CH. II.] NERVE-ACTIONS, EXTERNAL IMPRESSIONS. 233 



be seen that the latter afford a ready explanation of all the 

 phenomena which he refers to the vis insita. 



433. When an external impression is felt, it is also felt to be 

 either pleasing or displeasing (187). This difference lies in the 

 impression itself (186) ; consequently, the external impression, 

 which when felt is pleasing, is totally different in its nerve- 

 actions, and is of quite another kind, than the impression 

 which when felt is displeasing (189). Now since the nerve- 

 actions of an external impression are the same, whether it be 

 felt or not (364, iii), it follows that the impression which can 

 cause a pleasing sensation will excite the same animal move- 

 ments as the pleasing sensation itself will excite ; and so in 

 regard to the external impression of a displeasing external sen- 

 sation. Consequently, external impressions produce, as nerve- 

 actions, the same movements which their external sensations, 

 accompanied with sensual pleasure and pain, would develop as 

 sentient actions (186, 80). 



434. By means of the same force, an external impression 

 also produces the sentient actions of smarting, or tickling, 

 although not felt, provided it occur under the circumstances 

 when it would have caused titillation or pain (80). It cannot, 

 consequently, be inferred from the occurrence of those move- 

 ments which usually accompany an external sensation, par- 

 ticularly tickling or smarting, that the latter are felt, but only 

 that there is that present in the external impression which 

 can cause tickling or smarting. If an acephalous foetus, 

 or the headless trunk of a worm or insect, be irritated, the 

 same movements result as would have been considered the 

 direct and incidental sentient actions of the irritation, if it had 

 been felt, although this is impossible (25, 34). If it be so 

 irritated, that pain under ordinary circumstances would have 

 been caused, then those movements result which are the 

 ordinary direct and indirect sentient actions of pain. The 

 injured part contracts, is congested with blood, and swells and 

 inflames, and the animal writhes, tries to escape, leaps, flies, 

 defends itself, and exhibits all the signs of suffering, although 

 it is incapable of sensation. Titillation has a similar effect 

 imder similar circumstances, so that decapitated animals may 

 be excited to the performance of sexual acts, by the external 

 stimuli appropriate thereto (274, 396). 



