GENERAL NERI/E PHYSIOLOGY 225 



(r) The velocity of the conduction of the impulse through 

 the cells is much less than that through the fibres. 



From a physiological standpoint, the individual processes 

 of conduction through the cells differ from each other only 

 in the number of the impulses passing through the neurons, 

 and in the modification of the impulse in the cells. From a 

 psychical standpoint we may classify the processes of con- 

 duction through nerve cells into: 



1 . Conduction of impulses through the cells not accom- 

 panied by consciousness. This includes the reflexes, i.e. 

 the transferring of an impulse from a centripetal fibre through 

 a centre to a centrifugal fibre without resulting in conscious- 

 ness, which may indeed occur against the will. 



2. Psycho-physical processes, which are accompanied by 

 consciousness. The transferring of the impulse from the 

 sensory nerve fibre through the central nervous system to 

 the motor nerve, which occurs voluntarily, is called * ' volun- 

 tary reaction," in distinction from reflex action. 



The chemical processes which take place in the resting and the 

 active nerve cells are not known. That they are intense is apparent 

 from the fact that even temporary cessation of the blood supply soon 

 causes injury. Death of the nervous system through asphyxia 

 occurs in warm-blooded animals in a few minutes. 



