108 ELECTRO-PHYSIOLOGY CHAP. 



nerves (or striated muscle)." The same fact appears also in the 

 slender cutaneous (sensory) nerves of the frog's back, as shown by 

 Pick (65). "When, instead of giving single shocks, the spring of 

 the induction apparatus is set vibrating, no such enormous strength 

 of current is required to evoke (reflex) muscular contractions." 

 The same phenomena of summation have recently been investi- 

 gated by A. Ward (66), who found in the brainless frog that the 

 .application of electrical stimuli, as nearly as possible uniform in 

 quality and intensity, but not capable intrinsically of producing a 

 reflex twitch, did after a certain number of stimuli evoke the same, 

 if the excitation was repeated at intervals of about 0'5 sec. The 

 required stimulation -frequency was about the same on raising the 

 interval to 0'4 sec. 



These, like all similar phenomena, can obviously be explained 

 only on the presumption that a stimulus that is in itself in- 

 effective produces a certain alteration in the nerve-cell (as in 

 other cases in muscle, gland-cells, etc.), which is favourable to the 

 production of an effective excitation, or rather is itself a weak 

 excitation, and summates along with similar changes from 

 succeeding stimuli, until an effective discharge is produced. 

 Within the limits cited by Ward the time-interval is immaterial, 

 as appears from the fact that the alteration caused by excitation 

 is perceptibly of the same magnitude after 0'4 sec. In principle, 

 these central summations are in no way dissimilar from those 

 induced under certain conditions in the peripheral organs, and the 

 nerve-cells only differ in degree from muscle, gland-cells, etc. 



The same is sometimes expressed in a striking after-effect of 

 tetanisation. After prolonged tetanisation of the divided spinal 

 cord in the frog (near the cross-section), the same (descending) 

 opening current that was formerly quite ineffective is often found 

 to produce vigorous twitches, and this effect only dies out slowly 

 after an interval of several seconds (Fig. 170). This phenomenon 

 apparently stands in close relation with that termed by Exner 

 " canalisation " (" Balmunrj "), as opposed to " inhibition," in the 

 antagonistic action of excitation within the central nervous system. 

 If as can hardly be doubted we are here dealing essentially 

 with alterations of excitability in the conducting elements of the 

 gray matter of the lumbar region, an effect analogous to that of 

 canalisation might also be expected in cases where the modifying 

 and testing stimuli act in succession upon the two opposite poles 



