CH. II.] IMPERFECT EXTERNAL SENSATIONS. 79 



148. Erection of the nerve-points is a movement, or change, 

 which, when strong enough, may become an external impres- 

 sion (31), and on arriving at the brain, may excite a concep- 

 tion similar to external sensations ; but having no true material 

 impression as its basis, has consequently no real external cir- 

 cumstance. In this way, remarkably powerful impressions in 

 the brain (from very vigorous conceptions) may produce an 

 external impression on the nerves to which they have reference, 

 as if it came from a real thing without ; this reaching the brain 

 will excite the analogue to a real external sensation, and some- 

 times contains so much of the real, that the mind can only dis- 

 tinguish, by great attention, this class of sensations and their con- 

 comitant conceptions, from those which have a true external 

 origin. To distinguish these sensations from the latter class, 

 we shall term them imperfect external sensations. Hitherto 

 they have been confounded with very vivid imaginations ; but 

 their extraordinary distinctness shows that they differ from 

 these, inasmuch as often they are in no respect dissimilar from 

 true external sensations. They are necessarily, however, the 

 results only of very powerful impressions in the brain of other 

 conceptions, as shown above ; consequently, they can be the 

 actions of a too-active imaginative faculty, and may, there- 

 fore, accompany insanity, drunkenness, impressive dreams, 

 strong forebodings, and very strong and violent passions. 

 These imperfect external sensations are known as appearances^ 

 apparitions, delusions, spectres, ghosts, &c., and it is of not 

 less importance to pathology and practical medicine than to 

 the present subject, that these be traced to their origin, and 

 distinguished from the feeble images of imaginations and 

 foreseeings. If a vigorous effort be made to see with the eye- 

 lids closed, a red colour is seen : if the fear of a sudden fall 

 from a height excites the nerves of vision to give a false im- 

 pression of surrounding objects, they seem to move, or vertigo 

 occurs : a loud stunning noise is heard long after it has ceased, 

 but with decreasing indistinctness : if food be desired with 

 great longing, its taste is perceived beforehand : if a delightful 

 sensation be anticipated earnestly, the nerves become so sen- 

 sitive, that every slight impression causes it, although previously 

 they would have had no effect. All these are instances of 

 imperfect sensations of this kind, which happen to the most 



