228 



IRRITABILITY 



lus is necessary. Here the weakening of the excitation as well as 

 the prolongation of the reaction time is brought about by the 

 introduction of intercalated neurones. The reflex arc contains 

 more stations. (Figure 61.) If we accept the most plausible 



Fig. 61. 



Scheme of the simplest reflex arc from one to the other side, and 

 from a higher to a lower level. 



assumption that the central connection of antagonistic muscles 

 possesses like relations, then the effects discovered by Sherrington 

 are self-explanatory. In this case stimulation of the sensory 

 path, which brings about a strong reflex excitation of the motor 

 neurons of the anterior horns controlling a muscle, at the same 

 time stimulates the antagonistic muscle with sub-threshold stimuli. 

 The result of this as shown by the experiments of Vessi is not a 

 motor response of the antagonists, but an inhibition if the motor 

 neurons of the antagonists are at the time in a state of excita- 

 tion. It is, therefore, understandable that reflex excitation of a 



