[CH. II.] NERVE- ACTIONS, EXTERNAL IMPRESSIONS. 229 



which, in the natural condition, the nerve-actions of external 

 ipressions can possibly be prevented, and we will therefore go 

 [through them. 



427. All those natural obstacles which hinder external im- 

 jressions arising, prevent them also from developing direct 

 lerve-actions (47, i, ii, iii). 



i. If a part be not supplied with nerves, nerve-actions cannot 

 ike place in them : of this kind are bone, cartilage, 8zc. 



ii. If the nerves of a part be so protected by nature, by 



leans of membranes, mucus, &c., that they cannot receive 



irious impressions, they cannot excite the corresponding nerve- 



jtions, although they may be moved by internal impressions, 



md manifest, through these, either sentient actions or nerve- 



ictions. 



iii. If a nerve is naturally susceptible of certain impressions 

 mly, these alone can excite the direct or indirect nerve-actions, 

 id none others (424, ii ; 47, ii) . Various insects, as spiders, 

 5ndure the application of very acrid irritants, from which other 

 limals would experience violent inflammation and convulsions, 

 md yet feel the slightest irritation of another kind (413, 414) . It 

 often observed, in animals endowed with sensation, that some 

 lerves are only susceptible of certain impressions for a 'given 

 jeriod, consequently the nerve-actions dependent thereon cease 

 soon as the period of susceptibility terminates (265). The 

 )henomena of idiosyncrasy belong to this class. 



iv. If a mechanical machine be endowed with nerves, and 



'^et is naturally incapacitated for animal movement at the point 



[of impression, no direct nerve- actions can arise therein (424, iii) . 



^he liver, spleen, &c., are incapable of motion at the point 



firritated, namely, the substance of the viscus, and therefore no 



external impression excites movement in them. 



428, V. If the external impression be too feeble to reach a 

 point of reflexion, to be there changed into an internal impres- 

 sion, it may excite a direct, but not an indirect nerve-action, 

 although the body be in the natural state (425, i). Thus, in 

 a decapitated animal, a slight irritation of a muscle excites a 

 gentle contraction of its fibres, without any of those convulsions 

 resulting in other parts, which a more powerful irritation gene- 

 rally produces. 



vi. Doubtless there are cases in which, in the natural state 



