462 NERVE. 



the resistance of the circuit and the capacity of the plates, being slower 

 as either of these increases. The character of the discharge is thus 

 expressed by a fall from the full potential to nothing, which fall varies 

 in the time necessary for its completion. As condenser plates are liable 

 to allow leakage between the opening of the charging and the closure 

 of the discharging circuit, a useful method is to place the condenser 

 plates in direct connection, through the nerve, with the electromotive 

 source, and use one circuit only ; on short circuiting the electromotive 

 source the discharge of the plates occurs through the nerve. 



The elaborate investigations of d'Arsonval, Hoorweg, Cybulski, and Waller l 

 show that in this method of nerve excitation, the physical constants of the 

 stimulus are capable of being accurately determined, and thus the stimulus 

 energy in any one instance can be expressed in terms strictly comparable 

 with that in another. 



(4) The galvanic current. — The peculiar features of the excitation of 

 nerve by galvanic currents will be considered in detail later. As a mode 

 of stimulation, its employment has the disadvantage that the duration of 

 the current causes electro-chemical changes in the tissue of a more 

 pronounced character than those produced by the former methods. 



These changes may, however, be reduced to a small amount by 

 employing currents of very short duration, such as are produced by a 

 rapidly rotating striker which effects a closure by lightly striking a 

 stretched wire ; and if, by a further arrangement, a second similar 

 closure is also effected, two short galvanic currents opposite in direction 

 may be produced, thus reducing the possibility of prolonged electrolytic 

 changes to a minimum. By means of a rheochord the intensity of the 

 current, and thus the stimulus, may be graduated with great precision, 

 and a mode of stimulation can be thus obtained which has several points 

 in its favour. The duration and character of the currents used for 

 stimulation are constant, and can be determined with great precision, 

 whilst the electromotive force employed -is also readily ascertained. 



(5) Other modes of electrical excitation. — The passage of any electrical 

 current of sufficient intensity may act as a stimulus to a medullated motor 

 nerve, especially if the time of its development is sufficiently short ; thus the 

 discharge between the knobs of a frictional machine will excite the nerve of a 

 frog's muscle nerve preparation, when this is placed in air in the neighbourhood 

 of either pole. 2 An anomaly, however, appears to exist in the case of electrical 

 oscillations of very great frequency, such as those to which attention was 

 drawn by Hertz. The Hertz waves, even when adequate to cause an in- 

 candescent lamp to glow, will only stimulate an exposed nerve at the moment 

 of their onset, whilst, during their continuance, they appear to cause a paralysis 

 of conductivity. 3 



The currents produced by Bell's telephone have been made use of for 

 stimulation ; a striking instance is their employment by Hiirthle, in order to 

 record the time-relation of the sounds of the heart, the currents thus caused 

 being adequate to excite an exposed nerve. 4 



1 Cybulski, Arch. f. d. ges. Physiol., Bonn, 1894, Bd. lvi. ; Hoonveg, ibid., 1893, Bd. 

 liii.; ibid., 1895, Bd. lvii. ; Waller, Proc. Boy. Soc. London, 1899. 



2 Loeb, Arch. f. d. ges. Physiol., Bonn, 1897, Bd. lxvii. S. 483; Centralbl. f. Physiol., 

 Leipzig u. Wien, 1897, Bd. xi. S. 13. 



3 D'Arsonval, Compt. rend. Soc. de biol., Paris, 1893 ; ibid., tome cxvi. p. 630. 



4 Hiirthle, Proc. hitemat. Cony., Liege, 1892; Centralbl. f. Physiol., Leipzig u. Wien, 

 1892, Bd. vi. S. 398. 



