There is a large group of drugs which, like many of the foregoing, show marked pharmacodynamic action on several tissue systems of the body, and hence on more than one physiological function. Their chief use, however, is to modify, in some definite manner, the circulatory apparatus. They either bring about a marked regulating action on the rhythm of the heart and act on the vessels, as digitalis; or act almost alone on the blood-vessels, as adrenalin or ergot: still others acting purely as reflex stimulants to the heart action, as many of the xanthines, or act distinctly as cardiac depressants, as aconite.

These will be here taken up as a series of groups of drugs with which one can modify the conditions of the circulation. A large number of other factors enter into their full physiological activities, but these are laid aside for the moment.

In this present instance these remedies will be grouped as follows: (1) The Digitalis group, composed largely of drugs that exert a predominant stimulant action on the vessel walls, on the heart walls, and on the cardio-inhibitory apparatus; (2) Adrenalin group, in which the chief activity is exerted on the tissues of the walls of the blood-vessels; (3) the Xanthine or Caffeine group; (4) the Nitrite group, in which the main action consists of vessel dilatation; and (5) the Aconite group, in which heart-muscle and heart-ganglion depression are prominent and vasodilatation is pronounced.

It is preferred to eliminate the terms cardiac stimulants and cardiac depressants, hoping that the student should have in mind the general agents at his command to regulate the circulation, if such regulation is called for in treatment. As opportunity offers, the circulatory action of a number of the drugs already discussed under other headings will be considered. Thus, while alcohol exerts its main action on the nervous system, it plays an immensely important role as a remedy to modify cardiac and circulatory activity.