300 ANIMAL FORCES. [ii. 



them. But the question arises, whether the unfelt external 

 impression, which, by its reflection causes an indirect nerve- 

 action, can, when felt, excite the same movement as a direct or 

 indirect sentient action of its sensation ? It is not easy to 

 answer this question without going into details. 



583. When the toe of a decapitated frog is pinched, it 

 places its limbs in a position for leaping, and actually leaps, in 

 consequence of the indirect nerve-action caused by the external 

 impression on the toe (415, ii) ; and the question arises whether 

 the same impression, when felt, would cause the same leap as 

 a sentient action of the pain caused, either indirectly or 

 incidentally, in consequence of the excitement of an instinct ? 

 Probably it is so, for a healthy frog leaps when it feels the pain 

 caused by pinching its toe ; but in this case is the leap neces- 

 sarily a sentient action caused by the pain? for although it 

 may take place as such at the same time that it occurs as a 

 nerve-action (364), still it does not follow, that in the case of 

 the healthy frog, the leap is so produced. How can it be shown, 

 experimentally, whether it occurs from the sensation of the ex- 

 ternal impression, or from the impression only? Everything 

 which prevents the action of the external impression, also 

 partly prevents its being felt, and if felt, partly prevents the 

 same movement being excited as a sentient action of the sen- 

 sation. Other difficulties might be mentioned, and, in fact, 

 there is only one means of solving the problem. 



584. The sensational conceptions, namely, imaginations, 

 foreseeings, &c., are imperfect external sensations, which are in 

 relation to an external impression ; and their sentient actions 

 are the same as the actions of the external sensations, but are 

 imperfectly so, since the external impression and all its nerve- 

 actions are wanting (68 — 74). Now, if an imagination or fore- 

 seeing of a sensation excites imperfectly as sentient actions, the 

 same movements which the external impression of the sensation 

 usually excites, as its indirect nerve-action, the conclusion is 

 obvious, that the sensation of the impression itself will produce 

 the same animal movement ; particularly, as in the normal con- 

 dition it always accompanies sensation, which would not be the 

 case if it were always a nerve-action only of the external im- 

 pression. Now, the former proposition is established by ob- 

 servation ; and the latter must be true, since it does not appear 



