474 NERVE. 



has continued for six hours, with the sciatic nerve of the frog the 

 nerve still responds to the stimulation, for, on the removal of the 

 block, the muscular contraction reappears in its initial intensity. 



The propagation has been blocked by the local application of ether 

 vapour to a portion of nerve intervening between the excited region and 

 the muscle ; on removing the ether, the excitation of the nerve still 

 caused the muscle to respond after twelve hours. 1 The secretory nerve- 

 endings of the chorda tympani can be paralysed by atropin, and the 

 continuous excitation of these fibres is seen to become effective as the 

 atropin wears off. 2 



The electrical changes in the nerve have been taken as an index by 

 Waller. In this case the indication of each group of electrical responses 

 was obtained by photographing the galvanometric variation. The nerve 

 was submitted to a short series of rapid excitations of similar intensity 

 which recurred at regular intervals of one minute, and the resulting 

 effects showed no evidence of diminution through fatigue. 3 



Nerve fibres are thus remarkable in being so differentiated as to be 

 unaffected by repeated activity, and they offer a striking contrast in this 

 respect to other parts of the nervous system. Thus the nerve junctions 

 in the central nervous system and the neuromuscular connections are 

 readily fatigued, the latter more readily than muscle itself ; so that if a 

 muscle is indirectly excited, until it can no longer respond, it will be 

 found that it is still capable of contracting to direct stimulation. 



In these cases we may assume either that the terminal nerve fibres 

 differ from the fibres of the main trunk, or that the fatigue is localised 

 in the field of conjunction which lies on the further side of these 

 terminal fibres, whether in muscle, gland, or central grey matter. In 

 this connection the fatigability of the isolated electrical organ may be 

 referred to, the activity of which is in all probability that of the 

 terminal nerve fibres. 4 



The limits of efficient stimulation by rapidly repeated stimuli. 

 The second method of inquiry, although apparently of a different 

 character, involves to some extent the same issues. Can a nerve be 

 excited by a series of stimuli, when these recur at very rapid intervals, 

 and if so, at what point is a limit reached for such excitation ? The 

 muscular response is in this case an adequate index of nerve excitation, 

 and the simplest method of obtaining information is one in which two 

 electrical excitations of the nerve occur in rapid succession. Under 

 these circumstances, the two muscular responses must obviously be 

 fused, and when the second excitation occurs, at a very brief interval 

 after the first, this summated muscular effect may be so slight as to 

 be with difficulty distinguished from that of the single twitch. Those 

 cases in which such a difference can be detected furnish undoubted 

 evidence that the nerve has been adequately excited by the two 

 stimuli. Various methods have been employed for obtaining two ex- 

 citatory states in rapid succession. The nerve may be simultaneously 

 excited at two points, one near, the other remote from the muscle. 

 In this case, with a distance of 15 mm. between the seats of excita- 

 tion, the difference in the length of the nerve along which the 



1 Maschek, Sitzungsb. d. Jc. ATcad. d. Wissensch., Wien, 1887, Bd. xcv. Abth. 3. 



2 Lambert, Compt. rend. Soc. debioL, Paris, 1894. 



3 Waller, Croonian Lecture, Phil. Trans., London, 1896. 



4 Gotch and Burch, ibid., 1896, vol. clxxxvii. B, p. 347. 



