226 



SPECIAL PHYSIOLOGY. 



the cell B, upwards or downwards, through a portion of the nerve as 

 at 5, beyond the portion included in the galvanometric circuit, then a 

 current is immediately manifested in the part #, included in the gal- 

 Diagram E. 



Diagram E exhibits the experiments by which an altered condition, called electrotonus. is shown to be 

 produced in a nerve. 1, a, portion of nerve placed on the cushions of the galvanometric apparatus, within 

 which piece of nerve, the normal current passes, as shown by the arrow ; b, portion of the nerve beyond 

 the apparatus, stimulated by a constant current from a ^ell B, in the direction indicated by the upper 

 arrow, and therefore producing in this part of the nerve a current in the direction of the arrow b, i.e., cor- 

 responding with the normal nerve current at a. The result is to increase the force of the current in the part 

 a. 2, a. as before ; b shows the course of a constant current made to pass from the cell B, through the outly- 

 ing part of the nerve, running in the opposite direction to the normal current produced in the part a. The 

 result is to diminish the force of this hitter current in a. 3 shows a nerve so placed on the cushions, that 

 no current passes through the galvanometer from the part a; but when a constant current is made to 

 pass through the part b, from the cell B, then a current in the same direction is generated in the part, a, 

 as shown by the arrow. 



vanometric circuit, flowing, as shown by the two arrows, in a direction 

 corresponding with that of the exciting current. In other words, the 

 electrical condition of the whole piece of nerve, and not only that of its 

 excited portion, is altered by and obeys the direction of the exciting 

 current. To explain the controlling influence of the exciting current 

 on the normal nerve current, the molecules of the nerve are supposed 

 to be thus affected. As already stated, these are assumed to be, in 

 the quiescent state, either peripolar. Diagram D, or to be composed 



Diagram D (repeated). 



Diagram D is here repeated, as it shows the supposed peripolar condition of the nerve-molecules, as well 

 as that of the muscle-molecules, on which the normal current of those tissues is said to depend. 



of pairs of unipolar molecules, with their corresponding poles turned 

 towards each other, Diagram F, a; but under the influence of the ex- 



