Drugs in this section are conveniently divided into four classes, namely: heart stimulants, heart tonics, heart sedatives, and those which act on the blood-vessels (a) by contracting them, and (b) by relaxing them.

Heart Stimulants - Ammonia. Alcohol. Ether. Camphor.

Heart Tonics - Digitalis. Strophanthus.

Heart Sedatives - Aconite Belladonna.

Drugs which contract the Blood-Vessels - Acetate of Lead. Gallic Acid. Ergot. Witch-Hazel.

Drugs which relax the Blood-Vessels - Nitrite of Amyl.

Disordered blood-distribution may be due either to the heart or its blood-vessels acting improperly, or to both.

In another section the composition of the blood has been described, and the method of its distribution in health; but an animal may possess good blood and perfect vessels by which to convey the life-fluid to all parts of the body, and yet perish, if the heart fails in its work as a central pumping station, or if the blood-vessels of one organ are surcharged, while the nutrient supply fails to reach others.

The causes of disordered circulation are irregularities of the heart, whose contractions may be too forcible, too feeble, or irregular; or the heart may be capable of doing its work efficiently, but the vessels may offer undue resistance by reason of their calibre being abnormally contracted; or, on the other hand, they may be so far relaxed as to permit too free a flow of blood to a particular organ, causing congestion.

Remedies which affect the heart and blood-vessels are therefore of great importance, and deserve careful consideration.