CH. II.] SENSITIVE NERVES. 77 



receive and transmit both kinds of impressions (136, ii), is as 

 certain as that the vital spirits of the external impressions pass 

 on to the brain (18, 36), and that their laboratory is in the 

 latter (11). Since there must be an opening at the point of 

 origin of the nerve in the brain, at which the vital spirits must 

 enter to pass along the sensitive nerves to their terminating 

 fibrils, for the purpose of receiving external impressions, we 

 conclude that this is the course which the internal or cerebral 

 impression takes. In other words, the sensitive nerves have 

 fibrils like the motor nerves, which transmit internal impres- 

 sions from the brain to their terminating fibrils (126, 127). 



144. The external sensations develope their direct actions in 

 the sensitive nerves themselves (129, ii, 143) . But all sensational 

 perceptions (76) with their sensational stimuli (88), all sensational 

 instincts (90), and all the passions (91), efi'ect their sentient 

 actions through those nerves by which external sensation is 

 received. Now, since all these require such material ideas or 

 impressions in the brain, as the material external sensations 

 partly develope, and by which they are proximately determined 

 {66, 88) ; it follows, that their material ideas must excite at the 

 same time a corresponding impression at the origin of the 

 nerve, which is propagated downwards along the nerve, and 

 developes actions in it (123). On the other hand, the intel- 

 lectual conceptions, motives, volition, and involition (which 

 only depend remotely upon external sensations), have less 

 influence on the sensitive than on the motor nerves. 



145. By what means can the sentient actions, produced in 

 the nerves by the cerebral impressions of all these sensational 

 conceptions be ascertained, when their influence on the me- 

 chanical machines cannot be observed ? The nerves have in 

 themselves no visible movements ; yet the impressions of the 

 sensational conceptions can only act upon them as movements. 

 But these escape observation quite as much as those of external 

 impressions (31). They are probably only movements of the 

 vital spirits in the hollow fibrilli of the nerves (13). How can 

 the existence of these hidden movements be demonstrated? 

 VV^e infer their existence in the external impressions from their 

 action on the brain, inasmuch as they produce external sensa- 

 tions, and because compression of the nerve, in its course to the 

 brain, arrests or interrupts the propagation upwards of the im- 



