718 A MANUAL OF PHYSIOLOGY 



reference to muscle and nerve alone, and afterwards apply them 

 to other excitable tissues. 



1. All points of an uninjured resting muscle or nerve are approxi- 

 mately at the same potential (or iso- electric). In other words, if 

 any two points are connected with a galvanometer by means of 

 unpolarizable 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 per- 

 formed 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 



FIG. 266. A, uninjured, B, in- 

 jured, portion of nerve ; G, galvano- 

 meter. The large arrows show 

 direction of demarcation current or 

 current of rest, the small arrows 

 direction of negative variation or 

 action current. 



FIG. 267. DIAGRAM OF CURRENTS 

 OF REST IN A REGULAR MUSCLE, 

 OR MUSCLE CYLINDER. 



E, equator. The dotted lines 

 join points at the same potential, 

 between which there is no current. 



potential is such that a current will pass through the galvano- 

 meter from uninjured to injured point and through the tissue 

 from injured to uninjured point (current of rest, or demarcation 

 current, or injury response) (Fig. 266). 



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 potential from any more excited point. The difference 

 of potential is such that a current will pass through the galvano- 

 meter from the excited to 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 ' galva.nQ- 



