NEGATIVE AND POSITIVE AFTER EFFECTS. 



539 



The essential factor in the production of the modification is the 

 duration of the immersion of the nerve in 0'6 per cent. NaCl. 



The modified nerve giving positive variations can be rendered less 

 susceptible to such production by two agents, prolonged activity and 

 C0 2 gas, both of which agents augment the negative variation of 

 the freshly excised nerve. These agencies thus decrease the positive 



FIG. 277. Excitatory variations in fresh and stale nerves. A, Ex- 

 citatory variation in fresh nerve, large negative (down) effect 

 followed by oscillations and negative after effect; B and C, 

 excitatory variation in early stage of stale nerve, small negative 

 (down) effect, followed by conspicuous positive (up) effect ; in 

 C, there is a positive after effect ; D, excitatory variations in 

 later stages of stale nerve, the excitatory variation is now 

 positive (up) only. The tracings in the above series are to be 

 read from right to left. After Waller. 



and augment the negative one. If prolonged excitation of a nerve 

 in the fourth stage is interposed among the members of the series of 

 short groups of excitations, then the extensive deflection thus produced 

 is seen to be first positive, rapidly passing into negative. On the 

 cessation of the prolonged excitation, each excitation group evokes 

 an effect of the second stage, being first negative and then positive ; 

 this gradually changes into the third, and finally may again reach the 

 fourth stage, i.e. + only (Fig. 278). 



FIG. 278. The portion upon the left hand of the figure shows the in- 

 fluence of external C0 2 upon the variation obtained from modified 

 or stale nerve. Before the C0 2 the variations are positive (up), 

 followed by negative (down) ; after C0 2 they are large negative 

 (down), followed by positive (up). The right hand portion shows 

 the influence of prolonged faradisation upon such variation. 

 The faradisation occurs from a to w, the variation before is 

 positive (up) ; after it is negative (down), followed by positive 

 (up). After Waller. 



A nerve in a less modified stage, similarly treated, may give a 

 large negative effect only during the prolonged excitation ; on the 

 cessation of this, each group of the responses resembles that of the 

 fresh nerve, being negative only ; this gradually gives way to - + 

 and finally to + - effects. 



