CHAPTER IV. 



RELATIONS OF THE ANIMAL-SENTIENT [CEREBRAL] FORCES, AND 

 OF THE VIS NERVOSA TO EACH OTHER. 



JECTION I. ON THE SUBSTITUTION [eRSETZUNg] OF NERVE- 

 ACTIONS FOR SENTIENT ACTIONS. 



541. When an internal impression, caused by conceptions, 

 levelops sentient actions in the mechanical machines by means 

 )f the nerves, the material ideas must, firstly, be suitably in 

 contact with the cerebral origin of the nerve which regulates 

 the machine ; secondly, the internal impression must go un- 



jinterruptedly downwards along the same nerve to the machine 



itself; and, thirdly, there must be no impediment in the latter, 



^hen duly excited, to its performing the movements, which 



'e in accordance with its structure (129, 130). When an 



iternal impression, not derived from conceptions, develops 



lerve-actions in the mechanical machines, it acts under the 



fsame conditions (422, 496) ; only it is not necessary that it be 



lade at the cerebral origin of the nerve, or by means of 



laterial ideas there, but it may be made at any point of the 



lerve, provided it can go uninterruptedly from the point of 



impression to the mechanical machine, which it moves (493). 



In either case, the movement excited is the same, whether it 



he a sentient action or a nerve-action (360). 



542. When an internal impression, from external sensations, 

 |; develops direct sentient actions by means of the nerves, and, 



even, at the point where the external impression of the sen- 

 sations has taken place, it must be reflected, and sent downwards 

 .along the same nerve which received the external impression 

 (188). When an internal impression, by means of an indirect 

 nerve-action of an external impression, causes movements at 

 .the point where the external impression has taken place, it 

 -produces the same movements as nerve-actions (422), which, as 

 sentient actions, were produced under the circumstances just 

 mentioned; these are also produced by a reflected external 



