8 Foster, Physical Basis of Psychical Events. 



nucleus, or at all events towards the beginning of the axon. 



It is in our power to excite or stimulate by artificial 

 means, by electric currents or by other forms of stimulus, 

 these nerve fibres lying outside the central nervous system. 

 And we find invariably that, whatever part of the course 

 of a fibre we may artificially stimulate, we produce by 

 means of afferent fibres effects in the central nervous 

 system only, none in the periphery, and by means of the 

 efferent fibres, effects in the periphery only, none in the 

 central nervous system. The former acts apparently in 

 a centripetal manner only, the latter in a centrifugal 

 manner only. 



I may now return to the question — What events can 

 be ascertained to take place in a nerve fibre when it is 

 stimulated ? 



When we study the events taking place in other tissues, 

 in muscular fibres, gland cells, and the like, we find clear 

 evidence that the phase of activity, the contraction of a 

 muscular fibre, the act of secretion in a gland cell, is accom- 

 panied by chemical changes, by electric changes, and by 

 the setting free of energy in the form of heat. In the gland, 

 where chemical change is the goal of activity, the develop- 

 ment of heat is economised or even minimised ; in the 

 muscle, where energy is in like manner the goal, the 

 development of heat is larger, and is utilised for the good 

 of the economy. When, on the other hand, a nerve fibre 

 is stimulated, the most careful examination fails to detect 

 either chemical change or the setting free of heat. The 

 phase of activity of a nerve fibre is marked by one objec- 

 tive token and by one only, an electric change giving rise 

 to an electric current known as the current of action. This 

 electric change is witnessed during the activity of a nerve 

 fibre in all cases, not only when the nerve is excited arti- 

 ficially, but also when it is the instrument of natural 

 nervous events, of sensation, and of movement. Moreover, 



