FOURTH TREATISE 



ON MUSCULAR MOTION AND ANI- 

 MAL SPIRITS. INCIDENTALLY, 

 ON THE MOTION OF THE BRAIN, 

 AND ALSO ON THE USE OF THE 

 SPLEEN AND OF THE PANCREAS 



CHAPTER I 



EXAMINATION OF THE VARIOUS OPINIONS OF 

 AUTHORS AS TO THE WAY IN WHICH MUSCLES 

 CONTRACT 



That Nitro-aerial Spirit is, by means of respiration, 

 transmitted into the mass of the blood, and that the 

 fermentation and heating of the blood are produced 

 by it, has been elsewhere shown by us. But I shall 

 now further add concerning the use of that inspired 

 spirit, that it takes the chief part in the origination of 

 animal motions, an opinion which I published now a 

 good while ago, and still firmly hold ; not that I have 

 set myself to stick to it, as fixed to a preconceived 

 hypothesis, but because I consider it most agreeable 

 to reason. 



The cause of the production of any kind of motion 

 is so obscure, that the consideration of it may exercise 

 the minds of the anatomists nowadays, no less than 



229 



