The capillaries effect the final distribution of blood to the tissues. Their soft protoplasmic walls, through which the plasma, the oxygen, and the corpuscles pass into the tissues, have irritability of their own, and they are subject to many other influences, viz. those of the nervous system, of the blood which they contain, of the arteries and the veins at either extremity, and of the activity of nutrition. In the capillaries we discover the other element of the peripheral resistance.

The veins convey the blood back to the heart as comparatively passive tubes. They are probably subject to special nervous influences, but they are chiefly influenced physically by the volume of blood passing through them, that is, by the condition of the heart in front and of the arteries and capillaries behind. Thus, shortness of diastole, i.e. frequency of the heart, diminishes the time of emptying the veins, and raises the pressure within them. A low arterial pressure and a free flow through the capillaries have the same effect. Conversely, the veins react physically on the heart and capillaries; if they are dilated and full, the return of the blood to the auricle is delayed, and the force of systole weakened from lowness of the charge, whilst the capillaries are obstructed, and the flow of the plasma and metabolic products between the vessels and the tissues disturbed.

"We can now understand the meaning of the expression, the general blood-pressure. The elasticity of the arteries being taken as constant, the pressure of blood within the arterial system at any given moment will depend upon (1) the total quantity of blood in circulation; (2) the action of the heart; (3) the freedom of the flow into the veins, i.e. the peripheral resistance, due to vasor constriction and capillary obstruction. The arterial pressure is so far self-regulated, through the quantity of blood in circulation, by means of the Malpighian bodies of the kidney. In this mechanism, the general arterial pressure is brought to bear upon a length of unsupported arteriole, so as to press or excrete the water of the blood through the vascular wall into the uriniferous tubule. By the muscular and nervous Structures in the walls of the afferent and efferent arterioles, the pressure upon the glomerulus may be cut off, or thrown on, as the system requires, the result being less or more watery excretion, and corresponding rise or fall of the blood pressure. The perspiratory excretion, and, indeed, all exudations, probably act in the same way as the urinary, only less powerfully.

Another powerful influence on the circulation as a whole is muscular activity, exertion being attended by cardiac excitement and high arterial pressure, and muscular rest by calm action of the heart and a quiet pulse.

II. Pharmacodynamics

The circulatory system affords one of the most striking instances in the body of provisions for physiological change, and of functional reaction to influences of every kind which hear, or may he brought to hear, upon it. Herein lie at once its power of accommodation to circumstances and its vulnerability; and here, too, the therapeutist discovers his opportunity of influencing the heart and vessels at his pleasure.

1. The total volume of blood in circulation being one of the prime factors of the blood pressure, every change in this volume, whether by abstraction or addition, must alter the pressure. This can readily be accomplished by leeching, cupping or venesection on the one hand, or by transfusion on the other hand. As a matter of fact, however, the effect of either method on the circulation is but temporary. The tension of the pulse falls with venesection, only to rise again quickly by increased absorption of fluids from the tissue and bowels into the circulation. Transfusion raises the blood pressure for a time, but the compensating mechanisms soon restore the previous average pressure. Venesection is therefore the most powerful of all measures for quickly taking the tension off the whole circulation, and relieving the heart and lungs, but it is practically useless for the purpose of permanently reducing the blood pressure; and transfusion is similarly of inestimable value in rapidly restoring the pressure, if it have fallen dangerously low from loss of blood, and thus preventing death by circulatory failure.

2. The Heart

The Heart. a. The intrinsic nervo-muscular apparatus may be either stimulated or depressed. The first direct cardiac stimulant is an active coronary circulation, through which the heart responds to improved quality of the blood in oxygen and plasma, and thus, indirectly, to proper air and food, healthy digestion, and hepatic action. Direct cardiac stimulants include many drugs, such as Alcohol, Digitalis, Scilla, Strychnin, Ammonia, Ether, etc. The continuous battery current applied through the region of the heart acts similarly. Reflex stimulation is a ready and powerful means of increasing the activity of the heart, or of rousing it in actual arrest, and includes the various methods of local nervous stimulation described in chapter xi., especially irritation of the fifth nerve by Ammonia, the cold douche and flagellation, and counter-irritation of the praecordium. Cupping and leeching also exert a stimulant influence on the heart through the nervous system, as well as relieving it by abstraction of blood. Carminatives stimulate the heart, partly directly, partly by reflexion through the central nervous system of their impression on the gastric mucosa. The mind is a powerful instrument for invigorating the heart. Cheerfulness and encouragement may he more useful to a patient than many drugs. Lastly, all measures which lengthen the diastole (slow the heart) increase the cardiac strength by affording more time for rest.

The intrinsic nervo-muscular apparatus may he depressed or soothed by the opposite set of measures; by a low coronary pressure, the effect of low diet, purgatives, diuretics, and diaphoretics; by arresting reflex impulses by means of general, peripheral, and central nervous sedatives, such as Opium, warmth, or plasters applied to the praecordium, and the general warm bath; and by all measures which shorten the diastole, i.e., increase the rate of the pulse. Lastly, we have a number of drugs which are direct cardiac depressants, including Opium, Diluted Hydrocyanic Acid, Aconite, Antimony, Potash, Chloroform, Chloral, Ergot, Veratria, Ipecacuanha and many more.