Late Premature Beats. 327 upward deflection indicates negativity of the earthed lead, i.e., the lead from the pacemaker. Tuning fork, 1 d.v. = 10a. F ig. 3-A series of action potentials of normal and premature beats recorded by the Matthews oscillograph. In observations 1 and 5 premature beats were set up by an induction shock, the cathode of the stimulating circuit being also the earthed lead of the recording system. The main auricular deflection of the action potential occurs after the record

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... of the stimulus artefact (indicated by the arrow above observation 1). In observation 2 there is a record of the stimulus artefact which was too soon after the previous beat to set up a premature beat. The irregular series of deflections following this artefact is the ventricular action potential corresponding to the previous normal beat. Observation 6 is a more highly amplified record of a normal beat. Coupling condensers = 0*002pF. Fig. 4-A series of observations similar to fig. 3 but taken with a slower plate in order to show the disturbance of rhythm produced by premature beats (indicated by arrow). Fig. 5-Normal action potentials with leads in positions 4 and 6 of fig. 2. Coupling con densers = 4pF. The first arrow indicates the abrupt onset of the action potential and the second arrow indicates the ventricular T wave. 612.172.2. The disturbance of cardiac rhythm by a premature beat was first studied by Marey (1875), who found that the lengthening of the cycle following the premature beat usually just compensated for the shortening of the cycle preceding it, the two cycles together being equal to two normal cycles. The phase of the rhythm therefore suffered only a transient disturbance by the premature beat. Engelmann (1895) obtained similar results when premature beats were set up by stimulation of the frog's ventricles, but later (1897) he stated that there was no lengthening of the cycle following the premature beat set up by stimulation of the vena cava. On the basis of these observations he put forward the suggestion that the ventricle beats in response to discrete impulses discharged rhythmically from the sinus. A premature beat set up by stimulation of the ventricle would be prevented from reaching the sinus by the on July 19, 2018 http://rspb.royalsocietypublishing.org/ Downloaded from