154 THE MECHANISM OF LIFE 



, a " rational soul," while the bodies of the lower animals did not. 

 The latter could not feel pain or pleasure, and they acted quite 

 unconsciously ; but the rational soul in the human body felt the 

 stimulus and response, and could even modify the latter, or could 



♦ initiate movements of the body by its own volition. Neverthe- 

 less, the body of a man or woman was an automaton, and it could 

 do all that the body of a lower animal could do even when the 

 rational soul was inactive. 



To understand this conception we must try to forget our 

 modern notions of matter and energy, and we must study 

 Descartes' cosmogony. We shall return to the latter presently, 

 but in the meantime we must consider the physiology that was 

 current before his time and the ways in which he modified it. 

 Now the science of mechanics was very well developed at the 

 beginning of the seventeenth century, and craftsmanship had 

 reached a very high level of attainment. Anatomy had long 

 been studied, so that the general structures of the bodies of man 

 and the lower animals were very well known and had been 

 described in considerable detail. Thus the gross anatomy of the 

 skeleton, muscles, viscera, bloodvessels, brain, and nerves was 

 known to a degree that is not greatly inferior to our knowledge 

 (which excels that of medieval times mainly in its minutiae). 

 The microscope had not been invented, or at least had not been 

 improved to such an extent as to render it an aid to anatomy, 

 and therefore such organs as the brain, the muscles, nerves, and 

 viscera had then little of the complexity that we now know them 

 to possess. Our modern chemistry did not exist, so that there is 

 hardly what we should call a single chemical idea in all Descartes' 

 physiology. Finally, our kinetic energy was then called vis viva, 



I and there was nothing at all comparable with our essentially 

 modern conception of potential energy. 



The physiology was, then, largely that of Galen, but it was 

 modified very remarkably by Harvey's demonstration of the 

 circulation of the blood. The materials of the food taken into 

 the stomach were supposed to be converted by the agitation of 

 their particles into chyle, a turbid fluid which is present in the 

 small intestine. This was absorbed by the bloodvessels of the 

 gut, and was carried to the liver as the " natural spirits." In 

 the latter organ it became endued with the " vital spirits," and 

 the blood, containing this fluid, ascended to the heart, and was 

 poured into the right auricle. There was a " fire " or " innate 



