38 GENERAL ACTIONS OP DRUGS 



disorders characterized by loss of tone, as in shock and 

 collapse ; and in heart weakness or disease (digitalis and 

 strychnine) ; also in internal haemorrhage and inflammations 

 (ergot) ; and to aid the absorption of dropsies and cedemas. 

 The uses of drugs locally contracting vessels will be 

 described under Astringents, (p. 63) and of agents locally 

 dilating vessels under counter-irritants (p. 62). 



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 affect- 

 ing 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 depression and paralysis 

 after poisonous quantities. 



It is probable that alcohol, ether and chloroform are 

 cerebral depressants from the very beginning, contrary 

 to accepted ideas. Belladonna 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 condition of the patient 'has some 

 bearing on the action of a drug influencing the brain. Thus 

 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 

 organizpd the more powerfully is it influenced by drugs, it 

 follows thit drugs acting on the brain and cord are more 



Digitized by Microsoft® 



