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III.] VIS NERVOSA OF INTERNAL IMPRESSIONS. 257 



18 applied to the trunk of the nerves, and develops in them 

 primary internal impressions, which cause these nerve-actions. 

 Movements of this kind are observed_, for the most part, in 

 persons in whom the whole mass of humours is acrid, so that 

 when they come into contact with the medulla of the nerves, 

 they stimulate it. Of this class also are the movements which 

 occur if the trunk of a nerve be ulcerated, and the limbs to 

 which it is distributed are moved in various contra-natural 

 ways ; or when a tumour, a foreign body, &c., irritates the 

 nervous trunks, and thereby excites the muscles to contra- 

 natural movements. Examples of primary non-conceptional 

 impressions will be given subsequently, (515, 525, 532, &c.) 



492. We are as ignorant what peculiar irritation of the me- 

 dulla in the nervous trunks must be excited, to produce a certain 

 kind of internal impression and thereby certain definite nerve- 

 actions, as we are with reference to external impressions (413). 

 Thus much is known, however, that it is not every irritation 

 or impression on the trunk of a nerve that is effective 

 [sinnlich] ; or, in other words, it is not every irritation, 

 although directed from the brain downwards, that causes in 

 the nerve itself that mysterious movement which is propagated 

 downwards in the nerve, and puts the mechanical machines to 

 which it is distributed into motion. Every irritation which is 

 adapted to act on the nerves [or is sinnlich], develops animal 

 actions (121), and all others act simply as physical or mechanical 

 forces (7) . We have in the heart an example, in what a minor 

 degree many nerves receive certain internal impressions (488); 

 it is also known that nitric acid applied to a nervous trunk 

 corrodes the medulla, but does not excite the muscle to which 

 the nerve is distributed, although irritation of the same nerve 

 with a needle will throw the muscle into convulsions (Haller, 

 ' Opera Minora,^ torn, i, p. 364). The action of impressions can 

 be learnt only by experiment and observation. 



493. Just as a conceptional internal impression on a tied or 

 cut nerve does not pass beyond the injured point (128), so 

 also no other internal impression made above the injured point, 

 nearer the brain, passes beyond the ligature or section, to excite 

 nerve-actions in those parts only which are supplied with twigs 

 from the trunk between the point of irritation and of injury. 

 But if an internal impression be made on the nerve below the 



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