THE PHYSIOLOGY OF THE HEARTBEAT 171 



sidered as absolutely conclusive. Although some of "the proofs" may 

 at first sight appear to be conclusive, yet each of them breaks down when 

 subjected to a closer scrutiny. It is only after we have collected all the 

 evidence for and against each view that we shall be in a position to come 

 to any conclusion, and even then it will be plain that our conclusion can 

 be only tentative. 



Myogenic Hypothesis 



Taking first of all the evidence in support of the myogenic hypothesis, 

 the following stands out most prominently: 



1. The heart beats in the embryo chick before any nerve cells have 

 grown into it, and not only this, but if portions of heart muscle are re- 

 moved from the embryo and placed in blood plasma, they will continue 

 beating for many days It has also been observed that cells may wander 

 off from this mass of cardiac muscle and undergo multiplication and 

 differentiation, so as to produce isolated muscle cells which exhibit 

 rhythmic contractility. The rebuttal on the part of the neurogenists of 

 this apparently unassailable evidence is to the effect that, although em- 

 bryonic muscle cells may exhibit the power of rhythmic contraction, this 

 does not mean that the fully developed muscle cells will necessarily have 

 such power. In the eary stages of embryonic development, it is of course 

 evident that the functions which in the fully developed animal are del- 

 egated to various special organs and tissues should be performed by cells 

 having several such functions in common. The muscle cells of the heart, 

 for example, may to start with be possessed of a power not only of con- 

 tracting but also of initiating the contraction. It may be that they are 

 partly nervous in character and that only later, when the differentiation 

 is consummated, does the power of rhythmic contraction become dele- 

 gated to the nervous element and that of contraction retained by the 

 muscle itself. 



2. The nervous structure in the heart may be damaged either by me- 

 chanical means or by drugs without apparently interfering with the 

 power of rhythmic contraction; for example, in the heart of large tur- 

 tles it is possible to dissect out a considerable amount of nervous tissue 

 without any disturbance of the beat, and in all animals the administration 

 of atropine, which paralyzes the postganglionic fibers of the autonomic 

 nervous system (see page 226) found in the heart, does not affect it. 



3. The apex of the ventricle in such hearts as that of the turtle can 

 be shown, by careful histologic examination, to contain no nerve cells, 

 and although a few nerve fibers may be found, these are functionless 

 without nerve cells. This virtually nerveless piece of heart muscle can 

 be made to contract rhythmically by perfusing it with suitable saline 



