DEMARCATION CURRENT 



823 



two points are connected with a galvanometer by means of un- 

 polarizable electrodes, little or no current is indicated. (Although 

 it is scarcely possible to isolate a muscle without its showing some 

 current, the more carefully the isolation is performed, the feebler 

 is the current; and between two points of the inactive, uninjured 

 ventricle of the frog's heart no electrical difference has been found. 

 Frogs' nerves kept ten to twenty hours after excision in physiological 

 salt solution to which a little calcium salt and frog's blood have 

 been added, are absolutely iso-electric.) 



2. Any uninjured point of a resting muscle or nerve is at a different 

 potential from any injured point. The difference of potential is such 

 that a current will pass through the galvanometer from uninjured to 

 injured point and through the tissue from injured to uninjured point 

 (current of rest, or demarcation current, or injury response) (Fig. 292). 



3. Any unexcited point of a muscle or nerve is at a different potential 

 from any excited point, and any less excited point is at a different 



Fig. 292. A, uninjured, B, injured, 

 portion of nerve; G, galvanometer. 

 The large arrows show direction of 

 demarcation current or current of 

 rest, the small arrows direction of 

 negative variation or action current. 



Fig. 293. Diagram of Currents of 

 Rest in a Regular Muscle, or Muscle 

 Cylinder. E, equator. The dotted 

 lines join points at the same po- 

 tential, between which there is no 

 current. 



potential from any more excited point. The difference of potential 

 is such that a current will pass through the galvanometer to the 

 excited from the unexcited or less excited point (action current, 

 or negative variation, or excitatory electrical response). 



It has been customary in physiological writings to speak of the 

 electrical change in injured or active tissue as a negative one, because 

 when the tissue is led off to a galvanometer the current passes from 

 the galvanometer to the injured or excited portion of the tissue. 

 It may be called with greater precision ' galvanometrically negative.' 

 It is in this sense that we shall employ the term. 



The best object for experiments on the demarcation current is a 

 straight-fibred muscle like the frog's sartorius. If this muscle be taken, 

 and the ends cut off perpendicularly to the surface, a muscle-prism or 

 muscle-cylinder is obtained (Fig. 293). The strongest current is got 

 when one electrode is placed on the middle of either cross-section, and 

 the other on the ' equator ' that is, on a line passing round the longi- 

 tudinal surface midway between the ends. The direction of this 

 current is from the cross-section towards the equator in the muscle. 

 If the electrodes are placed on symmetrical points on each side of the 

 equator, there is no current. 



