344 PHYSIOLOGY 



system, so that response becomes general and inco-ordinate instead 

 of local and adapted to the stimulus. 



(6) FACILITATION OR ' BAHNUNG.' The passage of a nervous 

 impulse across a synapse or series of synapses in the central nervous 

 system has a twofold effect. If the passage be too often repeated 

 phenomena of fatigue are produced, and there is an increase of the 

 block at each synapse. If, however, the stimulus be not excessive 

 and the reaction not too frequently evoked, the effect of passage of 

 an impulse once is to diminish the resistance, so that a second applica- 

 tion of the stimulus evokes the reaction more easily. The process of 

 summation in fact is chiefly in the direction of removal of block. We 

 have a close analogy to this process of facilitation in the ' staircase 

 phenomenon ' observed in cardiac and unstriated muscle. In these 

 tissues the repetition of a sub-minimal stimulus renders it in time 

 effective, and then repetition of the now effective stimulus causes 

 a gradually increasing height of contraction, which depends on the 

 state of the contracting tissue itself and cannot be evoked by changes 

 in the strength of the stimulus. This process of facilitation or ' Bah- 

 nung ' is of great interest in connection with the development of ' long 

 paths ' in the central nervous system, and more especially with the 

 acquirement of new reactions by the higher animals. The Law of 

 Facilitation is really the Law of Habit. When an impulse has passed 

 once through a certain set of neurons to the exclusion of others it will 

 tend, other things being equal , to take the same course on a future 

 occasion, and each time that it traverses this path the resistances in 

 the path will be smaller. Education is the laying down of nerve- 

 channels in the central nervous system, while still plastic, by this 

 process of ' Bahnung ' along fit paths, combined with inhibition (by 

 pain) in the other unfit paths. Memory itself has the process of 

 ' facilitation ' as its neural basis. 



(7) INHIBITION. The constant occurrence of a reaction in response 

 to a given stimulus is obtained only if care be taken to isolate the seg- 

 ment of the central nervous system involved from the entry of other 

 afferent stimuli. As a rule, if two stimuli be applied simultaneously 

 at different points, the reaction which ensues will not be a combined 

 one, the resultant of the reactions which would be normally condi- 

 tioned by each single stimulus, but will be a response to one of the 

 stimuli, which we must therefore regard as the more effective. The 

 reaction to the other stimulus is either abolished altogether or comes on 

 after a considerable period of delay. The central nervous system 

 can apparently attend to only one thing at a time. In physiological 

 terms we should say that every effective reaction inhibits every other 

 reaction. In the spinal cord of the frog the normal withdrawal of the 

 foot in response to stimulation of the toe of the same side can be 



