| Plato's particles and Descartes' Animal Spirits | ||
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Plato had assumed that there were various kinds of particles governing the workings of the body. The best, the most nearly circular (and the lightest) ones governed the soul, the next best were to be found in he heart governing the emotions, while the roughest, crudest particles were to be found in the liver controlling things like lust and greed. This philosophical legacy explains why, two thousand years later, for Descartes, the heart produces a stream, of coarse and fine particles, carried along in the blood, which are then sorted out by the brain in such a way that the coarse particles are used to nourish it, whereas the small particles are separated out from the blood and become instead 'animal spirits'. These spirits can then enter the 'pores and conduits' of the brain and from there can stimulate nerves and cause movements. Descartes refers to this theory in the Discourse on Method (as part of a grisly description of cutting up a monkey) where he remarks: "I had explained all these matters in the Treatise which I formerly intended to publish. And afterwards I had shown there, what must be the fabric of the nerves and muscles of the human body in order that the animal spirits therein contained should have the power to move the members [ie. the arms and legs], just as the heads of animals [continue to function] a little while after decapitation... At the end of the seventeenth century, Descartes’ contemporary, Borelli and William Croone in London had refined the theory to replace ‘animal spirits’ with a supposed ‘nervous juice’ or fluid flowing along the nerves to inflate small bladders in the muscles. It was one century after Descartes that scientists like van Leeuwenhoeck and Fontana, using the new microscopes, observed the structure of nerves more directly. Fontana found little grains in the ‘cylinders’ making up the nervous fibres, but thought they hardly qualified as animal spirits as they ‘seemed too lazy to move’. (Also see History of the synapse, Max R. Bennett, Taylor and Francis 2001 for more on this) |
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