246 ELECTRO-PHYSIOLOGY CHAP. 



There is never an undulatoiy transmission of the contraction over 

 large sections of the worm-body. On the contrary, these remain 

 fixed in the expansion determined at closure as long as the 

 current is passing, and this applies as well to the anodic circular, 

 as to the kathodic longitudinal, muscular contraction. In the 

 leech also it is certain that the latter does not appear singly, but 

 is accompanied by a simultaneous localised contraction of the 

 circular muscles at the point of contact with the electrode. 

 A small but plainly visible swelling accordingly starts up under 

 the electrode, running transversely to the fibres of the circular 

 muscles, and, as it were, opposed to the expansion produced by 

 the contraction of the longitudinal muscles. No trace of excitation 

 is visible in the same bundle of circular fibres beyond the small, 

 sharply-defined swelling. Where a perceptible (" tonic ") contrac- 

 tion existed already before closure, it may be seen to expand into 

 the localised swelling at the point of current exit. At both sides of 

 this there will then be a visible relaxation of the circular muscles, 

 so that at times the lateral parts of the segment bulge out 

 bladderwise, convexly to the exterior, thus producing a very 

 characteristic, and more or less complicated, change of form in the 

 muscular integument at the proximity of the electrode. The 

 swelling caused by the local contraction of the longitudinal 

 muscles is also sharply defined on either side, although, as stated, 

 it extends over several segments. At break of the circuit the 

 changes described (Fiirst, I.e.) are either equalised simply, or 

 (with stronger currents, and longer duration of closure) a break 

 excitation makes its appearance, as in the earthworm, by a 

 shortening of the circular muscles, which extends over large areas 

 of the previously excited segment, producing a more or less pro- 

 nounced segmental constriction a change, of which the resem- 

 blance to the effects of anodic make excitation is ^riw /""' 

 evident, although it is difficult to demonstrate complete coinci- 

 dence in the two cases. While on bringing any points of the 

 upper surface of the muscular investment of Hirudo into 

 contact with the kathode, there is, in consequence of the simul- 

 taneous shortening of the longitudinal and circular muscles, a 

 general pressure of the muscle-substance from all sides towards 

 the point where the current leaves the muscle, a precisely opposite 

 effect appears with anodic excitation. It is even more evident 

 in Hirudo than in Lumbricus, that the segment in contact with 



