In the denervated state the mammalian heart, both in vivo and in vitro, is excited at very regular intervals, the coefficient of variance of the interbeat intervals not exceeding 2%. The pacemaker that is the source of this regular ex citation is localised normally within the sinus node (" sino-atrial node " node of Keith and Flack), a most intriguing small piece of tissue in the caval corner of the right atrium. A small portion of this node containing a group of probably only a few thousands of cells fires spontaneously, that means without any exter nal influence to trigger their activity....