64 THE HEART 



how difficult it is to induce degeneration in an isolated nerve 

 by a tight ligature. Anatomical evidence of such degeneration 

 in the heart must be brought, and at present this has not been 

 furnished. 



In regard to the proofs offered for the automaticity of the 

 heart residing in its muscle, considerable importance rests upon the 

 evidence obtained from the embryonic heart. Here one may object 

 that this embryonic tissue is probably neither muscular nor nervous. 

 Bethe claims that recent methods show that from it both muscle 

 and nerve cells develop. The action of muscarine upon the heart 

 before the inwandering of the ganglion cells is also brought as 

 evidence on this point. If this be the case, it leads on to the idea 

 that the automaticity of the adult heart may possibly reside in 

 a similar tissue neither muscular nor nervous. This tissue, akin 

 to nerve-muscular cells from which the heart is developed, may 

 be shut off at any early stage of development for this special 

 purpose of leading the automatic rhythm of the heart. May it 

 not also in some ways correspond to the form of tissue in the 

 myoneural junctions, in being easily affected by nervous influences 

 and manifesting the effect upon the adjacent muscle. Why, more- 

 over, may it not be acted upon by certain internal secretions, thus 

 providing the heart with a further reflex mechanism for preserving 

 the well-being of the body as a whole ? 



Again in regard to the conduction of the impulse, it is possible 

 that the conducting mechanism of the heart is developed from 

 such a tissue, becoming nervous in one form of heart, muscular in 

 another, according to the requirements of the organ in that special 

 genus. This would explain the discrepancies in the evidence from 

 the invertebrate kingdom. Then again the Purkinje fibres of the 

 sheep's heart are not seen in the human. This is probably some- 

 thing to do with the requirements of co-ordination. The Purkinje 

 fibres are totally different from the surrounding musculature it 

 is difficult to call them muscle ; but it is easy to understand how 

 such a fibre can be differentiated from a tissue capable of giving 

 rise to both muscle and nerve. From such a tissue, moreover, a 

 conducting mechanism can be evolved presenting different histo- 

 logical appearances, but having the requisite rate of conduction 

 for the co-ordination of thai heart. 



The above are the chief facts in regard to both the neurogenic 

 and myogenic theories. Neither theory is a dogma one may 



