CHAPTER II. 



ON THE VIS NERVOSA OF EXTERNAL IMPRESSIONS. 



408. After having considered the two kinds of vis nervosa in 

 general (355), we will consider them separately, and, firstly, the 

 vis nervosa of the external impressions, generally and specially. 



SECTION I. ON THE VIS NERVOSA OF EXTERNAL IMPRESSIONS 



IN GENERAL. 



409. An impression passes upwards to the brain, along the 

 nerves, from the point of impression. If it reaches the cerebral 

 origin of the nerves, it develops there an external sensation, 

 and the actions which it thus produces in the body are sentient 

 actions from external sensations (32, 34) . On the other hand, 

 all actions which it produces before it reaches the brain, or in 

 other words, before it becomes an external sensation, are nerve- 

 actions of its vis nervosa (98, i, and 353). It is the latter which 

 we have to consider here. 



410. When a nerve is irritated with a needle, it receives 

 both an internal and external impression ; if, for example, it be 

 a motor nerve, the latter acts on the muscles above the point 

 irritated, and the former (or the non-conceptional internal im- 

 pression) on the muscles situated below, or in a direction from 

 the brain. Both kinds of movement are equally nerve-actions. 



411. If when, as in many external sensations, the irritation 

 is so applied that the nerve is mechanically concussed below 

 the point of irritation, and this concussion of the lower portion 

 of the nerve acts as another external impression, and thus 

 develops movements in the parts below, they will be the same 

 as those induced by the first irritation (37). 



412. Since the external impressions that are made at the 

 same time on several nerves, do not hinder or confuse each 

 other in their course to the brain, but pass along the same 

 nerve, or through the spinal cord, unmingled with each other 

 (39) ; it follows, that they can produce their corresponding 



