IX 



CARDIAC MUSCLE AND NERVES 



303 



occur, not at regular intervals, but in groups, separated by long 

 pauses (Fig. 133). This strange effect may continue for two or 

 three hours, and usually exhibits a regular course. Often the 

 duration both of the groups and of the succeeding pauses declines 

 regularly; at other times it increases in the primary phase, and 

 declines in the next ; other cases again present irregular oscillations 

 with a constant tendency to decrease. 



The number of beats in each group has no apparent relation 

 with the duration of the respective pauses. Their frequency varies 

 usually in regular order. 



The more typical groups commence with rare contractions that 

 are gradually accelerated, and then again slow down into a long 

 pause. The height of the contractions in each group usually 



Fi<;. 134. Three groups of beats obtained from various frogs' hearts tied at the auricles. 

 (Luciani.) In A the beats form a descending staircase ; in B they are approximately the same 

 height ; in C the first four beats form an ascending staircase. 



forms a descending staircase; more rarely a straight horizontal 

 line ; more rarely still, a slightly ascending staircase (Fig. 134). 



Our experiments tend to show that the groups are of longer 

 duration, and the intervals between them shorter, when the 

 ligature is nearer the sinus. This fact agrees with Eckhard's 

 conclusions to the effect that the duration of the pause increases 

 in proportion as the incisions in the heart are made at different 

 heights, from limit of sinus to auriculo- ventricular groove. 



The periodic rhythm is an absolutely constant phenomenon 

 when the cannula is attached at any height whatever of the 

 auricles, and given all the other conditions of our method, in re 

 serum, temperature, and pressure. When the ligature falls on the 

 auriculo- ventricular groove, the phenomenon may appear in a 

 rudimentary form, or may be altogether absent. When it falls on 

 the upper limit of the ventricle (1-1*5 mm. below the auriculo- 

 ventricular groove) Bowditch's preparation is obtained, of which we 



