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 current. 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- 

 changed living substance elsewhere. 

 For this reason Hermann described 

 the current obtained as a demarcation 

 current ; others have called it the cur- 

 rent of injury. 







Fig. 37. Schema showing 

 the principle of construction of 

 the galvanometer: M, The mag- 

 net suspended by a thread; B, 

 the battery, with the wires lead- 

 ing off the current encircling the 

 magnet. 



Means of Demonstrating the Demarcation Current. The demarca- 

 tion current and other electrical conditions to be described require especial 

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

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

 passes through a wire in its vicinity. As shown in the accompanying dia- 

 gram, if a magnetic needle is swung by a delicate thread so as to move easily 

 it will come to rest in the magnetic meridian with its north pole pointing 

 north. If now a wire is curved round it and a battery current is sent through 

 this wire the needle will be deflected to the right if the current passes in one 

 direction and to the left if it passes in the opposite direction. The move- 

 ment of the needle is an indication of the presence and direction of the electrical 

 current in the wire. The extent of deflection of the needle may be used to 

 measure the strength of the current by ascertaining the amount of deflection 

 caused by a standard battery. The effect of the current upon the needle 

 increases with the number of turns of wire, so that delicate galvanometers 

 constructed upon this principle are spoken of as high resistance galvanom- 



