92 



THE PHYSIOLOGY OF MUSCLE AND NERVE. 



or negative pole. On joining the ends of the wires a current will 

 pass from positive to negative pole. 



A current of this character from an excised nerve or muscle 

 is, of course, small in amount and to detect it one must make 

 use of a delicate electrometer of some sort (see below). Du Bois- 

 Reymond considered that the difference in electrical potential 

 which gives rise to this current exists normally in the muscle, 

 although masked by an opposite condition in the tendinous ends, 

 and he therefore spoke of the currents as the natural muscle or 

 natural nerve currents. It has since been shown by Hermann 



that this view is incorrect; that the 

 perfectly normal uninjured muscle or 

 nerve has the same electrical potential 

 throughout and will therefore give no- 

 current when any two points are con- 

 nected by a conductor. Moreover, the 

 completely dead muscle or nerve shows- 

 no current. The difference in poten- 

 tial that is found in the excised 

 nerve or muscle is due, according to 

 Hermann, to the fact that at the cut 

 end the nerve or muscle is injured. The 

 chemical changes that take place as a 

 result of the injury make the tissue 

 electronegative as regards the un- 



Fig. 37. Schema showing . , , 



the principle of construction of changed living substance elsewhere. 



For this reason Hermann described 

 the current as a demarcation current; 

 others have called it the current 'of 

 injury. 



The nature of the changes at the injured end are not known. It is inter- 

 esting to note that Bernstein * has shown that the electromotive force of the 

 muscle current increases with the temperature, a fact which leads him to 

 conclude that the difference in potential between the longitudinal and cut 

 surface of the muscle depends upon a difference in concentration of the 

 electrolytes. The muscle, in fact, acts after the manner of a "concentration 

 cell." Such a difference in concentration may pre-exist in the normal mus- 

 cle, or, according to the view adopted above, is developed as the result of 

 injuring one end of the muscle. It may be supposed that the injury causes 

 changes which result in the formation of new organic or inorganic electro- 

 lytes and thus increases the concentration at that point. 



Means of Demonstrating the Muscle Current. The demarcation 

 current and other electrical conditions to be described require especial ap- 

 paratus for their study. To detect the existence of a current physiologists 

 use either a high-resistance galvanometer or a capillary electrometer. f The 

 galvanometers employed are usually of two types, the Kelvin reflecting 

 galvanometer or the d'Arsonval form. The principle of the galvanometer 

 lies in the fact that a magnetic needle is deflected when an electrical current 



*"Pfluger's Archiv," 1902, xcii., 521. 



the 

 ing off i 

 magnet. 



