1 84 THE ORIGIN OF THE NERVOUS SYSTEM 



the two. Positive ions will then migrate toward b, 

 negative ions toward a. In a given cell, n, of the mass 

 this migration will occur if the current passes through it, 

 and the ions will accumulate at two opposite regions of 

 the cell, the side toward b becoming more positive inter- 

 nally, the side toward a more negative. For convenience 

 in the following discussion this is called the first type of 

 polarization. 



We turn now to the other alternative, Figure 59, in 

 which the cell n is not in direct tissue-continuity with 

 the source of current a but suspended in a conducting 

 medium. The cell or tissue a is electropositive inter- 

 nally, because of its physiological condition and is 

 therefore electronegative externally, i.e., with respect to 

 the medium in which the cell n lies. Consequently in 

 this medium positive ions will migrate toward the 

 cathode on the surface of a, negative ions in the opposite 

 direction. The cell n will therefore undergo electrical 

 polarization with the positive pole toward a, the negative 

 in the opposite direction. This may be called for our 

 purposes the second type of polarization. Comparing 

 the two diagrams, Figures 58 and 59, it is evident at 

 once that although the relations as regards space and 

 direction between the region of high positivity a and 

 the cell n are the same in both cases, the polarization of 

 n is opposite. If the region of increased positivity in 

 the polarized cell becomes more active physiologically 

 and growth occurs in it in consequence of the polariza- 

 tion, the outgrowth in the first case (Fig. 58) will be 

 away from a and toward b, and in the second case 

 (Fig. 59) toward a. In the following sections a few 

 suggestions are advanced concerning the occurrence of 



