156 THE MECHANISM OF LIFE 



wards through the tubuli of the nerves, and so into the muscles. 

 Expanding in the latter, they produced the contractions and 

 relaxations which moved the parts of the body. 



Now in all this there is nothing but mechanism, in the strict 

 sense. A fluid (the blood) is expanded and rarefied by heat; 

 gross and cold, or slowly moving constituents are separated from 

 finer, more ardent, and more quickly moving constituents merely 

 by the difference of the motions ; larger and coarser particles are 

 separated from smaller and finer ones by sieves; liquids flow 

 through tubes, and the thinner parts flow through the narrower 

 tubes more quickly than do the thicker parts; there are valves 

 that are pulled open by stretched organs; there are muscles, 

 which swell or relax according to the way an expansible fluid is 

 thrown into them; these muscles act upon levers, the bones, 

 which thereupon move the limbs and other parts of the body. 

 All this is pure mechanism in our sense. 



What were the animal spirits ? It is very difficult to avoid 

 reading into Descartes' physiology our present-day notions of 

 energy; still, the spirits were a fluid, but a very rare and subtle 

 one, hot, and thin, and ardent — that is, they were kinetic in our 

 sense (like a highly heated gas under pressure). But they could 

 be confined in the thin-walled cerebral ventricles and nerve 

 tubuli, so that their energy must have been repressed in some 

 way — in our term, the energy was potential until it was released 

 by the afferent nervous impulse. As Huxley says, a relatively 

 slight change in Descartes' terminology would have brought his 

 physiology into line with ours. 



Obviously he did very much what we do in our chemical and 

 physical hypotheses of living activities — he pushed his specula- 

 tions to their limits, as the mathematicians would say. His 

 analysis of structure stopped at what could be revealed by dis- 

 sections, since there was no microscope, and therefore he assumed 

 that his mechanism held true, in the parts that were beyond his 

 observation, just as it did in the parts that he could see. It is 

 very doubtful, however, whether he would have modified his 

 speculations greatly even if he had known as much about the 

 minute structure of the animal body as we do. For instance, 

 the histology of muscle and nerve, as we know it, would lend 

 itself quite well to his mechanical explanation, and the structure 

 of the kidney, as we have sketched it on pp. 71-3, can easily 

 be assumed (in the absence of chemical data, it must be noted) 



