On Muscular Motion and Animal Spirits 257 



can, at their own will, give a luminous motion to the 

 nitro-aerial spirits in the eye, I cannot certainly 

 decide : it is so far an indication of this that the cat 

 can use its sense of vision even in the dark. 



It is in harmony with the hypothesis stated above 

 that animals struck by lightning are not unfrequently 

 killed without any injury, or vestige of a blow. But 

 that the reason of this may be understood, I may 

 repeat here what I have elsewhere endeavoured to 

 show, viz., that lightning is caused by the nitro-aerial 

 particles diffused through the whole atmosphere being 

 thrown by the violent concussion of the air into a 

 luminous, and sometimes really igneous motion. 

 Wherefore, if nitro-aerial particles constitute the 

 animal spirits, it may sometimes happen that they 

 in the brain follow the motion of the nitro-aerial 

 particles forming the lightning in the air ; so that 

 the animal spirits would seem not so much struck 

 by lightning as themselves to form lightning. And 

 hence it is that they, being violently moved and as it 

 were set on fire, are dissipated in a moment ; and so 

 on account of the flame kindled in its brain the 

 animal, deprived of the common light and breath, 

 is extinguished. But this will be dealt with later. 



We may note here in passing that animals have 

 need of more intense respiration for some time after 

 violent exercise. The reason of this seems to be that 

 the blood returned from the brain to the heart is to a 

 great extent deprived of nitro-aerial particles, inas- 

 much as it had deposited some in the brain and in 

 the cerebellum to supply animal spirits ; whence it 

 comes about that no small part of the blood goes 

 without its proper fermentation, because of the lack 

 of nitro-aerial particles, and is to some small extent 

 coagulated. For it has been elsewhere shown that 



R 



