36 CONDITIONS MODIFYING MEDICINAL ACTIONS 



IV. Climate and Temperature modify considerably the 

 actions of medicines. Heat increases the power of anti- 

 septic solutions. Narcotics are generally believed to act 

 more powerfully in warm than in cold climates. This fact, 

 as well as other differences in medicines observed in hot as 

 compared with cold climates, may depend upon slight differ- 

 ences in animal temperature, and in the varying amount of 

 excretion effected by the skin and kidneys. 



Moderate warmth favours chemical reactions and proto- 

 plasmic movements two conditions intimately connected 

 with the actions of medicines. ' Alexander von Humboldt 

 first observed that warmth not only acted as a stimulant to 

 the heart, increasing the power and rapidity of its contrac- 

 tions, but noticed that warmth increased the rapidity with 

 which alcohol destroyed the irritability of a nerve, and 

 potassium sulphide that of a muscle. . . . Many, if not all, 

 muscular poisons act more quickly with increased tempera- 

 ture. . . . Rabbits poisoned with copper or potassium salts 

 also die more quickly when placed in a warm chamber than 

 when left at the ordinary temperature ' (Brunton). On the 

 other hand, however, narcotic poisoning by alcohol or chloral 

 is retarded when the animals are in a warm atmosphere. 



Habit. The continued use of a medicine sometimes alters 

 the degree of its action. Caustics and irritants, which exer- 

 cise only topical action, exhibit, on their repeated applica- 

 tion, gradually increasing activity. But many medicines, 

 when continuously administered, have their ordinary power 

 considerably diminished. Thus, arsenic-eaters sometimes 

 use with perfect impunity twelve or fifteen grains of 

 arsenic daily a quantity sufficient to poison three or four 

 unhabituated persons. A like tolerance is observable among 

 horses which have been accustomed to receive arsenic. 

 Opium, and most general stimulants, when administered 

 for some time, gradually lose their effects. Virginian deer, 

 from habit, are said to thrive on tobacco ; some monkeys, 

 feeding on strychnine-containing nuts, are stated to become 

 insusceptible to strychnine (Wood). The tolerance thus 

 induced by the habitual use of a medicine occasionally 

 depends on retarded absorption or quickened excretion ; 

 sometimes, as in the case of many alkaloids, on the liver 



