The afferent nerves of the heart, which carry to the brain the impressions of common sensibility originating in the cardiac tissues, may be depressed by means of Opium, Chloral, Belladonna and its allies, and possibly by heat and cold.

b. The terminations of the vagus in the heart may be stimulated, and the cardiac action rendered less frequent, by Digitalis and Scilla. The same part of the inhibitory mechanism may be depressed, and the rate of the heat increased, by Belladonna, Hyoscyamus, Stramonium, Amyl Nitrite, and large doses of many drugs. These local measures act very powerfully.

c. The cardiac centre in the medulla is readily stimulated by certain drugs, such as Digitalis and Scilla, Ether, Alcohol and Chloroform at first, Strychnia, and Belladonna; and by many peripheral nervous impressions, such as counter-irritation and cold. On the other hand it can be depressed by warm applications to the surface, such as the hot bath, and by certain drugs, including Chloroform and Alcohol after the first stage, Aconite, Antimony, Opium, Chloral, Diluted Hydrocyanic Acid, Ipecacuanha, Nitrite of Amyl, Physostigma, and Conium. Our control of the inhibitory action of the vagus at either extremity, that is, of the frequency of the heart, is of much value from the power which it affords us of influencing the cardiac nutrition and strength, by lengthening or shortening the diastole or resting-time of the ventricle. Thus it will be found that all cardiac retarders are cardiac stimulants, whilst all cardiac accelerators prove in the end to be cardiac depressants.

In this connection muscular exercise and rest must be mentioned as the most powerful and available of all the measures which increase and diminish, respectively, the work and nutritive activity of the heart. Rest in bed, avoidance of walking, carriage exercise, movement on level ground, are a descending series of means of giving the heart rest, and the different kinds of wholesome muscular exercise are equally valuable means of throwing work upon the heart, when its condition demands increased activity.

3. The Arteries

The Arteries. The peripheral resistance in the arteries introduces us to a vast number of pharmacodynamical influences which we must be content simply to enumerate: a. The vaso-motor centre can be stimulated directly by Alcohol and Chloroform (temporarily), by Ether, Ammonia, Strychnia, Digitalis, and Scilla; by irritation of the sensory nerves in any accessible part of the body-for instance, by cold, counter-irritants such as mustard, etc., applied to the calves or soles, by stimulation of the trigeminus, the most ready and powerful means of which is Ammonia held to the nose. On the other hand, the vasomotor centre may be directly depressed, by Alcohol and Chloroform in the second stage, by Opium, Chloral, Diluted Hydrocyanic Acid, Antimony, Ipecacuanha, Aconite, Belladonna and its allies; by muscular rest; by emotional quiet and balance; and by local sedatives, such as anodynes, warmth, and gentle friction.

b. The local vaso-constriclor mechanism in the arterial walls is stimulated directly by Lead and Silver, Digitalis and Squill, in the first stage, Ergot; and by local cold, produced by irrigation with water, by Ether spray, or by evaporation of spirituous, acid, and saline solutions, such as lotions of Rectified Spirit, Vinegar, and Chloride of Ammonium. We call these measures vascular astringents.

Vascular dilatation may be effected through the same local mechanism by the Nitrites of Amyl and Sodium, Nitroglycerine, Alcohol, and Belladonna; by the local heat afforded by poultices and fomentations; by the whole group of Volatile Oils, of which Turpentine and Camphor are the types; by Acrid Oils, including Mustard and Mezereon; by irritant metals and metalloids, such as Zinc, Copper, and Iodine; and artificial carbon compounds, including Creasote, Carbolic Acid and their allies. Local vascular dilators are naturally local circulatory stimulants. The continuous current also causes local vascular dilatation.

4. The Capillaries

The Capillaries. As one of the causes of peripheral resistance, the condition of the capillary areas is an object of great interest to the therapeutist. We can dilate the capillaries and increase the flow through them by either local warmth or persistent cold, by friction, and by local nervous irritants, such as the confined vapour of Spirits, Mustard, Aromatic Oils, and other rubefacients. This is but an early stage of the process of in47° Materia Medic a and Therapeutics.

flammation, characterised by capillary dilatation and escape of the constituents of the blood, which can be induced by a continuation of the same measures, or by excessive heat, Cantha-rides, Croton Oil, etc. (vesicants and pustulants), and markedly modifies, as we shall see in chapter xiv (The Body Heat, And Its Regulation : The Skin)., the capillary circulation of neighbouring parts, and the general blood pressure.

On the other hand, we can contract the capillaries and diminish the flow through them by the application of excessive local cold (congelation and refrigeration), by Lead, and Silver, which are pure astringents; and by the constringents, namely, Tannic and Gallic Acids, and the many vegetables which contain them (Kino, Catechu, etc.), which constringe or "tan" the connective tissues supporting the delicate capillaries, by condensing their gelatinous and albuminous constituents. Some substances, such as Persalts of Iron, may also arrest the circulation in the capillaries, by promoting coagulation of the blood within them.

5. Our influence upon the walls of the veins appears to be but small. The veins of a part may be dilated by hot applications; contracted, and then dilated, by moderate local cold. Ergot is believed by some authorities to relax the venous walls. Indirect measures are more powerful in our hands. The heart a fronte, or the arterial pressure a tergo, may be employed, as we have seen, to increase or diminish the venous pressure. The processes of secretion and excretion are not less powerful in modifying the fulness of the veins. Thus, hydragogue purgatives, as we have seen, drain the portal system; and we shall afterwards find that saline diuretics relieve the renal veins in a very similar way.