Ixxx INTRODUCTION 



In addition to the reservoir of nervous fluid already men- 

 tioned, Lamarck imagined a nucleus {foyer) at which all the 

 afferent nerves meet. It is to be understood that he con- 

 sidered what we should now call the afferent and efferent 

 portions of the nervous system to be two entirely distinct 

 systems, though with a channel of communication for the 

 passage of nervous fluid between them. The efferent nerves 

 issued from their common reservoir ; the afferent nerves 

 terminated in a common nucleus, situated, he believed, in the 

 medulla oblongata. The relation between the nucleus and 

 the reservoir is nowhere definitely stated. 



Lamarck's idea of sensation, then, was as follows : some 

 stimulus affects a nerve-ending in some part of the body ; 

 say the stimulus of contact by some foreign body. The 

 nervous fluid at the termination of the nerve affected is 

 immediately thrown into agitation ; and this agitation travels 

 up the nerve till it reaches the nucleus. Thereupon it is 

 propagated through the nucleus into every other sensitive 

 or afferent nerve in the body. The disturbance is carried 

 down all these nerves to their endings ; it then recoils and 

 returns along them till once more it arrives at the nucleus. 

 Hence there is a simultaneous reaction upon the nucleus by 

 all the afferent nerves, save that which brought in the original 

 agitation. This nerve alone remains passive, while the rest 

 are undergoing the action and reaction of the disturbance 

 which it aroused. Hence when the reaction reaches the 

 nucleus, and thus throws it for the second time into agitation, 

 the entire effect of that second agitation is concentrated on 

 the single nerve, which alone had not reacted because it 

 brought the original impression. The agitation now travels 

 back along that nerve to its ending, where the original 

 stimulus was applied. Hence the location of sensation at 

 the point of the body first stimulated : but such localisation 

 is regarded by Lamarck as an illusion, for sensation is a 

 general effect depending upon an agitation throughout every 

 portion of the sensitive system, and not upon processes 

 occurring in any single part of that system. It is worth 



