VIII. BIOELECTRIC MEASUREMENTS 



237 



current in the circuit. Then as far as the resistances of the cell sus- 

 pension are concerned, from Ohm's law, E = Rf = 0. Thus as far 

 as the cells are concerned no potential exists across them and the}' 

 are subjected to no electrical influence. The field is entirel.y short- 

 circuited by the salt solution. It is no wonder then that such of 

 these experiments as have been done carefully have shown negative 

 results. The investigators could have saved themselves the trouble 

 of doing the experiment if they had bothered to draw an equivalent 

 circuit. 



■AA/WVWV 



Fig. 1. Elementary equivalent circuit for a test tube containing 

 a cell suspension between two d.c. electrodes. 



It would be difficult to ovei'emphasize the importance of drawing 

 a complete eciuivalent circuit for each measuring problem under con- 

 sideration and then assigning values to as many of the components 

 as possible. Indeed very little confidence could be placed in meas- 

 urements imless this had been done before the measurements were 

 imdertaken. For these reasons, the present chapter will be developed 

 entirely from the point of view of equivalent circuits. 



B. POTENTIAL MEASUREMENTS 



1. Origin of Bioelectric Potentials 



It has been known for a great many years that electrical potential 

 differences exist between the C3'toplasm and the surrounding fluid 

 of all living cells. Further it is found that for many tissues a poten- 

 tial difference exists between the two different sides of the tissue, for 

 example, the inside and outside of the intestinal mucosa or inside 

 and outside of frog skin. These potentials are not invariant quanti- 

 ties but depend upon the physiological state of the material and es- 

 pecially upon the concentration and composition of the salt solutions 

 in contact with the cells and tissues. 



A complete discussion of the origins of these potentials is beyond 

 the scope of this book. For a more complete treatment the reader 

 is referred to Bayliss (3), Ilober (16), or Maclnnes (19). However, 



