238 ANIMAL FORCES. [ii. 



the same reasons as laid down in § 201 with reference to 

 external sensation. 



443. The nerve-actions of an unfelt external impression may 

 be felt, and induce external sensations (184). This constitutes 

 in sensational animals, a new link between the nerve-actions of 

 external impressions and the sentient actions of external sen- 

 sations. A loaded state of the stomach, worms, or poisons 

 cause therein external impressions, which usually are not felt. 

 These have their direct nerve-actions in the stomach, producing 

 in it a contraction and contra-natural movement, and this 

 nerve-action it is which we feel, when we say that we are ill. 

 Vomiting follows upon this external sensation, as its sentient 

 action, and as the result of a nerve-action of an unfelt impres- 

 sion. So the headaches accompanying disorder of the stomach, 

 are felt nerve-actions from unfelt external impressions (419). 

 Examples of this kind are of daily occurrence. 



In specially considering the nature of the nerve-actions in 

 the different mechanical machines, it must not be forgotten 

 that the latter are by nature peculiarly adapted to certain 

 movements, and to none other (193). 



SECTION IT. THE VIS NERVOSA OF AN EXTERNAL IMPRESSION IN 



SPECIAL RELATION TO DIRECT NERVE-ACTIONS. 



444. The nerve-actions, produced indirectly by an external 

 impression, are really nerve-actions of a non-conceptional 

 internal impression (422). As these will be considered in the 

 next Chapter, our inquiry as to the vis nervosa of an external 

 impression need not extend beyond the direct actions it produces 

 in the mechanical -machines (418). 



415. Muscular fibre of all the tissues is most eminently that 

 in which an external impression excites direct nerve-actions. 

 The structure is peculiarly adapted to the latter, since the 

 fibrilli are easily excited to movement at any point of their 

 length, when the two ends either approach or separate from each 

 other (161). An external irritation which duly excites the nerve- 

 medulla distributed through the muscle can therefore very 

 readily impart an obvious movement at the irritated point, and 

 produce a direct nerve-action (418). An external impression 

 cannot so easily excite motion in other mechanical machines. 



