PERCEPTION. 511 



as from a dream, having no sense of the thne 

 which has elapsed since the moment of the 

 attack. All causes which disturb the healthy 

 condition of the brain, such as alcohol, opium, 

 and other narcotic drugs, or which disorder 

 more especially the circulation in that organ, 

 such as those inducing fever, or inflammation, 

 produce corresponding derangements of the in- 

 tellectual powers ; modifying the laws of the 

 association of ideas, introducing confusion in 

 the perceptions, irregularity in the trains of 

 thought, and incapacity of reasoning, and lead- 

 ing to the infinitely diversified forms of mental 

 hallucination, delirium, or insanity. Even the 

 strongest minds are subject to vicissitudes 

 arising from slighter causes, which aifect the 

 general tone of the nervous system. Vain, 

 indeed, was the boast of the ancient Stoics 

 that the human mind is independent of the 

 body, and impenetrable to external influences. 

 No mortal man, whatever may be the vigour 

 of his intellect, or the energy of his application, 

 can withstand the influence of impressions on 

 his external senses ; for, if suflticiently reiterated 

 or intense, they will always have power, if not 

 to engross his whole attention, at least to in- 

 terrupt the current of his thoughts, and direct 

 them into other channels. Nor is it necessary 

 for producing this eflect that cannon should 

 thunder in his ears ; the mere rattling of a 



