58 DESCARTES. 



day? And what more need be adduced to explain 

 nutrition, and the production of the different 

 humours of the body, beyond saying, that the force 

 with which the blood, in being rarefied, passes from 

 the heart towards the extremities of the arteries, 

 causes certain of its parts to remain in the members 

 at which they arrive, and there occupy the place of 

 some others expelled by them ; and that according 

 to the situation, shape, or smallness of the pores 

 with which they meet, some rather than others flow 

 into certain parts, in the same way that some sieves 

 are observed to act, which, by being variously per- 

 forated, serve to separate different species of grain? 

 And, in the last place, what above all is here worthy 

 of observation, is the generation of the animal 

 spirits, which are like a very subtle wind, or rather 

 a very pure and vivid flame which, continually 

 ascending in great abundance from the heart to the 

 brain, thence penetrates through the nerves into the 

 muscles, and gives motion to all the members; so 

 that to account for other parts of the blood which, 

 as most agitated and penetrating, are the fittest to 

 compose these spirits, proceeding towards the brain, 

 it is not necessary to suppose any other cause, than 

 simply, that the arteries which carry them thither 

 proceed from the heart in the most direct lines, and 

 that, according to the rules of Mechanics, which are 

 the same with those of Nature, when many objects 

 tend at once to the same point where there is not 

 sufficient room for all, (as is the case with the parts 

 of the blood which flow forth from the left cavity of 

 the heart and tend towards the brain,) the weaker 



