282 ANIMAL FORCES. [ii. 



are excited as nerve-actions, which usually are volitional; for 

 the animal raises itself, and creeps about to seek food. In 

 animals in whom the existence of a conceptive force is more 

 doubtful, the same thing is observed, for it appears from 

 Schaffer's experiments, that decapitated snails can obtain food, 

 and satisfy the instinct of hunger. (Schaffer, Versuche mit 

 Schnecken.) He placed headless snails under a glass, with 

 some bean-leaves ; on the following day he observed traces, 

 showing that they had crept about; on the fourth day, the 

 leaves were eaten into holes ; by the end of the month a new 

 head had grown. 



554. It must be remembered, however, that direct proofs of 

 the production as nerve-actions of sentient actions, by the vis 

 nervosa only, are not always possible. In many cases, de- 

 capitation arrests certain functions, which although purely 

 nerve-actions, require some influence from the vis nervosa pro- 

 duced in the head and cerebrum. (Compare §§ 515, 524, 532.) 

 Again, when the head is separated, many nerve-actions cannot 

 possibly take place, because the organs in which they ordinarily 

 occur as sentient actions, are removed with the head. Thus, 

 the flow of saliva cannot be excited in a headless animal, by 

 the sensational stimulus of hunger. Further, the proof in 

 many cases can only be indirect, or an inference, as in the ex- 

 ample of the headless tortoise just mentioned, which, after long 

 fasting, crept about as if in search of food. Certain actions 

 result from certain stimuli previously to decapitation ; the same 

 results follow on the same stimuli after decapitation ; hence 

 we conclude, that in both cases the actions result equally from 

 the stimuli. 



555. The external impressions that develop the instinct to 

 voluntary movements, excite unpleasant external sensations of 

 weariness, lassitude, indisposition, &c., which, being strong sensa- 

 tional painful feelings cause the vital movements to be feverish, 

 and stimulate the muscles of voluntary motion to perform their 

 proper functions, so that they jerk, move the limbs, and produce 

 complete movements. When the instinct is satisfied by the 

 performance of the movements, the agreeable sensation thence 

 resulting has also its peculiar results in the economy, and in- 

 duces a general healthy tone of the system (283). The instincts 

 for particular kinds of movements, as walking, sighing, laughing. 



