On Muscular Motion and Animal Spirits 251 



of the learned Dr Willis had no small weight with 

 me ; he supposes that the animal spirits are of the 

 nature of volatile salt. For this learned man in his 

 treatise on Diseases of the Brain^ chapter xi., speaks 

 as follows : Aiid indeed the Animal Spirits when in a 

 healthy and regular condition seem, to behave to some 

 extent as a spirituous liquor full of volatile salt which 

 distils from the blood. But, indeed, with all respect to 

 so eminent a man, since muscular contraction is pro- 

 duced by particles of diverse kinds mixed together 

 and mutually moving themselves (as is the view of 

 the learned author, and as also seems most consonant 

 with reason), if the animal spirits consist of volatile 

 salt, then the other motive particles supplied by the 

 blood must be acid salt, for otherwise the animal 

 spirits meeting them would not effervesce : but it is 

 scarcely to be supposed that acid salt can have a 

 place in a healthy body, much less in the motor parts, 

 as has been shown above. Should we admit that an 

 acid liquid is contained in the fibres, why should not 

 the animal spirits effervescing with it sometimes them- 

 selves turn into an acid liquid, as does happen, accord- 

 ing to the learned author's opinion, in Melancholia 

 and Mania? Wherefore it seems preferable to sup- 

 pose that the animal spirits consist of nitro-aerial 

 particles, which proceeding, on the determination of 

 the mind, from the brain into the motor parts, meet 

 there the saline-sulphureous particles, and that by 

 their mutual agitation taking place according to their 

 nature, the contraction of the fibrils is effected in the 

 way to be described below. Indeed, I imagine the 

 animal spirits to be of such a sort that they never 

 undergo change ; and as to the diseases which are 

 commonly believed to depend on their vitiated condi- 

 tion, I consider that they arise from the interrupted 



