THE LIMITS OF EFFICIENT STIMULATION. 475 



excitatory states are propagated to the muscle is such that at 15° C the 

 one will reach the motorial nerve endings only - 0005 sec. in advance of 

 the other. The results are complicated hy the conditions of the experi- 

 ment, since the part of the nerve subjected to the electrical stimulation 

 is changed as regards both conductivity and excitability, these changes 

 being dependent upon the direction and the intensity of the exciting 

 currents. In the most favourable conditions, it has been found that the 

 effect of the two excitations is to evoke a contraction which is larger 

 than that evoked by either excitation alone. This may mean that 

 two excitatory states recurring at 20V0 sec - interval can be propagated 

 along the nerve without blending ; in other words, the upset of nerve 

 equilibrium by the first stimulus does not interfere with the production 

 of a fresh disturbance by a second one at this interval. 1 



Another method is that of exciting the nerve at the same point by 

 two successive electrical stimuli. The alterations as regards both 

 conductivity and excitability are even more pronounced under these 

 conditions, but in spite of these it has been found possible, by the use 

 of minimal stimuli, to evoke, with two excitations T oVo" sec - apart, a 

 response which is larger than that produced by either single excitation. 2 

 Recent experiments carried out under the direction of the present 

 writer have, however, limited the extent of this capacity of the nerve 

 to respond to a second stimulus. 3 These show that a stimulated muscle 

 contraction cannot be evoked by double excitation of a nerve at 3° C, 

 unless the stimuli are o^ F sec. apart, whereas at 15° there is evidence 

 of summation with stimuli at l0 1 00 sec. to l0 2 0<) sec. apart. 



A large number of rapidly recurring excitations of the same point 

 of the nerve have also been used. The difficulty in this case is the 

 uniform production of the rapid series of electrical currents. By the 

 use of a small dynamo, it is possible to obtain a series of sinusoidal 

 current variations, at a rate of 1800 to 2000 per second. With this 

 rate the nerve at 15° C can be excited, and the muscular response is 

 a prolonged tetanic contraction. 4 It was observed by Bernstein that 

 with a rapid series of make and break induction shocks, the resultant 

 muscular response, when these recurred at 500 per second, was not 

 a tetanic but a single initial contraction. 6 It was therefore surmised 

 that only the first excitatory state was adequately produced. The facts 

 just mentioned are opposed to any generalisation from these experi- 

 ments. The failure of the tetanic response undoubtedly implies that 

 at the temperature employed a relatively smaller nerve disturbance 

 is produced by the successive excitations, as compared with that evoked 

 by the first of the series, but it does not mean that at intervals of -g^ sec. 

 nerve excitation is impossible. 6 This is shown by experiments in which 

 telephone currents, augmented by an appropriate microphone, are used 

 for purposes of excitation. With these, Roth found that it was pos- 

 sible to excite the nerve so as to obtain a tetanic muscular response, 

 with excitations recurring at from l0 1 00 sec. to w Vo sec. interval. He 

 observed, however, that as the rate increased the intensity of the stimuli 



1 Werigo, Arch. f. d. ges. Physiol., Bonn, Bd. xxxvi. 



3 Sewall, Joum. Physiol., Cambridge and London, 1880, vol. ii. ; 1881, vol. iii. 



3 Boycott, ibid., 1899, vol. xxiv. 



4 Langdon and Schenck, C'incin. Lancet-Clinic, 1896. 



5 Bernstein. " Ueber d. Erreg. Nerven- 11. Muskelsysteme," 1871. 



6 Engelmann, Arch. f. d. ges. Physiol., Bonn, 1871, Bd. iv., who also found that failure 

 of excitability occurred in nerve stimulated by rapidly recurring currents. 



