640 



A MANUAL OF PHYSIOLOGY 



A current led into the sheath tries, so to speak, to pass mostly by 

 the good conducting wire. If this is not polarizable if it is, e.g., a 

 zinc wire surrounded by saturated zinc sulphate solution there is 



little or no spreading of 

 the current outside the 

 electrodes : it passes at 

 once into the core, and 

 so on to the other elec- 

 trode. If, however, 

 there is polarization 

 when the current passes 

 from the liquid into the 

 wire, as is the case in 

 the platinum-zinc sul- 

 phate, or the zinc- 

 sodium chloride com- 

 binations, the stream 

 spreads longitudinally 

 in the sheath since the 

 polarization introduces 



FIG. 



a virtual resistance at 

 the surface of the wire, 

 in comparison with 

 which the difference in 

 resistance of an oblique 

 and a direct transverse 



207. RITTER'S 

 TETANUS. 



A strong voltaic current was 

 passed for some time through the 

 nerve of a muscle-nerve prepara- 

 tion. On opening the circuit, 

 the muscle gave one strong con- 

 traction, and then entered into path through the liquid 

 irregular tetanus, which con- hprnm^ crrmll TtJ^o 

 tinued for four minutes. (Only t Decomes sma11 ; 

 the first part of the tracing is been supposed that in 

 reproduced.) medullated nerve a si- 



milar polarization takes 

 place at the boundary 



between some part of the nerve-fibre which may be called a core, 

 and another part which may be called a sheath, for instance, between 

 the axis-cylinder and the medullary sheath, or between the latter and 

 the neurilemma. It is known that the electrical resistance of nerve 



FIG. 208. DIAGRAM SHOWING DIRECTION OF THE EXTRAPOLAR ELEC- 

 TROTONIC CURRENTS. 



in the transverse direction is much greater (five to seven times)* than 



* Since a part of the current is conducted by the connective tissue 

 and other structures lying between the nerve-fibres, and the longitudinal 

 and transverse resistance of these tissues may be supposed equal, the dis- 

 proportion between the longitudinal and transverse resistance of the 

 nerve-fibres themselves is probably much greater than this. 



