POLARISATION 81 



current starts from a, and of a descending current from b, in both 

 cases the kathode ; and, on the other hand, that the contraction on 

 break starts from B with an ascending, and from a with a descending, 

 current, these two points being the anodes in the two cases. The 

 differences are more clearly seen if the muscle be fatigued. 



Experiment 7. — Engelmann's experiment. Curarise a frog, dissect out 

 its sartorius, and suspend it by its pelvic end. Arrange two electrodes to 

 send a current transversely across this end. On closing the current the free 

 end moves towards the kathodic side of the muscle, on opening towards the 

 anodic. 



Experiment 8. — Prepare a sartorius from a curarised frog that has been 

 previously kept at a low temperature for some hours. Place it on a pair of 

 unpolarisable electrodes. On closing the current the rmiscle passes into 

 tetanus, and if the muscle be carefully observed, it will be seen that the only 

 part in persistent contraction is that around the kathode. On opening, the 

 muscle also commonly passes into tetanus, but in this case the contraction 

 is limited to the region of the anode. This experiment is all the more 

 striking if Biedermann's plan, of striping the sartorius transversely with black 

 lines made by a bristle dipped in china ink, be adopted. The region in 

 contraction is then clearly marked by the approximation of the black lines. 

 The non-contracted part is seen to be stretched out, either by an actual 

 stretching due to the contracted part pulling on it, or due to an active 

 relaxation in that region. The latter is probably the main cause, as is well 

 exemplified by the following experiment. 



Experiment 9. — A frog is pithed and its heart exposed. One electrode 

 is now placed on any part of the body, and the point of the other, which 

 should be a fine wire, is applied to the heart. If the electrode on the heart 

 be the anode it will be seen that at each contraction the part around the point 

 touched does not pale like the rest of the heart, i.e. that region does not con- 

 tract but is inhibited. Conversely, if it be the kathode, it is seen that that 

 spot remains pale during relaxation of the heart, which means that those 

 fibres immediately affected do not relax properly. This latter point is not so 

 easy to make out as the former. 



Polarisation of Electrodes. — If a pair of clean platinum wires be 

 immersed in water, and a current sent through them for a time, it is 

 found that both of the platinum terminals become covered with bubbles 

 of gas. That one in connection with the negative pole of the battery- 

 is covered with hydrogen, and the other with oxygen. If now the 

 battery be removed and the two electrodes connected to the two 

 terminals of a galvanometer, it is found that a current is shown by 

 the galvanometer, which has a direction, in the galvanometer circuit, 

 from the electrode covered with oxygen to that covered with hydrogen. 

 It is, in other words, in the reverse direction to that of the initially 

 employed current. The production of this state at the electrodes is 

 spoken of as polarisation of electrodes. The same usually occurs, 

 though to different degrees, if solutions of salts be tested instead of 

 distilled water, and no matter what metal the electrodes are made of. 

 Eegnault discovered, however, that if the electrodes were made of 



a 



