NERVE 85 



electric variation occurs in B, it does not occur in A. The 

 impulse has failed to cross the synapse. 



6. A nerve may be stimulated again and again at very short 

 intervals of time, the refractory period beinc; about -^ — 



-on .,. 6000 



sec. Kenex arcs manifest much more prolonged refractory 

 periods during which it is impossible to elicit another 

 response. This is of importance because a reflex act takes 

 a very appreciable time to be completed, and unless it is 

 completed before it is again started confusion of movements 

 would be produced. 



By the use of the spinal dog (p. 82) Sherrington has 

 been able to show how it is that definite reflex actions 

 are possible, and how, in spite of the number of stimuli 

 which are constantly fidling on the body, there is no confusion, 

 no mixture of reflexes. 



Fig. 35. — To show the method of demonstrating the Valve Action 

 in reflexes by means of the electrical response. 



He finds that the response varies with the character and 

 locality of the stimulus. 



Thus, if the hind foot of the dog be pinched, a flexor 

 withdrawal of the leg occurs varying in extent with the 

 strength of the stimulus. If the stimulus is strong, this 

 may be accompanied by an extension of the opposite leg — a 

 crossed extensor thrust. 



If, on the other hand, a finger is thrust into the pad of 

 the dog's hind foot, an extensor thrust, such as occurs in 

 the act of walking, is produced. 



If, while this extensor thrust is being produced, the foot 

 is pinched — i.e. if a harmful stimulus is applied, the 

 extensor reflex is checked and is replaced by the flexor with- 

 drawal reflex. As Sherrington puts it, nocuous stimuli are 

 prepotent. 



