34 GENERAL ACTION OF DRUGS 



DRUGS ACTING ON THE NERVOUS SYSTEM. 

 I. — Drugs Influencing the Brain. 



It is impossible to classify drugs according to their action on the 

 various centres of the brain, because our knowledge is insufficient. In a 

 general way, drugs affecting the nervous system fall into two groups, — 

 the excitant and depressant. But another difficulty arises in regard to 

 classification from the fact that the same drug usually both excites and 

 depresses. Many drugs influencing the nervous apparatus possess some 

 exciting action, and most drugs which excite in small doses cause depres- 

 sion and paralysis after poisonous quantities. 



Apart from a transient stimulation, alcohol, ether and chloroform 

 are essentially cerebral depressants, contrary to accepted ideas. Bella- 

 donna and its congeners, on the other hand, only excite the brain in large 

 amounts; while opium and cannabis indica may excite the brain in small 

 doses, but are used for their more common depressant action. The condi- 

 tion of the patient has some bearing on the action of a drug influencing 

 the brain. Thiis moderate doses of alcohol depress and stupefy healthy 

 animals while stimulating the enfeebled and ill-nourished. 



The brain of the lower animals is undeveloped compared to that of 

 man, and, in accordance with the general fact that the more highly a 

 portion of the nervous mechanism is organized the more powerfully is it 

 influenced by drugs, it follows that drugs acting on the brain and cord 

 are more prone to affect the cord in veterinary patients while impressing 

 the brain more potently in man. For this' reason we notice in the horse 

 that the primary period of excitability (sometimes seen in man) follow- 

 ing the administration of morphine is much prolonged and not infre- 

 quently completely obliterates the somnifacient action of the drug in this 

 animal. 



We shall be content to classify drugs acting on the nervous system 

 according to their most pronounced action in moderate doses. 



(a) Cerebral Excitants. 



Camphor Quinine 



Caffeine Cocaine 



Uses. — These drugs are rarely used simply to excite the brain, but 

 for other purposes. ■ Camphor, caffeine and quinine are employed to gen- 

 erally excite the nervous system in depressed condition. Camphor is 

 perhaps the best agent we possess to stimulate the heart and vital nerve 

 centres in emergencies. Caffeine is a valuable antidote to the depressing 

 cerebral action of opium in poisoning and is a potent heart stimulant. 



(b) Cerebral depressants. — It is fortunate that drugs progressive- 

 ly paralyzing the functions of the brain follow the so-called law of disso- 

 lution — i.e., paralyze the various functions of the brain in the inverse 

 order of their evolutionary development. The centres last to be acquired 

 are the first to be paralyzed (cerebral centres) ; while those of earliest 

 origin (the respiratory, vagus and vasomotor centres) are last to suc- 

 cumb to the action of cerebral depressants. The cerebral depressants 

 are used mostly to relieve paiUj whem they are called anodynes or anal- 



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