THE NERVOUS SYSTEM. 119 



too busy about other matters, to take notice of the impres- 

 sions made on them ; accordingly, there are instances known, 

 where persons have walked off a precipice, or into the water, 

 without noticing the danger till it was too late. 



26. Sensations, then, are the more vivid ; and the impres- 

 sions which they make, more durable, in proportion to the 

 degree of attention with which the mind is directed towards 

 them ; or the degree of activity of the brain. To obtain 

 ideas, which are|the pictures of sensible objects painted on 

 the brain) we must therefore endeavour to concentrate the 

 whole attention upon the subject before us, or the impres- 

 sions we receive will be weak and speedily fade away. We 

 see a great difference in this respect in different persons. 

 Two individuals will travel together through the same coun- 

 try, and apparently take the same degree of interest in the 

 objects which come under their notice. Yet, while one of 

 these persons will be able to describe minutely every thing 

 he has seen, the other can give only a confused and indistinct 

 account of what he has observed. It is, therefore, not only 

 necessary in the acquisition of knowledge that objects should 

 make an impression on the sensual organs ; but the brain 

 must act upon them, and that not in a slight and careless 

 manner, but vividly and energetically. 



27. It is possible even that sensations may be excited by 

 the action of the brain itself, without the intervention of any 

 sensible objects. AVe see this oftenfn diseases of the brain, 

 and especially in that form of insanity, called delirium ire- 

 mens, or drunken delirium.) Here, the wretched victim of 

 depraved habit sees serpents, and lizards, and bats, and all 

 creeping things, and devils, flying about ; and he hears sing- 

 ing and various sounds, to which he listens, and calls the 

 attention of others, and nothing can break the spell by which 

 he is bound, or dispel the illusory conviction, that what he 

 sees and hears is real and not imaginary. 



28. Such illusory and false impressions, are no doubt 

 sometimes excited in the minds of those, who cannot be said 





