IV 



ELECTROMOTIVE ACTION IX MUSCLE 



367 



segment (the hatched part of the curve, Pig. 114) is therefore 

 always cut out of the superficies of a variation curve, and 

 through its summation a deflection of definite proportions is 

 produced. And, since the time of galvanometer closure can be 

 prolonged as required over the entire interval between the two 

 stimuli, the form and magnitude of the curve of variation corre- 

 sponding to each single stimulus are easily determined. The 

 credit of having constructed apparatus that satisfied these require- 

 ments belongs to Bernstein (20), whose "differential rheotome " 

 has since found an extended application in experimental physio- 

 logy. The instrument consists essentially of a wheel (/) (Fig. 115) 



FIG. 114. 



revolving easily round the central axis, worked as uniformly as 

 possible (5-10 revolutions per second) by clockwork, or a small 

 motor. At the periphery of the wheel there are three isolated 

 metal-points (according to Hermann, brushes of copper- wire), one of 

 which (c) forms the exciting contact, the other two (</, c") effect the 

 closure of the galvanometer circuit. The former slides at each 

 revolution over a thin extended wire, or pool of mercury, and 

 thus closes the circuit (R\ li") of the primary coil of an induction 

 apparatus. The currents produced in rapid succession in the 

 secondary coil (make and break shock) are led into the prepara- 

 tion, and may be regarded collectively as a momentary stimulus. 

 Diametrically opposite to the exciting contacts, isolated from the 



