214 ANIMAL FORCES. [ii. 



extensive as that of the cerebral forces, and it would be difficult 

 to discover a movement which occurs as a sentient action, which 

 may not be also effected by the vis nervosa, although, in many 

 cases it is not practicable to show this by satisfactory experi- 

 ments. The greater number of movements take place by means 

 of muscles or muscular tissues, whether they be sentient or 

 not (379). That, however, which a muscle can effect when 

 stimulated by the cerebral forces, it can also effect very easily 

 solely by means of an unfelt internal or external impression 

 on its nerves (364, i), for in either case it is stimulated to the 

 same movement (356). It is not surprising, therefore, that 

 suitable impressions on the motor nerves can act as the primary 

 iucitants to all nerve-actions independently of sensation or 

 thought, and maintain the whole machinery of an animal body 

 in continued working and reciprocal functional activity, just as 

 is effected by external sensations and their sensational concep- 

 tions by means of internal impressions on the brain. For we 

 must remember that an impression not only develops the same 

 movement as it develops when felt, or as when excited by con- 

 ceptions ; but that, just as from external sensations and their 

 sentient actions, other sensational conceptions and their sen- 

 tient actions are produced and combined together, whence the 

 connected acts of the cerebral forces arise, so also unfelt external 

 impressions cause unfelt internal impressions, from which some- 

 times other external impressions originate; all which have 

 unitedly their special nerve-actions, constituting connected and 

 combined acts. To show this more distinctly, and to render it 

 by successive proofs more probable and obvious, how an animal 

 body can be regulated and excited, as well to automatic as to 

 what are usually volitional movements, independently of mind, 

 as regularly and connectedly as if directed by thought and sen- 

 sation, we must now consider the relations of the two kinds of 

 vis nervosa to each other ; and thus facilitate a comprehension 

 of this important matter, which will, for the first time, be placed 

 in its proper light in the succeeding Chapters. 



398. Either of the two kinds of impressions may reciprocally 

 excite the other, without the intervention of the conceptive 

 force ; and both can excite the same or other movements, either 

 consentaneously or consecutively; and from their reciprocal 

 connection whole series of acts may take place as the result of 



