Fig. 121.__Diagram to show the effects of heat and cold in lessening the pain of inflammation. The

Fig. 121.__Diagram to show the effects of heat and cold in lessening the pain of inflammation. The

Fig. 122.   Diagram to show congestion of the lung. The pulmonary vessels are shown dilated, and those of the thoracic wall contracted.

Fig. 122. - Diagram to show congestion of the lung. The pulmonary vessels are shown dilated, and those of the thoracic wall contracted.

It is probable that a similar condition occurs in man, and that when we apply a blister to the side we, sometimes at least, cause contraction of the vessels in the pleura and lung below, and thus relieve pain in the chest in much the same way as when we apply cold to an inflamed finger. It has been supposed that the action of a poultice or blister was simply to draw away blood from the inflamed part. We have seen that the poultice does this in the case of an inflamed finger, but in an inflamed lung or pleura the quantity which comes to the skin is insufficient to explain the relief. It is quite possible, however, that the vessels in the lung and pleura adjoining the inflamed district may be dilated by the application of a poultice or blister to the side, and thus relief is afforded in the same way as by the application of a poultice to the finger. It is not easy to say in which of these ways a poultice or blister acts in any particular case. Clinical experience seems to show that sometimes the blisters relieve acute inflammation by causing contraction of the afferent vessels (as represented in the accompanying diagram, Fig. 123) and thus lessening the tension in the vessels of the inflamed part. If the blister is too near to the inflamed part, it may increase instead of diminishing the congestion, and thus do harm instead of good.

Fig. 123.   Diagram to explain the action of counter irritation. A blister or other counter irritant is shown applied to the chest wall. The stimulus which it causes is transmitted up the afferent nerves to the vaso motor centre; it is thence reflected down the vaso motor nerves to the pulmonary vessels, causing them to contract, while it is reflected down vaso dilating fibres to the vessels of the thoracic wall and probably of other parts of the body also, causing them to dilate, and thus lessening the pulmonary congestion by withdrawing blood from the lungs.

Fig. 123. - Diagram to explain the action of counter-irritation. A blister or other counter-irritant is shown applied to the chest-wall. The stimulus which it causes is transmitted up the afferent nerves to the vaso-motor centre; it is thence reflected down the vaso-motor nerves to the pulmonary vessels, causing them to contract, while it is reflected down vaso-dilating fibres to the vessels of the thoracic wall and probably of other parts of the body also, causing them to dilate, and thus lessening the pulmonary congestion by withdrawing blood from the lungs. (Compare with Fig. 122.)

As a matter of practice, the rule is usually insisted upon, that in a case of pericarditis, for instance, the blister should not be put immediately over the pericardium, but at some little distance from it.

Counter-irritation is not only used, however, as a means of lessening congestion and pain in acute inflammation, it is also employed with much advantage to cause the re-absorption of inflammatory products. The use of the increased circulation which a blister causes in a chronic ulcer is unquestionable, and the rapid absorption of the thickened margins of the ulcer is perceptible to the eye. A similar absorption appears to occur in deeper-seated organs, such as the lung, on the application of counter-irritation to the chest, and painting with iodine liniment is useful in promoting absorption of liquid effused into the pleural cavity or of the product of chronic inflammation of the lung. The mode in which the irritation acts is probably the same both in the chronic ulcer and in the lung, i.e. by increasing the circulation through the part affected. Where the blister is applied, as in acute pericarditis, to lessen congestion, it is usually placed at a little distance from the inflamed part, but where we wish to increase absorption, as in consolidation of a part of the lung, we apply the counter-irritant directly over the consolidated part.

Rubefacients.

Mechanical, as friction.

Ammonia

Solution of ammonia, compound camphor liniment.

Alcohol (prevented from evaporating by oil-silk or a watch-glass).

Arnica.

Cajeput oil.

Camphor.

Capsicum.

Chloroform (prevented from evaporating, like alcohol); chloroform liniment.

Ether (like chloroform).

Iodine And Its Preparations

Iodide of cadmium, iodide of lead.

Menthol.

Mustard.

Oil of turpentine, of nutmeg, and many other volatile oils.

Vesicants.

Acetic acid (glacial). Heat of:

Boiling water.

Corrigan's hammer. Cantharides. - Solutions, plaster, cantharidin. Euphorbium. Mezereon.

Volatile oil of mustard. Rhus toxicodendron.

Pustulants.

Croton oil. Tartarated antimony.

Caustics.

Actual cautery. Acids : - Acetic (glacial). Carbolic. Chromic. Hydrochloric. Lactic. Nitric. Osmic. Sulphuric. Alkalies: Lime.

London paste (p. 346). Vienna paste (p. 346). Potash. Soda.

Ethylate of sodium. Alum (burnt). Antimony (chloride). Arsenic. Bromine.

Soluble compounds of the heavier metals; as : Copper sulphate. Mercuric chloride. ,, nitrate. Silver nitrate. Zinc chloride. Zinc sulphate.