IH. IV.] SUBSTITUTION OF NERVE-ACTIONS. 281 



I 



^^Wut direct sentient actions of the instincts are none other than 

 l^^bianges in the vital movements arising from pleasure and pain, 

 combined with those movements which take place more com- 

 pletely during the satisfaction of the instinct, and which are 

 properly the sentient actions of a foreseeing (271, 272). To 

 these may be added a number of incidental sentient actions of 

 the instincts, which the vis nervosa can develop as regularly 

 and as providently as those of the instincts themselves (436 — 

 439). Lastly, the sentient actions of the satisfaction of the 

 instinct, are simply those of the foreseeing and of the actual 

 sensation, or of other sensational conceptions (275, 276), and 

 these also can be developed by the vis nervosa. We will de- 

 monstrate these views with reference to some of the principal 

 instincts. 



553. The external impressions on the stomach, which excite 

 the instinct of hunger, excite an unpleasant external sensation 

 in the stomach of faintness, which, being a painful sensation, 

 changes the vital movements contra-naturally, and otherwise 

 stimulates to the performance of their functions, all the me- 

 chanical machines which co-operate in the mechanism and func- 

 tion of digestion (281). Incidental sentient actions accompany 

 these direct sentient actions of the instinct, as, for example, 

 that the animal shall go out to seek food, seize it, and carry it 

 to the stomach. Satiety, or the satisfaction of the instinct by 

 these means, is an external sensation in the stomach, which has 

 also its peculiar direct and incidental sentient actions, subser- 

 vient to the whole process of digestion. AU these actions may 

 be excited by the vis nervosa only, especially in those animals 

 whose organisms are so constituted that the vis nervosa can 

 take the place of the cerebral forces (439) . A headless tortoise 

 lives several months ; it cannot possibly feel the sensation of 

 faintness from emptiness of the stomach, yet the external im- 

 pressions must change the vital movements contra-naturally 

 like that painful sensation, because it becomes feeble and faint 

 from starvation. The digestive organs must be excited to the 

 movements which are requisite to digestion, by the external 

 impressions of emptiness, just as by the instinct, since the 

 bowels are moved peristaltically, and the digestive fluids are 

 secreted, so long as life continues (468). The most convincing 

 proof, however, of the general principle is, that those movements 



