CURRENTS OF INJURY IN MUSCLE AND NERVE. 



649 



required for their demonstration or a tangent mirror-galvanometer, for example 

 the electrogalvanometer (p. 384), with a damped periodic magnet. If the wires 

 of the multiplicator were placed in direct communication with the moist animal 

 tissue, they would give rise to a current by reason of their inequality, and, besides, 

 polarization would develop on the surface of the wires on the passage of a current. 

 Therefore unpolarizable electrodes, upon which the tissues may rest (Fig. 225, I, 

 P, P) , are always used in conjunction with the conducting wires. 



The capillary electrometer of Lippmann (Fig. 229) has been advantageously 

 employed for the demonstration of muscular currents. In this a thin column- 

 of mercury in a capillary tube lying in contact with a conducting fluid (dilute 

 sulphuric acid) is displaced by the galvanic current, the constant of capillarity 

 of the mercury undergoing alteration in consequence of the polarization at the 

 surface of contact. The displacement, which the observer (B} recognizes with 

 the microscope (M), takes place in the direction of the positive current. The 

 image of the capillary tube can be projected upon a screen and the oscillations 



III. 



FIG. 228. 



FIG. 229. Diagrammatic Representation of the Capil- 

 lary Electrometer. 



of the mercury may be photographed. In Fig. 229, representing such an ap- 

 paratus diagrammatically, R is a glass tube drawn out below to capillary fine- 

 ness, and filled from above with mercury and from c downward with dilute 

 sulphuric acid. The capillary tube extends downward into a wide glass tube, which 

 has a platinum wire fused into it below and is filled with mercury (<?) and dilute 

 sulphuric acid (\). The conducting wires are connected with unpolarizable 

 electrodes, which are applied to the transverse section and the surface of a muscle. 

 On closing the current the column of mercury is displaced downward from c in 

 the direction of the arrow. The electromotive force can be measured with the aid 

 of the capillary electrometer from the extent of the displacement of mercury. 

 On the other hand, when the electrical processes take place rapidly the movement 

 of mercury cannot follow rapidly enough on account of the resistance. 



The strength of the currents in animal organs is best measured by permitting 

 another current of graduated and known strength to pass through the electrom- 

 eter circuit in an opposite direction, so that the tissue-current present is reduced 

 to zero compensatory method of Poggendorf. 



