iv ELECTROMOTIVE ACTION IX MUSCLE 331 



simply cutting out the former. For this purpose two mercury 

 keys are introduced into the circuit, one between the cell and the 

 rheochord, the other between the latter and the muscle. The 

 former is denoted below as the key in the principal circuit, 

 the latter as key in the deriving circuit. If the key of this 

 deriving circuit is closed immediately after the thermic section has 

 been effected, the key of the primary circuit remaining open, a 

 perfectly visible, though usually weak, closure twitch is seen 

 under favourable conditions in very excitable preparations. The 

 results are more certain if the unpolarisable electrodes are placed 

 near together, in direct lateral contact with two points of the 

 surface of the muscle, whereby the resistance in the circuit can be 

 suitably reduced. If the upper half of the uninjured sartorius 

 is stretched on a cork plate, and one electrode placed at the 

 pelvic end, the other at a slightly lower point of the longitudinal 

 surface, then on leading a weak or medium current, descending 

 or ascending, through the muscle, a twitch occurs at every 

 closure, while on opening the circuit by the principal, or shunt 

 key, no trace of change of form in the muscle is apparent. 

 The result of the experiment is very different when an artificial 

 (thermic) section has previously been made at the pelvic end 

 of the muscle ; if the negative electrode is now in contact with 

 the heat-rigored end of the muscle, while the positive electrode 

 is applied to the nearest point of the uninjured surface, there is, 

 with rare exceptions, in excitable preparations, immediately after 

 the injury, a distinct twitch, the index of excitation in the freely- 

 depending half of the muscle, as soon as the deriving circuit is 

 closed the principal circuit remaining open. It is obvious 

 from the conditions of the experiment that this also is an 

 excitation resulting from the passage of the muscle current 

 through the shunt current. Whether this happens or not, there 

 is invariably, under these experimental conditions, a pronounced 

 shortening of the muscle when a weak battery current opposed 

 in direction to the muscle current (i.e. in this case ascending) is 

 made for a short time and broken in the principal circuit. Since 

 the physiological kathode is at the seat of injury, the make 

 excitation either fails altogether, or is very insignificant. This 

 result, however (given a sufficient distance of the leading-off, or 

 leading-in, electrodes), appears at the opening of the principal 

 circuit only, while there is no sign of mechanical change on 



