204 



THE CIRCULATION OF THE BLOOD 



It was shown along ago (by Merunowicz in 1875) that the isolated apex 

 of the ventricle of the frog remains quiet when filled with blood, but readily 

 gives good rhythmic contractions in physiological saline and other artificial 

 solutions. The inactivity in blood is not necessarily, therefore, due to 

 nervous isolation from the ganglionated parts of the heart nor to the 

 bundle system alone. Contractions occur in the small bits of ventricular 

 muscle as isolated by Gaskell, and these may continue for hours. It is. 

 well known also that the embryonic heart contracts rhythmically before 

 nerve cells have reached the organ or even before any blood is formed, as 

 shown in the embryos of certain fishes. 



FIG. 176. Stereoscopic photograph of a model of the atrioventricular nodal system, 

 in the calf's heart. Viewed from behind. The auricular network is not shown. Should 

 be examined through a stereoscope. (Lydia M. DeWitt.) 



The phenomena of heart block, the contractions of the ventricular apex 

 when physiologically isolated from the parts of the heart which contain the 

 ganglia, the behavior of isolated strips of the heart, especially of the ven- 

 tricle and the rhythm of the embryonic heart are all held to be in favor of 

 the myogenic theory. However, in light of recent developments we must 

 find our explanation of the block phenomena as well as of other facts used 

 in argument for the myogenic theory in the physiology of this 

 differentiation from the embryonic muscle, namely, the bundle system. 



PRESENT VIEWS OF THE AUTOMATICITY OF THE HEART. 



Whatever view one adopts of the heart's beat he has to explain not only 

 the periodic origin of the rhythm but also to explain the orderly sequence 

 of auricles and ventricles. Keith and Flack (1906) have ascribed the 

 initial rhythm to a center or node, the sino-auricular node as given above. 

 In the mammalian heart the normal beat under normal conditions is 



