CHAPTER I. 



ON THE VIS NERVOSA AND ON NERVE-ACTIONS IN GENERAL. 



^H 356. Only two primary animal forces of the animal machines 

 ^^[the brain and nerves) are known; namely, the two kinds of 

 impressions on the nerves (33, 121). These animal machines 

 being incorporated with the mechanical machines (155), render 

 them capable of animal operations; and it is from the action 

 of the animal forces on the former, and through them on the 

 latter, that all the phenomena of animal organisms are produced. 

 These animal forces constitute the animal- sentient forces when 

 acting in and through the brain of an animal endowed with 

 mind, and are the means whereby the reciprocal connection of 

 the body and mind is effected. It is the external impression 

 which supplies the mind with all its external sensations, but on 

 the condition that it reaches the brain, and produces there 

 material external sensations (46) ; while the internal impression 

 excites all the sentient actions of the body, with the condition, 

 however, that it be caused by conceptions (123). But if these 

 conditions fail, that is to say, if the external impression be not 

 transmitted to the brain, or at least do not excite materi^ ex- 

 ternal sensations therein; or if an internal impression at the 

 cerebral origin, or on the medulla of the nervous trunks, is not 

 caused by conceptions, but by other irritants, the two kinds of 

 impressions still excite actions in the organism, and these are 

 the nerve-actions of the vis nervosa (353) . Every animal move- 

 ment is, in reference to the animal force that develops it, 

 either a sentient action or a nerve action; and if it be pro- 

 duced at the same time by both the cerebral forces and the vis 

 nervosa, it belongs to both classes of actions (193). Under such 

 circumstances, it would be erroneous to consider the one class 

 as distinct from the other, since they are identical in both cases, 

 just as a musical clock is the same, whether put in motion by 

 a performer or by machinery. "A muscle, when its nerve is 

 irritated, contracts and performs a movement the same as that 



