26 CHANGES OF EXCITABILITY IN NERVES 



excitability is greatest. It increases with the strength of the 

 current till the latter has reached a certain grade. 



If the experiment is pretty frequently repeated the enhancement 

 of excitability remains much the same during each closure of the 

 current, even when the contractions elicited by the mechanical 

 stimulus alone fall off. 



EXAMPLES. 

 Experiment 6. 



+ P = 20 ; P = 40 ; E = 37-5 ; 3 Meidinger, Eh. = 92 ; 7=17. 



a. The mechanical stimulus produces weak muscular contractions ; 

 the polarising current only a make-contraction. Under its influence 

 the contractions grow to more than double their size, and during the 

 passage of the current show an exceedingly slight falling off. As soon 

 as the current is opened the contractions fall off, but are still some- 

 what greater than they originally were. The same phenomenon is 

 repeated if the experiment is gone through several times in suc- 

 cession. 



b. The nerve is stimulated somewhat farther up with the same 

 strength of mechanical stimulus (E = 38). The polarising current is 

 now stronger (3 Meidinger, Rh. = 150). With this arrangement the 

 contractions rise from a very low point to an equal height with 

 those in a. On opening the current they disappear completely, to 

 reach the same strength as before when the current is closed. In 

 other respects the phenomena of a are here repeated. 



Experiment 7. 



+ P = 10 ; P = 30 ; E = 29 ; I Meidinger, Eh. = 1950 ; 7 = 5. 



The mechanical stimulus produces exceedingly slight contractions ; 

 the polarising current only make-contractions. Under its influence 

 there occur strong, almost maximal, contractions, which keep through- 

 out at the same height, and instantaneously cease when the current is 

 opened. When the current is closed the strong contractions set in 

 afresh. After the nerve has rested for a minute the experiment is 

 repeated from the beginning in the same way. There now appear, 

 after the current has been twice closed and opened, small muscular 

 contractions, showing that the excitability of the nerve has increased. 



Under the influence of the polarising current the same strong 

 contractions as before take place ; they are now however not of so 

 uniform a height as at the beginning of the experiment, but display 

 slight irregularities. 



