THE CIRCULATION 237 



Even in the heart of animals with well-marked nerve cells 

 in the walls of the heart, and with nerve fibres coursing 

 among the muscular fibres, the conduction of the contraction 

 is purely a function of the muscles. For if the heart of a 

 frog be cut across and across, so that all nerve fibres are 

 severed, the contraction passes along it. The rate at which 

 the contraction travels is slow, only about 10 to 15 centi- 

 metres per second. 



Since in the mammalian heart muscular continuity be- 

 tween auricles and ventricles is partially interrupted, the 

 wave of contraction is delayed at this point, and in the dying 

 heart, and in various pathological conditions, the contraction 

 frequently fails altogether to pass this block, and thus the 

 ventricles either stop beating before the auricles, or respond 

 to every second or third auricular contraction. 



2nd. Starting Mechanism of Contraction. In the ascidian 

 heart no nerve structures have been found, yet it beats 

 regularly and rhythmically. In the apex of the ventricle 

 of the frog there are no nerve structures, yet, if the apex be 

 cut off and repeatedly stimulated at regular intervals with 

 galvanic making and breaking stimuli, it will, after a time, 

 begin to contract spontaneously, regularly, and rhythmically. 

 Not only so, but if the apex be tied on to a tube, and a 

 stream of blood passed through it, it will again start con- 

 tracting regularly and rhythmically. 



These experiments clearly show that regular rhythmic 

 contraction is a function of cardiac muscle. 



In the cardiac cycle in the frog each contraction starts in 

 the sinus. What part does the sinus take in initiating 

 contraction ? 



If a ligature be tightly applied between the sinus and 

 auricles (Stannius' Experiment), the sinus continues to 

 beat, and the auricles and ventricles usually stop beating 

 for a longer or shorter period. But ultimately they begin 

 to beat again. Hence it would seem that it is not any 

 special mechanism in the sinus which is essential in starting 

 cardiac contraction. A ligature subsequently applied between 

 auricles and ventricles sometimes starts the one, sometimes 

 the other, sometimes neither. Hence we see that any 

 part of the heart has the power of originating rhythmical 



