CHAPTEB XXI 



THE PHYSIOLOGY OF THE EEARTBEAT (Cont'd) 



THE ORIGIN AND PROPAGATION OF THE BEAT IN THE 



MAMMALIAN HEART 



As h;is been shown in tlic preceding chapter, there is no doubt thai 

 in the cold-blooded heart the beat originates at the sinus venosus, whence 

 it spreads to the rest of the heart. Very strong evidence has also been 

 presented to indicate that the beating power is inherent in the muscle 

 fiber itself and independent of nervous structure. This would suggest the 

 further possibility that the structures through which the beat is propa- 

 gated arc the muscle fibers and not the nerve fibers — in other words, 

 that the propagation of the heartbeat, like its origination, is myogenic 

 rather than neurogenic. Direct proof of this hypothesis is readily fur- 

 nished by numerous experiments, among which may be mentioned mak- 

 ing interdigitating cuts across the heart, or excising a ribbon of ven- 

 tricular muscle by an incision simulating the walls of Troy. In both 

 these cases the beat will be found to travel from one end of the muscular 

 band to the other, although it is evident that all the nerves proceeding 

 from base to apex of the heart must have been severed. Of course this 

 evidence is not irrefutable, for it might be argued that there are nerv- 

 ous structures disposed in the form of a plexus continuously all over the 

 heart, and that some branches of the plexus remain uncut in the above 

 experiments. It is only in the heart of Limulus that undoubted evidence 

 exists that the beat is transmitted by nerves, but as Ave have seen, this 

 heart in all its properties is probably the proverbial exception which 

 proves the rule. The balance of evidence stands in favor- of the view 

 that the propagation of the beat over the cold-blooded heart is myogenic 

 and not neuros;enic. 



■ ^> 



CONDUCTING TISSUE IN MAMMALIAN HEART 



When we at I cm pi to investigate the problems of the origin and propa- 

 gation of the beat in the warm-blooded heart, many experimental diffi- 

 culties of course face us. In overcoming these, the first thing Ave must 

 do is to establish tin 1 structural relationship between cold-blooded and 

 warm-blooded hearts. In the embryo of both classes of animals the 



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