244 ELECTRO-PHYSIOLOGY CHAP. 



no means so simple at the anode, nor are they more so with 

 kathodic excitation. There can be no doubt that the longi- 

 tudinal muscles of the directly stimulated section are excited, but 

 we may question, for reasons to be stated below, whether the 

 ring-muscles are not also, at least locally, excited at the point 

 of contact. 



The first desideratum, in an exact observation, is not to 

 employ too strong a current, since the problem will otherwise be 

 unduly complicated. It must be remarked once more that the 

 results of bipolar, coincide exactly with those of unipolar, 

 excitation. The more striking change is, as we have seen, the 

 shortening (decrease in height) of the body -rings implicated. 

 This is not, as a rule, uniform throughout the periphery, but is 

 essentially confined to the immediate proximity of the kathode. 

 Here, in consequence of the longitudinal muscular contraction, the 

 segment involved appears in relief as a swollen blister, between 

 the contiguous segments. The latter participate equally in the 

 stimulation with stronger currents, so that on closure of the 

 current the longitudinal muscular contraction, extending over 

 several segments, causes a more or less pronounced swelling to 

 spring up, which is mainly confined to the point directly 

 excited, rises most abruptly under the kathode itself, and falls 

 away tolerably quickly on either side of it. If the tract of 

 kathodic excitation is examined at an appropriate strength of 

 current, with a magnifying lens, particular attention being given 

 to the appearance of excitation effects in the segment directly in 

 contact with the electrode at the moment of closure, it is not 

 usually difficult to ascertain positively that there is also 

 at that point a contraction of the circular muscles, localised 

 to the kathode, which only remains unnoticed with less atten- 

 tive observation, because its spatial restriction prevents any 

 perceptible diminution of diameter in the muscle -ring. This 

 effect is limited, with the application of moderate currents, to the 

 segment directly excited, and in consequence a marked, lumpy 

 protuberance is often visible at the point of contact, which 

 is no doubt due to the disguised and local contraction of the 

 longitudinal and circular muscles. In order to attest the latter 

 it is important to note that in the muscular integument the layer 

 of circular muscles (conversely to the vertebrate intestine) lies 

 externally, and is thus directly accessible to observation. Other- 



