ELEMENTARY VITAL PHENOMENA 



267 



minuteness of the latter, but the rule must hold good as well 

 for the cell-complex, the tissue. As a matter of fact it may be 

 demonstrated in the latter, and Herman's " differential theory " 

 ('67 '68), according to which a current may be led off from a 

 tissue (muscle, nerve, mucous membrane, etc.) only when differ- 

 ent processes are taking place at the leading-off points, is merely 

 the expression of the actual relations. In a resting uninjured 

 muscle, e.g., the sartorms of the frog, which is the best object for 

 demonstrating the truth of this, no current is present, because 

 the same internal processes are taking place at every point 

 (Fig. 121). If, however, at two points in the muscle a difference 

 be produced artificially by warming one point, by cutting the 

 muscle across, which is associated with a local decomposition of 

 living substance, or by making a contraction- wave pass over the 

 muscle, an electric current is obtained; the warmed, dying or 

 contracting part becomes negative to all other parts. Tissues 



FIG. 120. Schematic. /. A cell in which at all points of the living substance similar chemical 

 processes are taking place is without a current. //. Polarised cells (e.g., cells of mucous mem- 

 brane) in which at one pole chemical processes are present that differ from those at the other 

 give a current. 



whose cells do not possess polar differentiation never show a 

 current in the undisturbed condition, but relatively strong 

 currents can be led off always from glands and mucous membranes, 

 even when undisturbed ; here the cells are polarised in such a 

 manner that the lower part of the cylindrical cell-body contains 

 different substances and transformations of substances from the 

 upper part (Fig. 120, II). The fact discovered by Mendelssohn 

 is interesting, that an excised nerve, when led off from both cross- 

 sections, shows an axial current which runs in a direction contrary 

 to the nerve-conduction, i.e., in motor nerves centripetally, in 

 sensory nerves centrifugally. 



All such currents may be demonstrated, like those arising ther- 

 mometrically, by means of the multiplier or the galvanometer 

 (Figs. 115 and 116, p. 259). But a special arrangement of the 

 leading-off electrodes is necessary to avoid false results. If a 

 current be allowed to pass for a time through a wire, the ends of 

 which dip into a moist conductor, electrolytic decomposition- 



