CH. II.] EXTERNAL SENSATIONS. 35 



36. True external sensations are conceptions excited by 

 external impressions on the nerves. Thereby the mind dis- 

 tinguishes at each act of attention the point where the external 

 impression takes place. Consequently, the sphere of action of 

 the external impression, which causes the external sensation, is 

 only between the point of impression and the material external 

 sensation in the brain; and since there is first the external 

 impression, and then its transmission, the vital changes which 

 cause the external sensations must be propagated from the 

 point of impression upwards to the brain, and not downwards, 

 from the brain, in so far as they are felt (31). If, consequently, 

 a branch of a nerve is irritated at a point nearest to the 

 brain, the external impression which ascends thence to excite 

 a material external sensation, can excite it in the most distant 

 branches, and by their means develope impossible animal actions. 

 And if these actions should arise in the distant branches, or 

 through them, they are the actions resulting from an impression 

 occurring contiguous to the brain which is sent downwards, 

 and contributes nothing to sensation. The probable motion of 

 the vital spirits accords with this view. 



37. It may, however, be quite possible, with regard to many 

 external impressions, that the impression on the nerve may so 

 take place, that it concusses it, or its lower or more distant 

 branches, only mechanically. The impressions excited by this 

 mechanical concussion of the nerves are sometimes duly received, 

 and, like other external impressions, propagated to the brain, 

 and there produce external sensations. Thus, an external 

 impression may appear to be transmitted downwards without 

 that being actually the case. An example of this kind is 

 afforded by the tingling which a blow of the elbow causes to 

 be felt as far as the tips of the fingers, the nerve being 

 mechanically concussed ; it cannot be said that the external 

 impression felt at the elbow had been propagated backwards, 

 and felt through the fingers. 



38. The mind determines the point of external impression 

 in external sensations by an act of the judgment. At first 

 it learns to distinguish the point of contact by a due observa- 

 tion of its external sensations, and a comparison of them with 

 the place where the external impression takes place ; but after 

 frequent repetition it determines it in a shorter way, by analogy. 



