CHAP, ii.] THE CONTRACTILE TISSUES. 113 



adduced in favour of the view that these currents are not 'natural 1 

 phenomena but essentially of artificial origin. It will therefore be 

 best to speak of them as currents of rest. 



67. Currents of action. Negative variation of the Muscle- 

 current. The controversy whether the 'currents of rest' observable 

 in a muscle be of natural origin or not, does not affect the truth 

 or the importance of the fact that an electrical change takes place 

 and a current is developed in a muscle whenever it enters into a 

 contraction. When currents of rest are observable in a muscle 

 these are found to undergo a diminution upon the occurrence of a 

 contraction, and this diminution is spoken of as 'the negative 

 variation ' of the currents of rest. The negative variation may be 

 seen when a muscle is thrown into a single contraction, but is most 

 readily shewn when the muscle is tetanized. Thus if a pair of 

 electrodes be placed on a muscle, one at the equator, and the 

 other at or near the transverse section, so that a considerable 

 deflection of the galvanometer needle, indicating a considerable 

 current of rest, be gained, the needle of the galvanometer will, 

 when the muscle is tetanized by an interrupted current sent 

 through its nerve (at a point too far from the muscle to allow of 

 any escape of the current into the electrodes connected with the 

 galvanometer), swing back towards zero ; it returns to its original 

 deflection when the tetanizing current is shut off. 



Not only may this negative variation be shewn by the galvano- 

 meter, but it, as well as the current of rest, may be used as a 

 galvanic shock and so employed to stimulate a muscle, as in the 

 experiment known as 'the rheoscopic frog.' For this purpose the 

 muscles and nerves need to be in thoroughly good condition and 

 very irritable. Two muscle-nerve preparations A and B having 

 been made, and each placed on a glass plate for the sake of insula- 

 tion, the nerve of the one B is allowed to fall on the muscle of the 

 other A in such a way that one point of the nerve comes in 

 contact with the equator of the muscle, and another point with 

 one end of the muscle or with a point at some distance from the 

 equator. At the moment the nerve is let fall and contact made, a 

 current, viz. the 'current of rest' of the muscle A, passes through 

 the nerve ; this acts as a stimulus to the nerve, and so causes 

 a contraction in the muscle connected with a nerve. Thus the 

 muscle A acts as a battery, the completion of the circuit of which 

 by means of the nerve of B serves as a stimulus, causing the 

 muscle B to contract. 



If while the nerve of B is still in contact with the muscle of A, 

 the nerve of the latter is tetanized with an interrupted current, 

 not only is the muscle of A thrown into tetanus but also that of 

 B ; the reason being as follows. At each spasm of which the 

 tetanus of A is made up, there is a negative variation of the 

 muscle current of A. Each negative variation of the muscle 

 current of A serves as a stimulus to the nerve of B, and is hence 



F. 8 



