THE ORIGIN OF THE HEART BEAT 335 



3. In the frog and turtle it is possible to remove practically the entire inter- 

 auricular septum, together with its ganglia and connecting paths, without inter- 

 fering with the character or rhythm of the cardiac contractions. 



4. A still stronger argument is contained in the fact that the embryonic heart 

 of the chick (2 to 5 days) or shark beat with perfect regularity at a time when as 

 yet no ganglion cells can be made out. If segments of the embryonic heart are 

 kept in. a medium of blood plasma, 1 they will continue to beat for a long time ; 

 indeed, the muscular units usually multiply under this condition and give rise to 

 new cells which possess rhythmic activity. While this, fact clearly proves that 

 the cardiac muscle is automatic, it may be contended that this property is primi- 

 tive and of short duration, and that it is eventually superseded by the auto- 

 maticity of the newly developed nervous elements. 



5. The excised bulbus aortse of the frog, and even portions thereof, usually con- 

 tinue to contract rhythmically. The same result may be obtained with small 

 segments of the veins entering the sinus venosus. 



6. Rhythmic contractions may be observed in the veins of the wing of the bat, 

 as well as in certain segments of the lymphatic system. Nervous elements have 

 not been demonstrated in these tissues. 



7. In the lower forms, the wave of contraction which normally starts in the 

 sinus portion of the heart, is propagated to the auricles and ventricles by means 

 of clearly recognizable strands of muscle tissue. Moreover, while the conducting 

 path in the mammalian heart, as represented by the bundle of His, is formed by a 

 type of tissue which cannot justly be classified as muscle tissue, it does not at all 

 possess the characteristics of nerve tissue. 



8. Waves of contraction may also be incited in other parts of the heart. Thus, 

 the stimulation of the apex most generally gives rise to a contraction in a direction 

 opposite to normal, namely, from ventricle to sinus. 



9. Engelmann has shown that the continuity of the nerve fibers of the heart 

 may be destroyed without materially changing the sequence of its contractions. 

 Thus, it is possible to convert the auricle of the frog's heart by several transverse 

 cuts into a zig-zag strip without blocking the wave of contraction as it passes 

 from the sinus to the ventricle. Very similar results may be obtained with the 

 ventricular muscle. If changed into a zig-zag strip by transverse incisions, a 

 contraction started in its basal portion eventually reaches the apex, while a 

 contraction incited at the apex also progresses to the base. 



The results of the experiments just enumerated indicate with 

 certainty that the nervous elements of the heart possess the power of 

 discharging rhythmic impulses and that cardiac muscle tissue is 

 equipped with rhythmic properties similar to those of other tissues. 

 Smooth muscle and, in a slight degree, also striated muscle are in 

 possession of this power. It may be contended, however, that -this 

 primitive functional characteristic of cardiac muscle prevails only 

 as long as no nervous tissue is present, and that it gradually loses its 

 dominating influence in the course of the development of the latter. 

 Thus, it may be said that the separation of the adult heart from the 

 central nervous system or the destruction of its nervous elements 

 again permits this primitive property of the cardiac musculature to 

 assert itself. Arguments of this kind are difficult to meet, because, 

 while an adequate proof of a supersedence or transfer of function of 

 this kind is not at hand, no perfectly definite reasons can be given 

 against this occurrence. It seems best, therefore, to leave this matter 



1 Burrows, Science, xxxvi, 1912. 



