( H. IV.] SUBSTITUTION OF NERVE-ACTIONS. 279 



and, consequently, the pleasing or unpleasing internal feelings 

 they excite in the mind, depend on the difference in the nature 

 of the external sensation to which they are related. Now, 

 since sensational pleasure or pain depends on a difference in 

 the nature of the impressions which are felt (80), so also the 



Ileasure or pain excited by all spontaneous sensational concep- 

 fons, differs accordingly as the external impressions differ that 

 Kcite the external sensations to which they are related (88). 

 jhe sentient action of sensational pleasure or pain is either a 

 Dnnatural or contra-natural change in the vital movements 

 ^51, 252) ; and an imagination, foreseeing, &c., in so far as 

 ; is agreeable or disagreeable, produces also a connatural or 

 Dntra-natural change by means of its internal impressions of 

 pleasure or pain, just as the external impression whereon it 

 depends produces change by means of its vis nervosa alone 

 (546, 547). The imagination or foreseeing of a titillation or 

 pain, changes the vital movements in the same way, but less 

 perfectly, as the titillation or pain itself. From hence we 

 conclude, that the vis nervosa of the external impressions, by 

 which the pleasure or pain in the imaginations, conceptions, &c., 

 is indirectly excited, can of itself excite vital changes, which 

 constitute the sentient action of the imaginations, foreseeings, 

 &c., themselves. In the example already given of the dream- 

 ing gouty patient, we have an illustration of this view; de- 

 capitated animals and acephalous foetuses seem also to be as 

 much distressed by violent external impressions merely, as they 

 would have been if those impressions had been felt, so that the 

 heart palpitates, and the pulse is manifestly quickened. This 

 doctrine will be also fully illustrated, when we consider the 

 relations of the instinctive and emotional acts to the vis 

 nervosa. 



549. From the preceding considerations (542 — 548), it 

 follows, that the actions of external sensations, or of imagina- 

 tions, or foreseeings, or of their sensational pleasure or pain, 

 may at the same time be both sentient actions and nerve- 

 actions, or may at another time be nerve-actions only ; or in 

 other animals, they may be solely nerve-actions resulting from 

 impressions independently of the co-operation of the cerebral 

 forces, so that neither head, nor brain, nor mind, is absolutely 

 necessary to their development; nay, if there be animals altogether 



