These are usually studied either by directly observing alterations in the size of the vessels in some thin structure, such as the ear of a rabbit, the mesentery, tongue, lung, web, or mylo-hyoid muscle of a frog, or the wing of a bat; or the rate of the flow may be observed. This can be conveniently done by cutting some part, as the toe of a frog, and noticing the rate at which the blood flows from the cut vessels with and without the administration of the drug to the animal. It is often necessary that an artificial circulation should be maintained; for if not, it might be difficult to prove that the alteration in the quantity of blood flowing from the cut surface was not due to influences acting on the cardiac mechanism. In order to determine if the changes are due to local or central effects, it is necessary to destroy the spinal cord, or to cut the nerves going to the part. When a drug is applied locally, as to the mesentery, and the vessels alter, if the nerves going to the part are not cut, it is difficult to say whether this alteration is direct or reflex.

Drugs are applied to the interior of vessels by injecting them into the circulation.

We know that each vessel is controlled by vaso-constrictor and vaso-dilator nerves, and that these proceed by different paths from the central nervous system, but we do not know by which set of nerves drugs act; probably some by the vaso-constrictor, and some by the vaso-dilator. We can only classify the drugs into those which dilate or contract the vessels by local action, and those which produce these effects through their action on the central nervous system. When a drug acts locally we cannot tell whether it "acts on the muscle in the wall of the vessel, or on the nerve terminations.

It of course follows that drugs acting on the heart, or on a large area, will considerably modify the blood-pressure.

A. Drugs acting locally on Vessels.

Drugs which, when locally applied to vessels, dilate them:

(1) Liquor Ammoniac.

(2) Silver nitrate

(3) Zinc chloride

(4) Copper sulphate

(strong).

(5) Mercuric nitrate.

(6) Arsenous acid.

(7) Antimony and potassium tartrate.

(8) Iodine.

(9) Chlorine.

(10) Mineral acids (strong).

(11) Alcohol.

(12) Ether.

(13) Chloroform.

If prevented from evaporating.

(14) Carbolic acid.

(15) Creosote.

(16) All volatile oils, as oils of turpentine, and many substances containing them, as mustard, horse-radish etc.

(17) Senega.

(18) Chrysarobinum.

(19) Ipecacuanha.

(20) Capsicum.

(21) Croton oil.

(22) Camphor.

(23) Cantharides.

(24) Phosphorus.

(25) Warmth, however applied, but usually as a poultice. This is true if its evanescent effect is desired. When long applied it contracts blood-vessels.

Irritants

All of the above, as they dilate the vessels, are often spoken of as vascular irritants.

Rubefacients are drugs which, when locally applied to the skin, cause it to become red because of the vascular dilatation induced. All the above drugs are rubefacients.