96 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



current leaving or entering the body and consequently the intensity of its 

 action. The application of the anode to a region of increased excitability, by 

 decreasing the irritability, may for the time lessen irritation; on the other 

 hand the kathode may heighten the irritability of a region of decreased 

 excitability. The sending of a strong polarizing current through a motor 

 nerve, by lessening the conductivity, may prevent abnormal motor impulses 

 from reaching muscles, and so stop harmful "cramps;" or the sending of 

 such a current through a sensory nerve may, during the flow of the cur- 

 rent, keep painful impulses from reaching the central nervous system. In 

 applying a strong battery current to lessen irritability or conductivity it 

 must be remembered that the after-effect of such a current is increased 

 irritability. 



(/) Effect of Conduction. Many experiments have been made in the hope 

 of detecting some form of chemical change as a result of conduction. The 

 nerve has been stimulated for many hours in succession with an electric cur- 

 rent, and then been examined with the utmost care to find whether there had 

 been an accumulation of some waste product, as carbon dioxide, or some other 

 acid body. The gray matter of the spinal cord, which is largely composed of 

 nerve-cells, is found to become acid as a result of activity, 1 but this cannot be 

 found to be the case with the white matter of the cord, which is chiefly made 

 up of nerve-fibres, nor has an acid reaction been obtained with certainty in 

 nerve-trunks. 2 



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. 3 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 grnrff 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 



1 Funke : Archivfiir Anatomie und Physioloyie, 1859, p. 835. Ranke : Centralblatt fur medicin- 

 ische Wissenschaft, 1868 and 1869. 



2 Heidenhain : Studien aus dem physiologischen Institut zu Breslau, ix. p. 248 ; Centralblatt fur 

 Medicin, 1868, p. 833. Tigerstedt : " Studien iiber mechanische Nervenreizung," Ada Societatis 

 Scientiarum Fennicce, 1880, torn. xi. 



3 Helmholtz : Archiv fur Anatomie und Physiologic, 1848, p. 158. Heidenhain : op. ciL 

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



