644 



'PRINCIPLES OF GENERAL PHYSIOLOGY 



these electrical responses by means of a sensitive galvanometer. In doing this, 

 any current already existing in the circuit is balanced by an opposing potential 

 difference from the slide wire, in order to bring the deflection to zero before 

 stimulating the tissue. No mere change of resistance can, in such circumstances, 

 cause a deflection ; there is no potential difference to create a current. 



Let us next examine the possible sources of potential difference in living 



tissue. 



We may take it that these potential differences must be due to elect 

 charges on ions. It does not seem probable that phenomena of frictional electricity 

 play any part. In any case, these phenomena themselves can generally be traced 



FIG. 203. NORMAL CURVE OF CAPILLARY ELECTROMETER. 



Produced by applying a potential difference of 0-01 volt between its terminals at the point P of 



the tracing. Considerable resistance was introduced into the circuit, so that the rate of 



movement is slowed. 



Ordinates position of meniscus. The divisions along the axis Oy are in O'OOl volt. 

 Abscissa; time. Ot is the axis and also the asymptote of the curve. The portion of the curve 



above the line through 5 is the entire normal curve for 0"005 volt, and the portion above P, is 



that for 0-00165 volt. 

 PN and P, N Ordinates at P and P f 

 PT and P, T Tangents to the curve at P and P.. 

 JtT and N, T, Subtangents. These are equal to one another. 

 The equation to the curve is 



\og y -=ct, 

 a 



where y is the vertical distance of any point from the asymptote of the curve, and t is the 

 horizontal distance from the origin of abscissae, a and c are constants of a particular capillary 

 tube. 



(Burch, 1890, p. 91.) 



to the production of some kind of ions. The question was discussed to some 

 extent in connection with the charge on colloidal particles (page 89). 



The part played by electric charges on surfaces is treated of in the paper by 

 Mines (1912, 1). 



That the source is ionic is indicated by the temperature coefficient of the 

 electromotive force of tissues. This was determined by Lesser (1907), for the skin 

 of the frog, and found to be proportional to the absolute temperature. If the 

 source were some kind of chemical reaction, a much higher temperature coefficient 

 would be found. 



If the ions arising from electrolytic dissociation of a substance are free to move 

 and intermix, it is clear that no potential difference could be detected. Moreover, 

 the increase of their numbers, whether by increased dissociation or by production 

 of new ones by the splitting up of larger molecules, or by the setting free from a 

 state of adsorption, in itself cannot give rise to any change in potential difference, 



