TRANSMISSION AND CONDUCTION 221 



chick (unpublished) the heart of the earlier stages shows 

 a distinct susceptibility gradient, the end which is to 

 become the sinus end being the high end of the gradient. 

 This end becomes functionally the dominant region, the 

 pacemaker, and the only conceivable basis for this 

 dominance is the fact that it is originally the high end 

 of the gradient. Its dominance depends essentially 

 upon the fact that it is able to give rise to impulses with 

 a more rapid rhythm than other levels under the same 

 general conditions, and this ability is unquestionably 

 associated with its position in the gradient. It makes 

 little difference for this conception whether we accept 

 the myogenic or the neurogenic theory of the heart beat. 



If the sinus region is cooled or otherwise inhibited, 

 the beat may be initiated by lower levels of the gradient, 

 but the rhythm is slower. Any level of the heart is able 

 to initiate a beat when sufficiently isolated from the 

 sinus region, either by inhibition of the latter, by block- 

 ing the passage of the beat, or by sufficient stimulation 

 of the level concerned. Even reversal in direction of 

 transmission of the beat is possible to a greater or less 

 degree under conditions which inhibit the sinus region 

 and excite the arterial end, that is, which tend to reverse 

 the gradient. In all these respects the heart as a con- 

 ducting path and as a gradient resembles the plate row. 

 The invertebrate heart differs more or less in form in 

 different groups, but is in general tubular, and, though 

 it has not yet been demonstrated to be a physiological 

 gradient, it behaves in general like one. 



The ascidian heart shows periodic reversal in the 

 direction of transmission of the beat, the reversal 

 occurring after a few or a considerable number of beats, 



