PHENOMENA OF MUSCLE AND NERVE. 315 



of current gives predominantly double, -f- , effects. The obser- 

 vations are very variable, owing- to inequalities in the surface of the 

 mercury, so that I have not followed out these experiments further. 

 They sufficiently establish the above-mentioned * turning point.' 



(b). THEORETICAL. 



I have previously shown that a current led through a portion of 

 the schema spreads, on account of resistances round the core, very 

 widely into the extrapolar regions, so much so that + currents are 

 indicated in galvanometer circuits connected with the schema at 

 considerable distances from the led-through part ; these correspond 

 to the electrotonic currents of nerve and muscle. At the same 

 time polarisation occurs through the formation of ions round the 

 core, the distribution of which may be expressed by a double curve. 

 This curve has its positive maximum at the anode of the polarising 

 current, the negative maximum at the kathode ; it cuts the abscissa 

 at a point in the intrapolar region (the indifference point), and 

 approaches the abscissa in the extrapolar regions asymptotically. 



This distribution of polarisation is the key by means of which 

 the physiological effects described as electrotonic appearances become 

 intelligible. 



On breaking the polarising current this polarisation equalises 

 itself by inner currents, which are indeed present during the passage 

 of the led-through currents and can weaken or strengthen the latter 

 according to their direction. 



If we take an extrapolar region by itself without reference to 

 the rest of the conducting schema, then this must, as is easily 

 seen, show a + after-current (i.e. in the same direction as the polaris- 

 ing current). On the side of the anode, for instance, the core is, in 

 the extrapolar region, polarised positively (the platinum wire, if in 

 zinc sulphate, being covered with zinc), in less and less degree 

 the further from the anode, consequently every point nearer the 

 anode is positive to one further off. A leading-off circuit connected 

 with this region will therefore lead off a current directed through 

 it, from the anodal to the further point of connection ; in the same 

 direction, that is to say, as the polarising current has in this circuit. 

 Similarly, on the side of the kathode, every point nearer the kathode 

 is negative to one further off (in tfye schema becoming covered with 

 SO 4 , i.e. sulphuric acid and oxygen), so that here again a circuit 

 must lead off a + current. 



