53 GENERAL ACTION OF DRUGS 



and phenacetin, act directly on the heat-regulating centres, either to 

 increase their resistance to the action of the toxins of infections, or to 

 lower the point at which the temperature of these centres is set — to use 

 the thermostat analogy. Such agents also strengthen the control of the 

 heat-regulating centres over the centres for heat-loss, and may or may 

 not cause sweating. In excessive doses their depressing effect on the cir- 

 culation is so marked that collapse may occur. That they act centrally 

 is shown by the fact that they fail to act when the spinal cord is cut. 

 They do not act in health because of the absence of disturbance of the 

 heat-regulating centres by toxins. They do not act by diminishing heat- 

 production — as has been hitherto taught — since metabolism is not notably 

 lowered by them. Acetanilid increases metabolism. They do not depend 

 upon sweating, as they act under the influence of atropine which prevents 

 diaphoresis. 



Uses. — Antipyretics are used to diminish fever. It is wiser to employ 

 the coal-tar products for this purpose because they are not only more 

 effective but also promote comfort by their sedative influence on the nerv- 

 ous system. They are, however, generally contra-indicated unless fever. 

 is high or long-continued, since it has been pointed out that high tempera- 

 ture is a natural protective agency in destroying toxins and bacteria. 

 Cold is the best antipyretic measure, when it can be • employed, in not 

 only lowering fever and increasing the elimination of toxins in the urine, 

 but in powerfully stimulating the vital nerve centres. (See p. 497.) 



DRUGS ACTING ON THE SKIN. 



(A) Drugs Influencing the Blood Vessels of the Skin 



(1) Locally dilating superficial vessels. 



IRRITANTS OR COUNTER-IRRITANTS. 



Cantharides Corrosive mercuric chloride 



Iodine Arsenous acid 



Mustard Silver nitrate 



Capsicum Zinc chloride 



Croton oil Carbolic acid 



Oil of turpentine and other Mineral acids 



volatile oils Caustic alkalies 



Ammonia water Anesthetics and alcohol (when evap- 

 Camphor oration is prevented) 



Red mercuric iodide Heat 



Drugs are classified as follows, according to the degree of irritation 

 they produce: 



Rubefacients are drugs which cause vascular dilatation and redness 

 of the skin when locally applied, such as mustard and iodine (and heat). 



Vesicants are drugs producing inflammation of the skin and exuda- 

 tion of serum und?r the epidermis (vesicles), when locally applied, such 

 as cantharides. 



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