CHAPTER III. 



OF MOTKICITY. 



In the case we have just mentioned at the end of the preceding 

 chapter, the series of phenomena may, spite of its complexity, be 

 summed up in a brief formula. A peripheric excitation is trans- 

 mitted by nervous fibres to multipolar cells, -which transform it 

 and send it back under the form of motory incitations along 

 other nervous threads. We may in a sort of rough way compare 

 the agitated nervous cell with a mirror which reflects an incident 

 ray. But when the shock is transmitted through the brain, its 

 reflection is not accomplished so simply, and along the path are 

 awakened phenomena altogether special, conscious psychical 

 phenomena. It is wholly otherwise when the conscious nerv- 

 ous centres do not come into play. Then everything comes to 

 pass silently ; the molecular shock comes from the periphery, 

 and is sent back thereto, but its passage is not seen. There is 

 thus truly what is called in physiology a reflex action. As a very 

 great number of nervous phenomena of every order may be 

 regarded as simple reflex actions, it is indispensable to speak 

 with some detail of these primordial acts of the physiology of 

 the nervous system. 



Three principal cases can present themselves : the centre, 

 which is the seat of the motory reflection, is either a simple 

 mvdtipolar cell, or a ganglionary nervous centre, or the spinal 

 marrow of a vertebrate. 



