44 CEREBRAL FORCES. [i. 



ii. An impression^ often repeated on a nerve, may render 

 it unfit to receive that particular impression, although it may 

 receive every other; as when one who, accustomed to cold, 

 neither feels the cold, nor has goose-skin produced, and yet 

 would feel a tickling from the slightest touch of a feather. 



iii. The frequent repetition of the same external sensation 

 may render a nerve insensible (47, iii.) 



iv. When the frequent repetition of the same external im- 

 pression (48), renders the nerve so insensible, that thereby 

 the ganglia and points of anastomosis are so changed that 

 they retain an impression which they previously allowed to 

 pass. This may only be observed in the cases in which an 

 external impression, in an unweakened nerve, excites both 

 sensation and movements at the same moment ; but in the ab- 

 normal condition excites the latter only. In such a case the 

 occurrence of the animal movement (a proof wanting in other 

 instances), shows that the external impression is really received 

 by the nerve and transmitted to the point, where, on its way 

 to the brain, it is reflected and sent downwards along the 

 trunk of the nerve, being the direction taken by an impression 

 transmitted from the brain itself (31, 122). This is the only 

 explanation admissible, since as the nerve is not enfeebled, the 

 principle just laid down (iii) cannot apply. Instances will be 

 remembered of persons who experienced spasms in their limbs 

 from various external impressions made on nerves in a distant 

 part, and in whom the same spasms continue to occur, 

 although the mind has become at last habituated to the pain, 

 and it is no longer felt. So, also, many epileptic and gouty 

 patients — the paroxysms they suffer being excited by worms or 

 gouty humors, causing external impressions on the nerves of 

 the stomach — can foretell an attack from the sensations thus 

 excited. After a time, however, when the disease is rendered 

 chronic, these sensations are no longer felt, and the paroxysms 

 come on quite unexpectedly. 



V. Lastly, the frequent repetition of the same external im- 

 pression can weaken or destroy the external sensations. The 

 point in the brain, where the impression ought to excite the 

 material' external sensation, undergoes such a change, that the 

 development of the material idea is prevented (49). This is 

 the case when a miller becomes so accustomed to the noise 



