388 PHYSIOLOGY 



shows signs of fatigue, and the minute change in locus of stimulus 

 which is required to reinduce a practically identical action shows that 

 the seat of fatigue must lie chiefly on the afferent side of the arc ; 

 perhaps in the first synapses through which the impulse has to pass. 

 This easy incidence of fatigue tends to cut short any given reaction 

 and to render it easier for other reactions to take its place. 



Just as excitation causes fatigue and therefore furnishes a hin- 

 drance to repetition of the same act, so the reverse process of inhibition, 

 which is a large component of every reaction, is followed by a condi- 

 tion of increased excitability, or diminished resistance to the passage 

 of impulses. In each case there is a tendency for a ' swing-back ' to 



FIG. 173. * Mark-time ' reflex in spinal dog, inhibited by slight stimulation 

 of the tail (duration of stimulation shown by signal). Note the augmenta- 

 tion of the mark-time reflex following the inhibition (successive spinal 

 induction). (SHERRINGTON.) 



take place from inexcitability to over-excitability, from excitation to 

 inexcitability. This ' successive spinal induction,' as it has been 

 termed, may be seen on inhibiting some movement by the excitation of 

 an independent reflex. Thus the scratch reflex is excited, and then 

 while the excitation is still continued the reaction is inhibited by 

 excitation of the extensor or stepping reflex. As soon as the ' step-, 

 ping ' reflex has passed off, the scratch reflex returns with an intensity 

 greater than before. This successive spinal induction explains the 

 tendency which exists in the spinal cord to an alternation of response ; 

 every act tending to come to an end by fatigue and, as a result of 

 negative spinal induction, inducing the opposed or antagonistic act. 

 Thus if a spinal dog be held up in the vertical position, so that the 



