cii. II.] VIS NERVOSA OF EXTERNAL IMPRESSIONS. 223 



nerve-actions, without the one kind interfering with the other. 

 Experiments on decapitated animals establish this fact. 



413. It is as difficult to discriminate the various kinds of 

 impressions from their nerve- actions as from their external sen- 

 sations (40, 41). An irritant may often act when we cannot 

 detect it, or when we think it not an irritant ; and hence it is 

 that the phenomena of idiosyncrasies are so inscrutable (52). 

 The heart is more stimulated by blood than by acrid irri- 

 tants, the urinary bladder by water, the intestinal canal by air 

 (Haller's 'Physiology,' §402). An irritant which a priori 

 would be expected to be more active than another apparently 

 less irritating, is in fact less active : many parts that remain 

 unchanged when the most acrid chemical spirits are applied 

 to them, are excited to convulsive movements on being irritated 

 by the point of a needle. It is in fact impossible to infer from 

 mechanical or physical laws, what nerve-actions will follow on 

 certain kinds of irritants ; the laws of their action can only be 

 known by experiment and observation. 



414. It is not every impression on the nerves that is adapted 

 to their structure, but only that which excites animal actions 

 (31, 32) ; nor is it adapted if it be not made so as to excite that 

 hidden movement in the nerves, which when propagated to the 

 brain, excites external sensations ; or excites movements in the 

 mechanical machines when propagated to them. We have 

 already discussed the former (42, et seq.) : we will now treat of 

 the hidden movements in the latter, as disclosed by nerve- 

 actions, and inquire under what circumstances they take place. 



415. i. If, at the point where a nerve receives an external 

 impression, it be completely incorporated with a mechanical 

 machine which is capable of performing certain movements at 

 that point, as in muscles for example (161), it excites these 

 directly to perform their animal movements ; and the nerve- 

 action thus excited requires nothing more than the external 



I impression, whether it proceeds further or not. Thus a mus- 

 cular fibre in an excised muscle contracts immediately at the 

 point where a point of a needle irritates it, or a particle of salt 

 is dissolved upon it. 

 ii. If a nerve causes nerve-actions, by means of an external 

 impression in parts remote from the point of impression, or even 



