I. Physiological Relations

The function of the heart is to drive a certain amount of blood through the whole length of the circulatory system within a given time. In its flow through the small arteries and capillaries, the blood meets with great peripheral resistance, and is dammed back, as it were, upon the larger arteries, which by virtue of the elasticity of their coats are constantly distended, and exert an equal and opposite pressure on the blood. The intermittent action of the heart is thus converted into a continuous force, the arterial blood-pressure, which (thanks to the aortic valves) urges the blood forwards in a steady stream.

The surface of the blood-stream is broken only in the arteries by the wave raised by each fresh discharge from the heart, and this wave is called the pulse.

The heart performs its work by virtue of being a nervo-muscular organ, freely supplied with blood by the coronaries. The muscular tissue is normally stimulated to contract by the intra-cardiac ganglia, which, whilst automatic in action, are excited by impressions coming from the inner surface of the heart-chiefly impressions of pressure or resistance; and the vigour of systole is in direct proportion to this pressure, which in turn is referable, partly to the auricular charge, and partly to the resistance ahead. The movements of the heart are regulated by the cardiac centre in the medulla, which is that part of the nervous system where afferent impressions are first received, and then reflected as motor impulses to the heart, either by the vagus or by the sympathetic, the terminations of which are connected with the cardiac ganglia. An impression made upon the terminations of the vagus diminishes the frequency of the nervous discharges from the ganglia, that is, inhibits the contractions of the heart; an impression made on the terminations of the sympathetic accelerates them. With regard to the heart- or pulse-rate, it is highly important to observe that the length of systole varies very little: whatever the work done or to he done, the ventricle takes 4/10 to contract. The part of the cardiac revolution that varies in length is the diastole, which is sometimes long, giving an infrequent pulse-rate, say 50, sometimes short, giving a frequent pulse-rate, say 100. Now, during diastole the nervo-muscular apparatus rests and is nourished, and the ventricles are filled from the auricles and veins. An infrequent pulse is thus (to a certain extent) an indication that the heart is being rested and filling well, whilst the force of the systole is not weakened, probably the reverse, by these two effects. Agencies which thus affect the rate of the heart through the terminations of the vagus and sympathetic, either reach them through the coronary blood, such as drugs, or are transmitted from the central nervous system through the nerve-trunks. Central impulses affecting the force of the heart probably reach it through the same channels.

The cardiac centre in the medulla is the centre of an area of impressionable matter, which is as extensive as the nervous system itself. Into this centre there pour constant streams of impressions from the vessels, abdominal viscera, skin, muscles, central nervous system (including the seat of mind), from the lungs, and indeed from every organ, including the heart itself; and thence the resulting impulses descend through the vagus and sympathetic to the heart, which is thus subject to every influence, however slight, to which the body may he exposed. Further, the cardiac centre is affected by its blood-supply, including both the quality and pressure of the blood within it.

Amongst the afferent impressions reaching the cardiac centre, those from the heart itself travel through the vagus. These are partly impressions of common sensibility, which pass through the medulla into the convolutions; and although normally too feeble to be perceived, may, if powerful, give rise to sensations of pain, distress, weight, and palpitation, referred to the praecordium.

The arteries are active, irritable muscular tubes, whose calibre can be modified by a variety of influences. A local nervous mechanism guides the vasor muscles; vaso-motor and vaso-dilator nerves pass between the local mechanism and the central nervous system; and there is a great central point in the medulla oblongata, called the vaso-motor centre, as well as other lower centres in the cord and brain, which collect impressions from every part of the body, and reflect them through the vaso-motor or vaso-dilator nerves, as the case may be, to the vessels. The muscular coat of the arteries, being constantly exercised to a degree, gives so-called "tone" to the vessels, which is one of the elements of that cardinal factor of the circulation, the peripheral resistance. The more active the vaso-motor nerves or centres, the greater the resistance and the higher the blood pressure; the more active the dilator, the lower the pressure; and the influence of each upon the heart respectively corresponds. Particular vascular areas, e.g. those of the skin and mesentery, may also be dilated or constricted independently of others. Manifestly local dilatation will admit more blood to the part, and so lower the general arterial pressure; local constriction will increase the local resistance, and so raise the general pressure. Amongst the impressions which influence the vaso-motor centre are mental states, visceral conditions, surface temperature and sensations of all kinds. It is also stimulated by deficiency of blood within itself, and by poverty of the blood in oxygen, and drugs act directly upon it as we shall presently see.

The afferent impressions which reach the vaso-motor centre from the heart are so important to the therapeutist that they demand special mention. When impressions originating in overdistension, distress, or failure of the heart, reach the cardiac centre through the vagus, they are transferred to the vasor centre, whence they are reflected to the vessels through the dilator nerves. The vessels are thus relaxed; the arterial pressure, which the ventricle has to overcome, falls; the heart empties itself more readily, and is relieved. This arrangement 2 E-8 for reducing the intercardiac pressure is called the depressor mechanism of the circulation.