CIKCULATION 251 



In considering what this mechanism is, it must be borne in 

 mind that two distinct questions have to be investigated. 



1st. How does the contraction, once started, pass in regular 

 sequence from one part of the heart to the other ? 



2nd. What starts each rhythmic contraction '( 



1st. Propagation of the Wave of Contraction. In the 

 heart of many of the lower animals, and in the embryo of 

 mammals, no nervous structures are to be found, and the 

 rhythmic contraction is manifestly simply a function of the 

 muscular fibres. 



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 between 

 auricles and ventricles through the band of His is of small 

 extent, 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 early foetal 

 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 ? 



