ELECTRO-PHYSIOLOGY 



639 



Suppose that the nerve in Fig. 206 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 r 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. 596) 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 excitation. The portion of nerve at E being negative 

 relatively to that at E I} an action current will pass through the 

 galvanometer from EJ to E, and through the nerve in the same 

 direction as the original stimulating stream ; that is, it will have 

 the direction of the positive polarization current. 



Under certain conditions a state of continuous excitation in the 

 anodic region of a nerve is shown by a 

 tetanus of its muscle (Rifter's tetanus, 

 p. 656, and Fig. 207). 



Griitzner and Tigerstedt have put 

 forward a different theory of the break 

 contraction. They say it is really a 

 closing contraction due to the closure 

 of the negative polarization current 

 through the tissue itself, as soon as the 

 polarizing current is opened. In fact, 

 they admit only one kind of electrical 

 stimulus, the kathodic, or make. 



Electrotonic Currents. If a cur- 

 rent be passed from the battery 

 through a medullated nerve (Fig. 

 208) in the direction indicated by 



FIG. 206. DIAGRAM TO SHOW 

 DISTRIBUTION OF 'POSITIVE 

 POLARIZATION ' AFTER OPEN 

 ING POLARIZING CURRENT. 



B, battery ; G, galvanometer. 

 The dark shading signifies that the 

 excitation to which the positive 

 polarization current is due is 

 greatest in the immediate neigh- 

 the arrOWS, while a galvanometer bourhood of the anode, and fades 



, away in the intrapolar region. 



is connected with either of the 



extra-polar areas, as shown in the figure, a current will pass 



through the galvanometer, in the same direction in the nerve 



as the polarizing current, so long as the latter continues to 



flow. 



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

 electrotonic ; in the anodic, anelectrotonic]. The exact mode of their 

 production 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 should be polarization (separation of ions) at the 

 boundary between the core and the sheath, i.e., between the wire and 

 the liquid, where the current passes from the one to the other. 



