446 MANUAL OF PHYSIOLOGY. 



a block of frog's muscle the measurement of the electric currents 

 requires considerable care, because they are so difficult to detect 

 that a most sensitive galvanometer must be used ; and such an 

 instrument can easily be disturbed by currents due to bringing 

 metal electrodes into contact with the moist saline tissues. Spe- 

 cially constructed electrodes must be used to avoid these currents of 

 polarization taking place in the terminals touching the muscle. 

 These are called non-polar izable electrodes, and may be made on 

 the following plan : Some innocuous material moistened in saline 

 solution (.65 per cent.) is brought into direct contact with the mus- 

 cle, and, by means of saturated solution of zinc sulphate, into elec- 

 trical connection with amalgamated zinc terminals from the gal- 

 vanometer. Thus the muscle is not injured, and the zinc solution 

 prevents the metal terminals from producing adventitious currents. 

 Small glass tubes drawn to a point, the opening of which is 

 plugged with moist china clay, make a suitable receptacle for the 

 zinc solution, or, instead of the china clay, a camel-hair brush 

 set iu plaster of Paris may be used to keep the zinc solution in 

 the tube, and the hair moistened in salt solution forms a suitable 

 point of contact with the muscle. If a pair of such electrodes be 

 applied to the middle of the longitudinal surface at (e) (Fig. 182), 

 and of the transverse surface at (p) respectively, and then be 

 brought into connection with a delicate galvanometer, it is found 

 that a current passes through the galvanometer from the longitu- 

 dinal to the transverse surface. A current in this direction can 

 be detected in any piece of muscle, no matter how much it be 

 divided longitudinally, and probably would be found in a single 

 fibre had we the means of examining it. The nearer to the centre 

 of the longitudinal and transverse sections the electrodes are 

 placed, the stronger will be the current received by them. If 

 both the electrodes be placed on the longitudinal or on the trans- 

 verse surfaces, a current will pass through the galvanometer from 

 that electrode nearer the middle of the longitudinal section (called 

 the equator of the muscle cylinder) to the electrode nearer the 

 centre of the transverse section (pole of muscle cylinder). If the 

 electrodes be placed equidistant from the poles or from the equator, 

 no current can be detected. 



