96 AN AMERICAN TEXT- BOOK OF PHYSIOLOGY. 



Not only has an attempt to discover this or other waste products which 

 might be supposed to result from chemical changes within the nerve-fibre 

 failed, but observers have been unable to obtain evidence of the liberation 

 of heat, which one would expect to find were the nerve-fibre the seat of chem- 

 ical changes during the process of conduction. 1 Stewart writes: "Speaking 

 quite roughly, I think we may say that in the nerves of rabbits and dogs there 

 is not even a rise of temperature of the general nerve-sheath of 2Q 1 00 of a 

 degree during excitation." 



.Many experiments have been made to ascertain whether a nerve would 

 fatigue if made to conduct for a long; time. Most of these have been made 

 upon motor nerves, the amount of contraction of the muscle, in response to a 

 definite stimulus applied to the nerve, being taken as an index of the activity 

 of the nerve. Since the muscle would fatigue if stimulated continuously for 

 a long time, various means have been employed to block the nerve-impulse 

 and prevent it from reaching the muscle, except at the beginning and end of 

 the experiment. This block has been established by passing a continuous 

 current through the nerve near the muscle, thus inducing an electrotonic 

 change and non-conducting area; 2 or the nerve-ends were poisoned with 

 curare (see p. 26), and the nerve excited until the effect of the drug wore off, 

 and the nerve-impulse was able to reach the muscle '/ or the part of the nerve 

 near the muscle was temporarily deprived of its conducting power by an 

 anaesthetic, such as ether. Another method of experimentation consisted in 

 using the negative variation current of a nerve (see p. 150) as an indication 

 of its activity, the presence of the current being observed with the galvanom- 

 eter.* Other experimenters have examined the vagus nerve, to see if after 

 long-continued stimulation it was still capable of inhibiting the heart, the 

 effect of the stimulation being prevented from acting on the heart muscle 

 during the experiment by atropin, 5 or by cold, applied locally to the nerve. 6 

 Still another method was to study the effect of long-continued stimulation on 

 the secretory fibres of the chorda tympani, the exciting impulse being kept 

 from the gland-cells by atropin. 7 Most of these experiments have yielded nega- 

 tive results, and it is doubtful whether nerves are fatigued by the process of 

 conduction. 



These results, of course, do not show that the nerve-fibres can live and 

 function independently of chemical changes. As has been said, nerves lose their 

 irritability in time if deprived of the normal blood-supply, and undoubtedly 

 they arc, like all protoplasmic structures, continually the seat of metabolic 



1 Helmholtz: Archiv fiir Analomie und Physiologie, 1848, S. 158. Heidenhain : op. cit. 

 Rolleston: Journal of Physiology, 1890, vol. xi. p. 208. Stewart: ibid., 1891, vol. xii. p. 424. 



2 Bernstein : Pfiiiger's Archiv, 1877, xv. S. 289. Wedenski : Centralblatt fur die medic inischen 

 Wwaenschqften, 1884. 



• , Bowditch : Journal of Physiology, 1885, vi. p. 133. 



* Wedenski : loc. cit. Maschek : Sittungsberichte der Wiener Academic, 1887, Bd. xcv. Abthl. 3. 



5 Szana: Archiv fur Anatomic und Physiologie, 1891, 8. 315. 



6 Howell, Budgett, and Leonard: Journal of Physiology, 1894, xvi. p. 312. 



7 Lambert: Comples-rendus dc la Societe de Biologie, 1894, p. 511. 



