DESCARTES AND PHYSIOLOGY 39 



ated by specific agencies known as the vital 

 and vegetative 'spirits.' 



Descartes was the first to put forward a 

 thorough-going mechanistic theory of the 

 working and development of the animal 

 body. His De Homine and De Forma 

 tions Foetus are works of great biological 

 interest, on account of their influence on 

 subsequent thought. The body is repre 

 sented, in so far as physiological knowledge 

 extended at the time, as nothing but a piece 

 of mechanism ; and its development is also 

 represented as a simple mechanical process. 

 The animal spirits are depicted as a subtle 

 fluid separated and filtered off into the 

 ventricles of the brain by a purely mechanical 

 process. The afferent nerves are supposed 

 to be fine fibres leading up to the ventricles 

 of the brain and connected with valves open 

 ing or closing the upper ends of the efferent 

 nerves. These latter are supposed to be 

 tubes conveying the animal spirits from the 

 ventricles. As a result of any sensory or 

 afferent stimulus the threads are pulled, and 

 the connected valves consequently opened, 

 with the result that the ' animal spirits ' rush 



