336 



ELECTRO-PHYSIOLOGY 



CHAP. 



wire, while the point led off on the longitudinal section is con- 

 nected with a metal slider (c) leading to the wire of the 

 rheochord, the galvanometer (B) will be affected on the one 

 hand by the difference of potential between the rheochord points 

 a and c, on the other by that between the transverse and longi- 

 tudinal surfaces of the muscle. It is easy at any moment by 

 moving the slider (c) to compensate the deflection produced by 

 the muscle current. The P.D. between longitudinal and trans- 

 verse section of the muscle will then obviously be the P.D. 

 between the points a and c of the rheochord wire. And in the 



FIG. 107. Round Compensator. (Du Bois-Reymond.) 



latter each millimeter corresponds with a given fraction of the 

 E.M.F. of a Daniell cell. 



In order to carry out these measurements quickly and con- 

 veniently, du Bois-Reymond constructed the " round compensator," 

 in which the rheochord wire a, b is attached to a circular disc of 

 ebonite. Its terminals are connected with screws I. and II. ; from 

 I. a wire also goes to IV., III. being connected with the metal 

 pulley (r), which slides upon the rheochord wire, so that a 

 L-iven length of it can be included by moving the slider (Fig. 

 107 a, I). 



By this method du Bois-Reymond took numerous measure- 

 ments of the electromotive force between longitudinal and 

 transverse sections of striated frog's muscle. The average was 

 0'035-0'075 Dan. According to Matteucci, the muscle- 



