lilt 



CH. II.] NERVE-ACTIONS, EXTERNAL IMPRESSIONS. 231 



origin of the nerves has received the internal impression of the 

 conception ; and if the ordinary sentient actions do not result, 

 we conclude that the internal impression cannot have reached 

 the mechanical machines (137). But with regard to the non- 

 pnceptional internal impressions, we have not the direct proof 



brded hy sensation, but only secondary evidence, when their 



direct nerve-actions remain unperformed, whether they be 

 "reflected at the point of reflexion [Reflexionspunkte] of their 

 nerve ; and, consequently, whether the cause of the failure is in 

 their course to the mechanical machines or not. We can only 

 incidentally conclude in a case of this kind, that the reflexion 

 has taken place, if another organ situate below the point of 

 reflexion, and receiving its innervation from the same nerves, 

 is excited to action by the same external impression which 

 ordinarily excites the other organ, but now fails to do so. It 

 is difficult to adduce instances, hut reasoning from analogy, 

 (compare §§ 138, 360,) it is probable that the reflexion takes 

 place, and it is the transmission downwards of the reflected 

 impression that is arrested. 



430. Just as habit enfeebles and prevents many sensations 

 and their sentient actions, in like manner it influences nerve- 

 actions from external impressions, as follows : 



i. The terminating fibrils may be so changed by the frequent 

 repetition of an external impression, that they are no longer 

 capable of the irritation requisite to the production of the im- 

 pression j and, consequently, no nerve-actions result from the 

 application of the stimulus. 



ii. The sensibility to certain stimuli only, may be destroyed 

 by frequent repetition of them, whilst with regard to others it 

 is unaltered (51, ii), and thus the former are no longer able to 

 excite nerve-actions (427, ii, iii). 



iii. The frequent repetition of the same external impression 

 on the same part of a nerve, may so enfeeble it, that whereas 

 previously it could reach the point where it was reflected, and 

 thereby excite indirect nerve-actions, after long habit it cannot 

 reach that point, and thus its indirect nerve-actions cease, 

 while its direct continue (428, v) . 



iv. Frequent repetition of the same external impression on 

 the same nerve, can so afi'ect the point of reflexion, that re- 

 flexion and transformation of the impression no longer take 



