62 NEGATIVE VARIATION. [BOOK i. 



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 

 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 very irritable and in thoroughly 

 good condition. Two muscle-nerve preparations A and B having 

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

 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 the 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 

 J5; 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 in the muscle- 

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

 the cause of a spasm in the muscle of B ; and the stimuli following 

 each other rapidly, as being produced by the tetanus of A they must 

 do, the spasms in B to which they give rise are also fused into 

 a tetanus in B. B in fact contracts in harmony with A. This 

 experiment shews that the negative variation accompanying the 

 tetanus of a muscle, though it causes only a single swing of the 

 galvanometer, is really made up of a series of negative variations, 

 each single negative variation corresponding to the single spasms 

 of which the tetanus is made up. 



But an electrical change may be manifested even in cases when 



