THE MECHANISTIC CONCEPTION OF LIFE 155 



heat " in the heart similar, Descartes says, to that which may 

 be observed in fermenting, damp hay, and this fire was fed by 

 the vital spirits brought to the heart from the liver. As the 

 blood fell into the right ventricle it became expanded, " just as 

 all liquids do when allowed to fall, drop by drop, into a highly 

 heated vessel " in other words, it was made to boil, but not 

 simply to boil, as we should say, for it yielded nutriment to the 

 fire of the heart at the same *ime. From the right-hand side of 

 the heart the blood went to the lungs, where it lost some of its 

 vapours, and became thick again by contact with the respired air, 

 " without which process it would be unfit for the nourishment 

 of the fire " of the heart. Returning then to the left ventricle 

 it became distilled, or highly rarefied, with the production of a 

 " very subtle wind, or, rather, a pure and vivid flame," which 

 was the " animal spirits." This was the most agitated part of 

 the blood, and as it ascended into the narrowing arteries leading 

 to the brain the grosser parts were left behind, while the " very 

 subtle wind " or " flame " entered the brain to become stored in 

 the cerebral ventricles. 



The more sluggish and thicker parts of the blood, which were 

 nevertheless a hot and nutritive fluid, then became distributed 

 to all the other parts of the body (except the lungs and muscles). 

 Nutrition and the formation of the " humours " (or secretions, 

 as we should say) depended on the existence of sieve-like struc- 

 tures at the extremities of the arteries, so that when the blood 

 was forced through these the larger and more sluggish particles 

 were left behind " in the same way that some sieves are observed 

 to act, which, by being variously perforated, serve to separate 

 different species of grain." The secretions and nutritive parts 

 of the blood were, therefore,* filtered off, in the modern sense. 



The animal spirits are meanwhile stored in the cavities, or 

 ventricles, of the brain. Now the nerves were described by 

 Descartes as minute tubuli which contained axial threads (our 

 axons). They had a twofold function: on the one hand they 

 transmitted something from the extremities to the brain (our 

 afferent impulses), and on the other they transmitted something 

 (our efferent impulse) from the brain to the extremities. When 

 a sense organ was stimulated, the axial nerve thread was affected 

 in the same way as when a wire is pulled or shaken, and this 

 motion acted upon valves in the walls of the cerebral ventricles, 

 allowing the animal spirits to escape. The latter flowed out- 



