ELECTRO-PHYSIOLOGY 



629 



Since it is in muscle and nerve that the phenomena of 

 electro-physiology are seen in their simplest expression, and 

 have been chiefly studied, we shall develop the fundamental 

 laws with reference to muscle and nerve alone, and after- 

 wards apply them to other excitable tissues. 



i. All points of an uninjured resting muscle are approximately 

 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 performed the feebler is the current ; and between two 

 points of the inactive, uninjured ventricle of the frog no 

 electrical difference has been found.) 



FIG. 196. 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. 197. 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. 



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

 higher potential than any injured point; so that a current will 

 pass through the galvanometer from uninjured to injured 

 point, and in the tissue from the latter to the former (current 

 of rest or demarcation current Fig. 196). 



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

 potential than any excited point, and any less excited point is at a 

 higher potential than any more excited point. 



The injured or excited muscle may thus be compared to a Daniell 

 cell, the injured or active tissue corresponding to the zinc, which is 

 connected with the negative pole of the battery, and the uninjured 

 or inactive tissue to the copper, which is connected with the positive 

 pole. 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. 197). The 



