228 ELECTRO-PHYSIOLOGY CHAP. 



The universal validity of the law of polar excitation being 

 thus unequivocally established in the case of striated skeletal 

 muscle, there is a priori no doubt of its further applicability 

 to cardiac, as well as to smooth, muscle. In view of the structure 

 of the heart (consisting, like all other smooth muscle, of innumer- 

 able cells in close juxtaposition, connected together by cement- 

 substance), the question may fairly be asked, what relatively 

 to the previous definition must here be understood by the 

 " physiological anode or kathode ? " To take a simple case, if we 

 imagine a strip composed of parallel fibre-cells, as is approximately 

 shown in a preparation of molluscan adductor muscle, we may 

 expect such a preparation when traversed longitudinally by 

 current to behave like a polymerous, cross-striated muscle, the 

 several parts of which must be regarded anatomically, and 

 physiologically, as independent individuals. This is well exhibited 

 in the M. rectus abdominis of the frog. If current is sent 

 through this muscle, when it has been exposed and stretched 

 between two corks, there is, as might be expected, at and during 

 closure, on the anodic side of each tendinous intersection (if ex- 

 amined with transmitted light under the magnifying lens) a clear 

 and sharply-delimitated swelling of the ends of the fibres corre- 

 sponding with the persistent kathodic closure contraction. It 

 disappears at the moment of breaking the circuit, eventually 

 making way for persistent anodic opening contraction on the 

 other side of the intersection. It follows of course that the closin^ 



o 



and opening twitch of each part must proceed from the same 

 point. 



The whole segmented muscle-band will thus be excited at as 

 many points in its continuity as it has divisions, since each 

 element of the muscle circuit has its .kathode and anode respect- 

 ively. And if the adjacent cells of the heart, or any other 

 smooth muscle, conduct themselves like the constituents of a 

 polymerous muscle, and if the interstitial, or cement, substance 

 plays the same part as the tendinous intersection, it may be pre- 

 sumed that the electrical current will produce excitation at 

 closure (or opening) at as many points in the continuity of the 

 tract as there are cells present. For obviously the latter would 

 each have their proper kathode and anode, so that, in conse- 

 quence of the inferior length of the cell elements in question, 

 the excitation (contraction) would in fact begin simultaneously at 



