On Muscular Motion and Animal Spirits 253 



itself with so much force, unless elastic particles were 

 present in sufficient abundance for its contraction ; 

 and hence it is that in violent exercises the respira- 

 tion is so much increased that the great expenditure 

 of animal spirits made in motive effort may be re- 

 paired. 



And not only for muscular motion but also for 

 sustaining life itself, it seems that there is need of an 

 ample supply of animal spirits ; for it is probable that 

 life cannot be sustained without a certain series and 

 continuous flow of animal spirits passing constantly 

 through the brain, or at all events through the 

 cerebellum. And that is the reason why, if the 

 respiration or the motion of the heart or of the blood 

 be stopped even for a moment, the animal at once 

 dies. But more fully of this below. 



In confirmation of what has been said, there may be 

 adduced what the learned Malpighi has observed as 

 to the Respiration of Insects : viz. — Insects which live 

 when their head is cut off^ and the separated portions of 

 which live^ have pneumonic vessels distributed through 

 the whole duct of their spinal m,arrow. For this 

 eminent man observed that the black points which are 

 to be found on each side of insects close to the spine, 

 are so many spicula or tracheae through which the 

 insects draw air ; insomuch that if the said spicula be 

 smeared with oil the animal soon dies from want of 

 breath. Further, he found out that some branches of 

 the said tracheae are inserted here and there into the 

 spinal marrow. 



But that we may apply these observations to the 

 present subject, I think we must allow that the 

 animal spirits in the more perfect animals are elabor- 

 ated only in the brain, and that they are dissemin- 

 ated from that source to the spinal marrow and to 



