CHAPTER XX 

 THE PHYSIOLOGY OF THE HEARTBEAT 



THE ORIGIN AND PROPAGATION OF THE BEAT THE PHYSIO- 

 LOGICAL CHARACTERISTICS OF CARDIAC MUSCLE 



The origin and propagation of the heartbeat are studied on the excised 

 heart of a frog or turtle, or on the mammalian heart by perfusing it 

 under suitable conditions, which have already been described. The results 

 obtained on the cold-blooded heart apply more or less directly to the 

 warm-blooded. In the first place it is clear that the rhythmic contrac- 

 tility of the heart is not at all dependent upon the central nervous sys- 

 tem, for if it were so, the excised heart could not continue beating. This 

 fact does not, however, necessarily imply that the beating power is in- 

 dependent of nervous structures, for in the heart itself an extended net- 

 work of nerve cells and connecting nerve fibers can readily be demon- 

 strated. It might quite well be the case that the rhythmic beat is de- 

 pendent upon the transmission to the muscle fibers of the heart of 

 impulses generated in the nerve cells and transmitted along the nerve 

 fibers of this local nervous system. Such is the neurogenic hypothesis of 

 the heartbeat. 



On the other hand, it may be that these nervous structures are not at 

 all responsible for the origination of the beat, but serve merely as sta- 

 tions on the pathway of the nerve impulses, transmitted to the heart 

 from the central nervous system along the vagus and sympathetic nerves, 

 for the purpose of altering the rate of the heartbeat so as to adjust it 

 to the requirements of blood supply in the various parts of the body. In 

 such a case the rhythmic power would reside in the muscular tissues of 

 the heart that is, each cardiac muscular cell would have the power, 

 not merely like skeletal muscle of contracting in response to a stimulus 

 transmitted to it, but also of originating that stimulus within itself. 

 This is the myogenic hypothesis. Much controversy has raged around 

 these two hypotheses and although space will not permit a detailed study 

 of the question, it will be necessary, on account of the great importance of 

 the subject from the physiological standpoint, briefly to review the main 

 arguments of each school of thought. 



There is no piece of evidence offered by the advocates of either the 

 neurogenic or the myogenic hypothesis that can, taken singly, be con- 



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