On Muscular Motion and Animal Spirits 259 



that would come from without. Why, then, should 

 the animal spirits not rather be derived from air than 

 from food taken in ? Undoubtedly the air is im- 

 pregnated with most active and subtle particles ; and 

 there is such a necessity of inhaling it that not for a 

 moment can we live without it. And indeed it does 

 not seem possible that the immense expenditure of 

 animal spirits can be supplied from any other source 

 but the air. 



But as to the artifice of the animal mechanism, 

 it consists in this, that the parts of the body are 

 formed with such perfect adjustment that quite 

 stupendous effects are produced in it by common 

 causes. 



I may note here, by the way, that while I hold that 

 nitro-aerial particles are the animal spirits, I do not 

 wish to be so understood as if I thought nitro-aerial 

 spirit to be the sensitive soul itself: for we must 

 suppose that the sensitive soul is something quite 

 different from animal spirits, and that it consists of a 

 special subtle and ethereal matter, but that the nitro- 

 aerial particles, i.e.^ the animal spirits^ are its chief 

 instrument. For, indeed, as to the sensitive soul, I 

 can form no other notion about it than that it is some 

 more divine aura^ endowed with sense from the first 

 creation and co-extensive with the whole world, and 

 that a little portion of it, contained in a properly 

 disposed subject, exercises functions of the kind which 

 we observe and admire in the bodies of animals ; but 

 that that spiritual material, existing out of the bodies 

 of living things, is not to be supposed either to 

 perceive or to do anything but to lie quite dormant 

 and inert, being much as is the case with the sensitive 

 soul when the animal is buried in sleep. 



The afflux of arterial blood does not seem sufficient 



