THE CIRCULATION OF THE BLOOD 



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. 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. 



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 ventricle 

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

 genic theory. 



Automaticity of the Heart. Whether one adopts the neurogenic 

 or myogenic theory of the heart's beat, he has still to explain the origin of 

 the heart's rhythm. In the former case one must look to the nervous 

 apparatus for the origin of the rhythm; in the latter case, the muscular ap- 



FIG. 176. Stereoscopic photograph of a model of the atrioventricular system in the 

 calf's heart. Viewed from behind. The auricular network or knoten is not shown. 

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



paratus, a fact to which Brown-Sequard long ago called attention. In the 

 former view the problem is to explain not only the periodic origin of the 

 nerve discharges from local cardiac ganglia, but also to explain the orderly 

 discharge of nerve impulses which maintains the proper sequence between 

 sinus, auricle, and ventricle. 



To perhaps the majority of physiologists the facts are best explained 

 by the myogenic theory. The origin of the rhythm is here supposed to be 

 due to the automatic property of the muscle itself. The sequence is ex- 

 plained on the observed facts, first, that muscular contraction in cardiac 

 muscle is conducted throughout the continuity of the mass, and second, 



