508 NERVE. 



There is little doubt that this statement expresses the general 

 conditions underlying polar excitation of medullated nerves ; instances, 

 however, occur, due to the interference of unsuspected currents in 

 the nerve, which have led some writers to attempt to associate all 

 excitation with catelectrotonus only. 



Influence of the demarcation current. The opening contraction is 

 generally obtained only on the cessation of currents of some intensity ; 

 it can, however, be obtained by much weaker currents, under the 

 following conditions 1 : 



If the nerve is divided from the spinal cord, and the cathode placed 

 near the cross section, the opening excitation is produced with very 

 weak ascending currents. As these increase in intensity, it fails to 

 reappear again with currents of moderate strength. The earlier opening 

 excitation of the ascending current may be due to the unmasking of 

 the descending demarcation current. There are, however, other currents 

 which will reinforce this. The galvanic current is succeeded by polar- 

 isation after effects; these polarisation currents are such as to make 

 the anode into a cathode ; they are unmasked by the cessation of the 



current flow, and may 

 be adequate to evoke 

 an excitation, especially 

 if they coincide in direc- 

 tion with the feeble 

 demarcation current. 

 The opening excitation, 

 if caused in this way, 

 is in reality based not 



FIG. 265. ^^key in derived circuit of rheochord (rh) ; upon the disappearance 



#2 -key in battery circuit of rheochord. The darker O f anelectrotonus, but, 



arrow indicates the nerve demarcation current, the thin i -\ n 1.1 i 



arrow indicates the battery derivation. the Coring 0116, On 



the production of cat- 

 electrotonus, through feeble polarisation currents in the opposite direction 

 to the original current summing with that of the nerve demarcation 

 change. 



That the demarcation current when it exists in some intensity, as in 

 freshly injured nerve, can profoundly modify the results which are ex- 

 pressed in Pfliiger's law, is shown, further, by Bering's experiments. 2 

 In these a nerve was used with a fresh cut section, and it was observed 

 that the minimal intensity of the battery current adequate for excita- 

 tion varied with the position. K and K% of the key which effects the 

 closure and opening (see Fig. 265). If this is in the electrode circuit 

 of the rheochord, at K lt its closure makes a circuit not only for the 

 derivation of the battery current, but in addition for that of any nerve 

 current present. If it is at K 21 in the battery circuit of the rheochord, 

 it makes a circuit for the voltaic current only, whilst the nerve current 

 is continually flowing through the derived part of the rheochord circuit. 

 It thus depends upon the respective directions of the battery current 

 and nerve current what the precise variation will be as regards closing 

 and opening excitation. 



If the rheochord derivation is about equal to the nerve current, and 



1 Griitzner, Arch. f. d. ges. PhysioL, Bonn, Bd. xxvii. S. 1882. 



2 Bering, Sitzungsb. d. k. Akad. d. Wissensch., Wien, 1882, Bd. Ixxxv. Abth. 3, 

 S. 237. 



