152 



EXPERIMENTAL PHYSIOLOGY 



character. This is because the galvanometer cannot respond to the 

 rapid alternations, the inertia of the magnets being too great, and it 

 therefore only gives a constant deflection proportional to the mean 

 value of the alternations. 



The production of this current proves that a part of a muscle 

 which is contracting is electro-negative to one which is at rest or in 

 a less degree of contraction. The injured part contracts less forcibly 

 than the non-injured, and it is because the injured part does not 

 respond so well to stimuli that its negativity remains practically 

 constant, whilst the negativity of the central active part becomes 

 greater relatively to its previous state. 



MEASUREMENTS OF POTENTIAL 



The method adopted by Du Bois-Eeymond for measuring the 

 E.M.F. of an animal current was that of compensation, in which the 

 current through the galvanometer is balanced by a fractional part of a 

 constant current of known E.M.F. sent through the galvanometer in 

 the reverse direction. The principle of the method is illustrated by 

 fig. 120. A Daniell battery is connected to the two ends of a long 

 platinum wire, a c, a commutator, k, being interposed so that the direc- 

 tion of the current through 

 the wire can be reversed. 

 The current to be measured 

 is sent through the gal- 

 vanometer and a part of 

 the platinum wire, viz. a b, 

 the connection at b being 

 movable. The muscle 

 current in the arrangement 

 of the figure passes from a 

 to b, and the Daniell current 

 is, by placing the commu- 

 tator in the proper position, 

 directed from b to a. A posi- 

 tion of b is now sought at 

 which the galvanometer 

 remains undeflected, when it follows that the branch circuit from 

 the Daniell through the galvanometer is equal to the muscle current, 

 and consequently that the E.M.F. between b and a due to the Daniell 

 is equal to the E.M.F. of the muscle current. 



"With the high external resistance the difference of potential 

 between a and c is practically that of the Daniell cell ; and as the wire 



Fig. 120. — Plan of DuBois Reymonb's Method 

 of measuring the muscle currents. 



