252 PHYSIOLOGY. 



and ventricles in the auriculo-ventricular groove. Von Bezold's 

 ganglion has its seat in the interauricular septum. 



The heart of a mammal differs from that of an amphibian only 

 in that there are several groups of ganglia in the mammals, while 

 but one exists in the amphibians. However, these several ganglia of 

 the mammal are believed to be automatically and physiologically 

 equivalent to the homologous single ganglion or group of ganglia of 

 the amphibian. The same general laws may be applied to both. 



Cause of Cardiac Rhythm. The rhythm of the ventricle is a 

 property of the cardiac muscle. In the maintenance of this rhythm 

 the nervous system does not intervene except as an ordinary excitant of 

 muscle. It is known that, if the apex of the frog's heart be cut away, 

 it is then separated from all ganglia. The excised portion does not 

 beat spontaneously, while the rest of the heart, the auricles and the 

 base of the ventricles, continue their rhythmical action. Thus it 

 seems that the ventricles can contract under the persuasion of irri- 

 tations which arise in them. 



If now the isolated and immovable portion of the heart be placed 

 under a cardiograph and subjected to opening of the induction current, 

 there will result a pulsation from each isolated induction shock. 



It is a remarkable fact that, if this same excised portion be excited 

 by frequent breaks (at least thirty per second), the muscle beats 

 rhythmically. Ordinary striped muscle responds to isolated and 

 separate breaks of the induction current by manifesting isolated con- 

 tractions. Heart-muscle cannot be tetanized. 



Hence this observation would force us to the conclusion that the 

 heart's rhythm does not depend upon the ganglionic cells of the heart. 

 The rhythm is the property of the cardiac muscle to react to the 

 frequent excitations which it receives. 



In this respect cardiac muscle is completely differentiated from 

 ordinary striated muscle. It is a mistake to seek to make the rhyth- 

 mical property of the cardiac muscle a property of ordinary muscle. 



Theory of Cardiac Rhythm. Numerous experiments have been 

 performed upon the hearts of animals (the frog chiefly) for determin- 

 ing the causes and means of control of the rhythmical movements of 

 the heart. The experiments consist, for the most part, of ligaturing 

 various portions of the heart, and are performed by tightening and 

 then relaxing the ligature so that the physiological connection is 

 destroyed, while its anatomical and mechanical functions are still 

 intact. The most important, as well as best known, of the ligature 

 experiments is the one known as; 



