xj of the Nervous System. 265 



"upon the parts of the brain whence they take origin, and 

 "by this means open up the orifices of certain pores which 

 "exist on the internal surface of the brain. Through these 

 "pores the animal spirits which are in the ventricles imme- 

 " diately begin to make their way and thus pass into the nerves 

 "and so into the muscles which carry out in the machine of 

 " which we are speaking movements exactly like those to which 

 " we ourselves are incited when our senses are affected in the 

 "same way. 



"If for example fire comes near the foot, the minute 

 "particles of this fire which as you know move with great 

 " velocity, have the power to set in motion the spot of the skin 

 " of the foot which they touch, and by this means pulling upon the 

 " delicate thread which is attached to the spot of the skin, they 

 "open up at the same instant the pore against which the 

 " delicate thread ends, just as by pulling at one end of a rope 

 " one makes to strike at the same instant a bell which hangs on 

 " the other end." 



According to Descartes then the movements of the body 

 viewed as 'an earthly machine' are brought about by that part 

 of the brain which forms the walls of ventricles serving as 

 centre where the play of sensitive impulses communicated by 

 the delicate threads of the marrow of the nerves determines 

 the outflow of the motor animal spirits along the tubular 

 channels of the nerves. And making use of the physical 

 properties of the delicate threads on the one hand, and of 

 the subtle but powerful fluid, the animal spirits, on the other 

 hand, by the help by various devices such as valvular arrange- 

 ments in the nerves, the existence of which he takes for 

 granted, he gives a detailed exposition of the varied working 

 of the machine. He does not hesitate to assume the existence 

 in the nerves of various physical properties and to explain by 

 means of them various nervous and even psychical phenomena. 

 He finds for instance the physical basis for habit and memory 

 in the following : 



" Consider also that an important feature of these delicate 

 " threads is the property of being easily bent in every kind of 

 " way by the mere force of the spirits which press upon them 



