360 THE SENSORIAL FUNCTIONS. 



on the return of consciousness, the person awakes as from a 

 dream, having no sense of the time 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 

 intellectual 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 

 leading to the infinitely diversified forms of mental halluci- 

 nation, delirium, or insanity. Even the strongest minds are 

 subject to vicissitudes arising from slighter causes, which 

 affect the general tone of the nervous system. Vain, in- 

 deed, was the boast of the ancient Stoics that the human 

 mind is independent of the body, and impenetrable to ex- 

 ternal influences. No mortal man, whatever may be the vi- 

 gour of his intellect, or the energy of his application, can 

 withstand the influence of impressions on his external 

 senses; for, if sufficiently reiterated or intense, they will al- 

 ways have power, if not to engross his whole attention, at 

 least to interrupt the current of his thoughts, and direct them 

 into other channels. Nor is it necessary for producing this 

 effect that cannon should thunder in his ears; the mere rat- 

 tling of a window, or the creaking of a hinge will often be 

 sufficient to disturb his philosophical meditations, and disse- 

 ver the whole chain of his ideas. " Marvel not," says Pas- 

 cal, " that this profound statesman is just now incapable of 

 reasoning justly; for behold, a fly is buzzing round his head. 

 If you wish to restore to him the power of correct thinking, 

 and of distinguishing truth from falsehood, you must first 

 chase away the insect, holding in thraldom that exalted rea- 

 son, and that gigantic intellect, which govern empires and 

 decide the destinies of mankind." 



Although we must necessarily infer, from the evidence 

 furnished by experience, that some physical changes in the 

 brain accompany the mental processes of thought, we are in 





