DEUGS ACTING ON THE NEEV0U8 SYSTEM 39 



prone to affect the cord in veterinary patients, while impres- 

 sing the brain more potently in man. For this reason we 

 notice in the horse that the primary period of excitability 

 (sometimes seen in man) following the administration of 

 morphine is much prolonged and not infrequently com- 

 pletely 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) CerebralJSxcitants. 



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 generally excite the nervous system in 

 depressed conditions. Caffeine is a valuable antidote to the 

 depressing cerebral action of opium in poisoning. 



(b) Cerebral Depressants. — It is fortunate that drugs 

 progressively paralyzing the functions of the brain follow 

 the so-called law of dissolution — i.e., paralyze the various 

 functions of the brain in the inverse order of their evolu- 

 tionary 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 succumb to the action of cerebral depressants. 

 The cerebral depressants are used mostly to relieve pain, 

 when they are called anodynes or analgesics. Pain is due 

 to irritation of any sensory nerve, or the sensory tract in 

 the spinal cord, or of the sensory centres in the brain. Pain 

 may be relieved by paralyzing any portion of this path 

 and destroying connection with the perceptive centres in 

 the brain. 



(1) Anodynes, by reason of their action on the brain. 

 Opium Alcohol Cannabis Indica 



Morphine Anaesthetics Gelsemium 



Codeine Chloral Bromides 



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