ELECTROTONIC CURRENTS 803 



physical, and therefore ultimately on the physiological, structure of 

 the tissues, they have been deeply studied, especially in nerve. 

 There is no question that they depend upon the presence in the 

 tissues of membranes presenting a relatively great resistance to the 

 passage of ions. When a current is passed by means of unpolarizable 

 electrodes (Fig. 230, p. 705) through a muscle or nerve for several 

 seconds, and the tissue connected to the galvanometer immediately 

 after this polarizing current is opened, a deflection is seen indicating 

 a current (negative polarization current) in the opposite direction. 



This (negative) polarization, like the polarization of the electrodes 

 seen after passage of a current through any ordinary electrolytic con- 

 ductor, dilute sulphuric acid, e.g., depends on the liberation of ions 

 (p. 423) at the kathode and anode. It is seen not only in muscle, nerve, 

 and other animal tissues, but also in vegetable structures, and indeed, 

 to a certain extent, in unorganized porous bodies soaked with electro- 

 lytes. In muscle and nerve, however, it is particularly well marked; 

 and although it is not bound up with the life of the tissue, and may be 

 obtained when this has become quite inexcitable, it is nevertheless 

 dependent on the preservation of the normal structure, for a boiled 

 muscle shows but little negative polarization. 



When the polarizing current is strong, and its time of closure short, 

 we obtain, on connecting the tissue with the galvanometer after opening 

 the current, not a negative, but a positive deflection, indicating a 

 current in the same direction as that of the polarizing stream. This is 

 really an action stream, due to the opening excitation set up at the 

 anode (p. 715). It is only obtained when the tissue is living, and is far 

 more strongly marked in the anodic than in the kathodic region. 



Suppose that the nerve in Fig. 293 is stimulated by the opening of 

 the battery B, and that, immediately after, the nerve is connected with 

 the galvanometer G by the electrodes E, E x . Suppose, further, that 

 the shaded region near the anode remains more excited for a short time 

 than the rest of the nerve, and we have seen (p. 761) that after the 

 opening of a strong current there is a defect of conductivity, especially 

 in the neighbourhood of the anode, which would tend to localize excita- 

 tion. An action current will pass through the galvanometer from EJ 

 to E, and through the nerve in the same direction as the original stimu- 

 lating stream. Under certain conditions a state of continuous excita- 

 tion in the anodic region of a nerve is shown by a tetanus of its muscle 

 (Ritter's tetanus, p. 715, and Fig. 294). 



Electrotonic Currents. If a current be passed from the battery 

 through a medullated nerve (Fig. 295} in the direction indicated by 

 the arrows, while a galvanometer is connected with either of the 

 extrapolar areas, as shown in the figure, a current will pass through 

 the galvanometer, in the same direction in the nerve as the polar- 

 izing current, so long as the latter continues to flow. 



These currents are called electrotonic (in the kathodic region katclectro- 

 tonic ; in the anodic, anelectrotonic}. The exact mode of their produc- 

 tion is obscure. Similar currents can be detected in artificial models 

 consisting of a good conducting core and a badly conducting envelope; 

 for example, a platinum wire in a glass tube filled with saturated zinc 

 sulphate solution, or a zinc wire covered with cotton-wool soaked in 

 salt solution. In such models it appears to be essential that there 



