194 ANIMAL FORCES. [n. 



to the brain, so that they may there excite material ideas, 

 giving rise to sensations, and of receiving internal impressions 

 caused by conceptions, they also possess another and entirely 

 different property, and are intended by nature to effect by means 

 of the external impressions they receive, whether the latter reach 

 the brain and are felt or not, the same movements which are 

 effected when they do reach the brain and are felt ; and to effect 

 by means of an internal impression, which they receive from a 

 touch or irritant caused by no conception whatever, the same 

 movements as are effected by means of the cerebral forces, when 

 the same internal imp^^ession is produced by a conception. The 

 animal machines are mysteriously and inscrutably endowed by 

 the Creator with these two distinct motor forces derived from 

 impressions, in addition to the equally-inscrutable animal force 

 originating also from them, partly that they may put the 

 animal-sentient forces into action, and partly that through 

 these they may move the organism animally ; and the greater 

 proportion of animal movements are so closely dependent upon 

 them, even when these are at the same time sentient actions, 

 that they must be considered as the most fundamental and most 

 general principium of the whole animal mechanism. But it is 

 also obvious, that these two kinds of vis nervosa have an essen- 

 tially distinct nature : that they produce their nerve actions in 

 some degree in an antagonistic manner; that the external 

 impression, considered also as simply a vis nervosa, is neverthe- 

 less a force as entirely different from that of the internal im- 

 pression as it is from the cerebral forces (32, 121) : that the 

 two kinds of vis nervosa are excited into action in opposite 

 ways : that they are regulated by different laws in their opera- 

 tions : and that all nerve actions of one and the same kind 

 cannot be all explained by one kind only of the vis nervosa, 

 nor that the existence of the one kind implies or excludes the 

 existence of the other. Hitherto, both have been generally 

 confounded too much with each other. Nevertheless, both 

 kinds oivis nervosa have certain properties in common, which 

 are now to be considered : those which are peculiar to each, 

 will be investigated in a subsequent chapter. 



362. No vis nervosa requires necessarily the co-operation of 

 the cerebral forces (358, 360). Further, all nerve-actions may 

 take place in the animal machines [the brain and nerves], and 



