ELECTRICAL CHANGES IN MUSCLE 



253 



FIG. 79. Current of rest. 



or ' demarcation current ' (Hermann), disappears. The current is 

 due to the electrical differences at the junction of living and dying 

 (not dead) tissue. If the sartorius of the frog be cut out and immersed 

 for twenty-four hours in 0'6 per cent. NaCl solution made with 

 tap water (i.e. containing lime), all the injured fibres die, and the 



uninjured fibres are then found to be 

 iso-electric and therefore currentless. 



The existence of this current may be 

 // demonstrated without using a galva- 

 nometer. If the nerve of a sensitive 

 muscle-nerve preparation (a, Fig. 80) be 

 allowed to fall on an excised muscle b, 

 so that two points of the nerve are in 



contact with the cut end and with the surface of the second muscle 

 b, the muscle a will contract each time the nerve touches b so as to 

 complete the circuit. 



Whatever be the explanation of this current of resting muscle, 

 there is no doubt that a very definite electrical change occurs in a 

 muscle when it contracts. To show this change, we may lead off 

 two points, one on the cut end and one on the surface of the muscle 

 of a muscle -nerve preparation, to a galvanometer. We shall then 

 obtain a deflection of the mirror of the magnet, due to the current of 

 rest or demarcation current. If now the nerve be stimulated with 

 an interrupted current so as to throw the muscle into a tetanus, the 

 ray of light from the galvanometer mirror 

 swings back towards the zero of the scale, 

 showing that the current which was present 

 before is diminished. When the excitation 

 of the nerve is discontinued, the galvanometer 

 indicates once more the original current 

 of rest. This diminution of the current of 

 rest during activity of a muscle is spoken of 

 as the ' negative variation.' 



FIG. 80. 

 Rheoscopic frog. 



In carrying out this experiment it is usual to compensate the demarcation 

 current by sending in a small fraction of the current from a constant cell. 

 The arrangement of the apparatus is represented in the accompanying dia- 

 gram. Two non-polarisable electrodes np are applied to the surface and 

 cross-section of a muscle m. These are connected with the shunt of the 

 galvanometer, one of the wires, however, being connected with a Pohl's reverser 

 p, and this in its turn with the shunt s. The two end-terminals of the 

 reverser are connected with a rheochord, through the wire of which ab a 

 constant current is passing from the Daniell cell D. By means of the rider c 

 the fraction of current passing through the reverser can be modified to any 

 extent. The key k being open, the muscle is connected with the shunt and 

 galvanometer, and the direction and extent of the swing noticed. THe 



