CH. II.] CEREBRAL IMPRESSIONS. 75 



shocks must overcome the other which is totally unsupported ; 

 in like manner, the natural hinderance is destroyed, and the 

 impression takes its uninterrupted course to the production of 

 its sentient action. 



138. V. Lastly, mechanical machines may act as impediments 

 to the production of sentient actions, when a change in them is 

 necessary before the impression in the brain can produce its 

 sentient action. This is the case with the mountebank, whose 

 gestures and postures cannot be imitated by another until his 

 joints are stretched, his muscles practised, and even his viscera 

 partly displaced, so that the machines can instantaneously 

 follow the act of willing. 



139. The sentient actions which are produced directly by 

 external sensations, can also be partly enfeebled, partly pre- 

 vented, in various ways, by the habit of reception of sensations 

 (50, 51, 134, i, 54); and since the actions of sensational con- 

 ceptions depend proximately upon external sensations (66), the 

 habitual reception of external sensations may have a consider- 

 able influence on their development. But it is unnecessary to 

 quote instances of this kind, as they are of daily occurrence. 



140. The sentient actions which ideas excite through the 

 nerves in dreaming and in insanity, partly accord with those 

 of former external sensations (69) ; so also those of sensa- 

 tional memory (72). Those of the sensational foreseeings, 

 expectations, and forebodings, as well as those of dreams and 

 of mania (75) partly produce the actions of coming future 

 external sensations (74). These actions are forcible in pro- 

 portion to the strength of the conceptions exciting them 

 (69, 74) ; abstraction of the mind can cause many actions of 

 the understanding to cease ; attention on the other hand can 

 excite and maintain many. The sentient actions of pleasure 

 or suffering are powerful in proportion to the strength of these 

 internal sensations (80, 96). The sentient actions of a desire 

 or aversion are composed of those of a foreseeing, of an 

 expectation, of a pleasure or pain, and of an effort of the 

 conceptive force ; and are powerful in proportion as these are 

 vigorous (85,87). So, also,, those of the sensational instincts 

 and emotions (93), and of willing and not willing (96), all 

 these sentient actions result according to general laws, partly 

 physiological, partly psychological (111). 



