b) accumulation of carbonic acid in the blood, both of which act as irritants to the vagus roots;

3. Reflexly, through irritation of sensory nerves, e.g. irritation of the intestines; of the sympathetic nerve; of the depressor; or of certain afferent fibres in the vagus. Reflex irritation is only likely to be caused by drugs having a powerful local action.

Drugs may also paralyse the inhibitory, or the ends of inhibitory, nerves in the heart, and thus quicken the heart.

Inhibitory ganglia have been supposed to exist in the heart, and certain drugs, such as muscarine, are supposed to slow its pulsations by their action on these ganglia. They have been supposed to be distinct from the ends of the vagus (p. 313), although generally when the ends of inhibitory nerves in the heart are spoken of, the inhibitory ganglia are included in the term.

III. Quickening Nerves. - These belong to the sympathetic system. They have their origin in the brain or medulla, pass down through the cervical part of the spinal cord to the last cervical and first dorsal ganglion (which in many animals are united), and thence through the third branch of the ganglion to the heart. Quickening fibres are said by some to run also in the cervical part of the sympathetic cord. In the frog the accelerating fibres pass from the spinal cord in the anterior root of the third nerve into the ganglion on the trunks of the glossopharyngeal and vagus and thence in the vagus trunk to the heart (Gaskell). Unlike the inhibitory nerves, the quickening nerves are not normally in constant action in mammals.

The accelerating centres may be stimulated 1. By the direct action of drugs upon them.

2. Indirectly by the drugs producing a diminution in the blood-pressure. Such a diminution acts as a stimulus to them.

Fig. 93.   Diagram to show the supposed relation of motor ganglia in the heart to accelerating fibres. A, accelerating fibres proceeding from the cerebro spinal or sympathetic nervous systems to the motor ganglia of the heart.

Fig. 93. - Diagram to show the supposed relation of motor ganglia in the heart to accelerating fibres. A, accelerating fibres proceeding from the cerebro-spinal or sympathetic nervous systems to the motor ganglia of the heart. G, motor ganglion. a, accelerating fibres passing from the endocardium to the motor ganglion. m, motor fibres to the cardiac muscle. h, the cardiac muscle. [For the sake of simplicity in this diagram all hypotheses regarding separate motor and accelerating ganglia hare been disregarded.]

It is probable that accelerating fibres also pass to the cardiac ganglia from the endocardium, for irritation of the interior of the heart, either mechanically or by the injection of irritating drugs into it, causes acceleration. The supposed relationship of the various accelerating fibres to the cardiac ganglia is shown in the accompanying figure (Fig. 93).

IV. Vaso-motor Nerves, which cause the smaller arteries, and probably also the capillaries, to contract. These belong to the sympathetic system; and the most important of them are contained in the splanchnics, which when stimulated produce contraction of the intestinal vessels. As these vessels can, under certain circumstances, hold all the blood in the body, the influence of the splanchnics over the blood-pressure is very great; and division of them can lower it, or stimulation of them increase it, very much. The intestine being much longer in herbivora than carnivora, the splanchnics have a greater influence over the blood-pressure in the former. The chief centre of the whole vaso-motor system seems to be in the medulla oblongata; and it is generally in constant action, keeping up a certain amount of contraction or tone in the vessels. There are also, however, subsidiary centres in the spinal cord, and possibly also in the ganglia of the sympathetic system.

The activity of the vaso-motor centres may be increased (cf. p. 276), and the vessels made to contract 1. By direct irritation of these centres.

2. By reflex irritation through (a) the cervical sympathetic, (b) the vagus, when the brain is intact, and the animal not narcotised, (c) sensory nerves, including the splanchnics themselves. When the medulla is separated from the rest of the body by dividing the spinal cord at the atlas, it can, of course, no longer exert any influence over the vessels; they consequently become dilated throughout the whole body, and the blood-pressure usually sinks very low. If the lower end of the divided cord be then irritated, the vaso-motor nerves which pass through it from the medulla to the body are stimulated, and the blood-pressure rises.

It is probable that the peripheral ends of the vaso-motor nerves in the vessels themselves may be either stimulated or paralysed by the action of drugs conveyed to them by the general circulation.

V. Depressor nerves. - Irritation of these nerves is conducted to the vaso-motor centres, and acts on them in such a way as to cause a reflex dilatation of the small vessels, either (1) generally throughout the whole body, or (2) locally in one particular part of it.

1. The chief nerve which causes dilatation, especially affecting the intestinal vessels, is one which runs from the heart to the medulla, and is called, from its power of diminishing blood-pressure, the depressor nerve. Its fibres seem to be included in the vagus in the dog; but in the rabbit it generally runs separate from the heart to the level of the thyroid cartilage; here it divides into two so-called roots, one root going to the superior laryngeal, and the other to the vagus nerve. These are generally called roots, though, as the nerve conveys impressions from the heart to the brain, they are, physiologically, really branches. There seem to be also depressor fibres in the vagus itself; but the vagus contains fibres of many kinds, and, among others, some which cause reflex contraction of the vessels and rise of blood-pressure - hence called pressor-fibres. The depressor-fibres of the vagus seem to act on the vaso-motor system through the medulla itself, while the pressor-fibres affect it through a centre in the brain, so that, when the brain is perfect, irritation of the central end of the vagus causes increased contraction of the vessels and raised blood-pressure; but, when the brain is removed or its functions abolished by opium, it causes dilatation of vessels and diminished pressure.

2. When a sensory nerve is irritated, the action of the vasomotor centre is suspended in the part supplied by the nerve, and in those which immediately adjoin it, so that their vessels become dilated, while at the same time contraction of the vessels in other parts of the body is produced. The blood-pressure is thus increased generally, and produces in the locally dilated vessels a very rapid stream of blood. This fact was first discovered, and its therapeutics indicated, by Ludwig and Loven.

The causes of alteration in blood-pressure as well as in the pulse-rate, will perhaps be more easily seen from the table on the next page.