136 A TEXTBOOK OF PHYSIOLOGY 



it cannot be denied that this connection contains nerve fibres a, 

 point insisted upon by the upholders of the neurogenic theory. 



3. If in the heart of the Li mums a section of the median nervous 

 strand be made, it immediately abolishes the synchronism of the 

 different segments. The parts on either side of the section continue 

 to beat, but no longer with the same rhythm. If a section be made 

 of the muscle of this heart, it produces no effect. 



In estimating the value of the experiments on Limulus it must be 

 remembered that it is an invertebrate evolved in the epochs of 

 geological time, and that the muscle of its heart seems more akin 

 to mammalian smooth muscle. 



The myogenic theory is based upon the following points : 



1 . The different chambers of the heart beat with a different rhythm 

 when separated from each other. There is no differentiation of any 

 nervous tissue of origin and conduction in these chambers, so far as 

 is known, while there is a marked difference in the histological appear- 

 ance of the muscle of the different heart chambers. 



FIG. 44. RABBIT'S HEART, A.-V. BUNDLE CUT, SHOWING EFFECT OF STIMULATION op 

 RIGHT VAGUS NERVE. (W. Cullis and E. M. Tribe.) 



The independent rhythm of auricles and ventricles is seen at the beginning, but 

 particularly well at the end of the tracing. The nerve acts upon the auricles but 

 not upon thi ventricles. 



2. The experiment in regard to the A.-V. ganglia is incorrect. 

 Excitation of the ganglion cells of this group causes no contrac- 

 tion. It is the excitation of the musculature of the auricular 

 ring which evokes contraction. 



3. Isolated parts of the great veins of cold-blooded hearts con- 

 taining no ganglion cells beat automatically. Thus, a piece of the 

 sinus of the frog beats for four days, and no less than 17,000 con- 

 tractions were recorded. 



4. The apex of the mammalian heart, said to have no nerve ganglia, 

 when suitably fed exhibits slow rhythmic contractions. 



5. The embryonic heart pulses before muscle and nerve have 

 become differentiated in it. 



6. It is possible to cause hearts to beat again several hours after 

 death. The heart of a boy, dead of pneumonia, was resuscitated 



