REFLEX ACTION 



493 



It would be premature to attempt an explanation of why the synaptic membrane is per- 

 meable to excitation in one direction only. It may be that it is permeable to one ion only of 

 an electrolytically dissociated colloid, in the way described on page 141 for the system of 

 Congo-red and parchment paper. In such cases, an electrical current can only pass in one 

 direction. But further knowledge is needed. 



Refractory Phase, This characteristic property of muscle and of nerve trunks 

 has been described above. In many reflexes, such as the scratch reflex, it is 







FIG. 154. RECIPROCAL INNERVATION OF THE EYE MUSCLES, ACCORDING 

 TO DESCARTES. Gutschoven's sketch to illustrate Descartes' description. 



(Descartes, 1677, p. 15.) 



very marked. This reflex consists of alternate flexion and extension at a rate 

 of about four times per second. This rate is independent of the frequency of the 

 stimulation, so that it is the same when the stimulus is a constant current. 

 High-frequency currents are also very effective (Sherrington, 1906, pp. 48 and 49). 

 Alteration in strength of stimulus has no effect on the rate of discharge. Suppose 

 that the stimuli are applied at the rate of one hundred shocks per second, it is 

 clear that the greater number of these must be ineffective ; in other words, they 

 fall in a refractory period. 



We have already seen that the reflex arc in this case consists of at least three 



