These are substances which cause dilatation of the peripheral vessels,1 and thus render the flow of blood through them more rapid. The most important are :

Heat.

Alcohol in its various forms.

Ether.

Nitrous ether.

Dover's powder. Acetate of ammonium.

Alcohol and ether, by stimulating the heart at the same time that they dilate the vessels, render the peripheral circulation very vigorous. From its stimulant action on the vaso-motor centre, ammonia is less useful than alcohol.

Vascular stimulants are useful in equalising the circulation and preventing congestion of internal organs. Thus, from exposure to cold generally so that the whole surface of the body is chilled, or from a local chill due to a draught, or to the combined action of cold and moisture, as in wet feet, congestion of the respiratory tract, or of the stomach, intestines, or pelvic organs may occur. This frequently evidences itself immediately either by rigors or by localised pain. If the congestion be not relieved inflammation may occur, but if alcohol be taken either in a concentrated form or diluted with boiling water, the vessels of the surface dilate, a warm glow is felt throughout the body, the shivering and pains disappear, and frequently all injurious results of the chill are averted. If the external cold, however, is very excessive, and the exposure is to be prolonged, alcohol must be so that it can easily be removed. This allows the heat to come gradually through without burning the skin. For a small gratuity the engine-driver or stoker is usually willing to fill the bag with hot water, and the bag can be refilled if necessary at each station where there is a sufficiently long stoppage. This is sometimes a very great boon to invalids on long railway journeys such as they are often compelled to make on their way to winter health resorts.

used with great care, as the blood becomes much more rapidly cooled when the cutaneous vessels are dilated than when they are contracted; and in arctic temperatures a person is much more readily frozen to death after the free use of alcohol. Dover's powder is also a useful vascular stimulant, though less powerful and rapid than alcohol. It is of use in similar cases to those just described, and may be given after the alcohol to supplement and continue its action.

1 From this definition it will be observed that while cardiac stimulants increase the functional activity of the heart, vascular stimulants do not increase the contractile power of the vessels, nor the activity of the vaso-motor centre, but, on the contrary, diminish the contraction of the vessels.

Slighter cases of chill may be treated by Dover's powder alone, and ten grains of it taken at night will often cut short commencing coryza, and will frequently prevent slight increase of consolidation occurring round a cavity after a chill in persons suffering from phthisis. Patients suffering from this disease should not omit to take a Dover's powder or some other vascular stimulant at night whenever they feel as if they had caught cold, and before any local mischief can be detected.

All nitrites dilate the blood-vessels and thus act as vascular stimulants. The one most commonly employed is nitrite of ethyl in the form of spirits of nitrous ether. This remedy, taken in hot water or along with acetate of ammonium, is a useful vascular stimulant, and is often used for the same purposes as Dover's powder.

Camphor is frequently used as a popular remedy instead of alcohol or Dover's powder in order to cut short coryza or catarrh, about ten drops of the tincture being taken on a piece of sugar. Local vascular stimulation is useful in removing chronic inflammation or consolidation. For a more detailed account of its action and uses, vide Irritants and Counter-irritants (p. 343).