278 ANIMAL FORCES. [ii. 



internal impressions imitate tlie complete sentient actions of 

 external sensations, so also their nerve-actions resemble the 

 sentient actions of all other sensational exceptions : and inas- 

 much as the dominion of the vis nervosa extends over all 

 the mechanical machines capable of any sentient actions, it 

 follows, that all the sentient actions of imaginations and fore- « 

 seeings are imitated by the vis nervosa only. This view is ■ 

 supported by observation. A gouty person dreams that he 

 has an attack of gout, and this foreseeing is accompanied by 

 the sentient action of retraction of the limb : in this case, the 

 obscure external sensation caused by pressure of the toe 

 against the bed-post, induced the foreseeing. But if the toe 

 of a decapitated frog be pinched, it makes the same movement, 

 although the irritation is quite unfelt. The same movement 

 is a sentient action in the one case, and a nerve-action in the 

 other. 



546. By the same cause that a sensational conception is 

 rendered agreeable or disagreeable, but especially by means of 

 the impressions of sensational pleasure or suffering, its sentient 

 actions are also so ordered, that at the same time they cause 

 changes in the vital movements (251); and these changes are 

 connatural, if resulting from a moderate sensational pleasure, 

 and contra-natural, if from an immoderate sensational pleasure, 

 or from sensational suffering (252). The vis nervosa can 

 induce similar changes. Examples of this kind are afforded 

 by changes in the heart's movements — especially in the 

 circulation through the thorax — and in the mechanism of re- 

 spiration (519, 520, 525). 



547. A change is caused in the vital movements — a sentient 

 action — by external sensations, in so far as they are pleasant 

 or unpleasant, or cause titillation or pain (80, 250). Now, the 

 cause why an external sensation is pleasing or unpleasing, con- 

 sists in a difference in the external impression itself, and this 

 difference exists whether it be felt or not (189). Consequently, 

 the vis nervosa of external impressions alone causes those 

 changes in the vital movements, which were caused when its sen- 

 sation was titillation or pain. For illustrations, see §§ 433, 434, 

 but the number might be readily increased. 



548. Spontaneous sensational conceptions, imaginations, fore- 

 seeings, &c., are only portions of external sensations (228, 239), 



