THEORIES OF THE HEART-BEAT 



175 



investigators, and now one, now the other theory has attracted followers 

 as new facts have been discovered. The hypotheses that have been ad- 

 vanced to explain the heart-beat are known as the neurogenic theory and the 

 myogenic theory, respectively. 



The heart has long been known to have the power of rhythmic contrac- 

 tions after removal from the body and after all connection with the central 

 nervous system has been destroyed. The isolated heart, even of man, will 

 contract with good rhythm when kept at the proper temperature and given 

 the proper nutritive fluid. 



The Neurogenic Theory, The neurogenic theory attributes the remark- 

 able power of the heart to continue its contractions after removal from the 

 body, and presumably while in the body, to the presence of the local collec- 

 tions of nerve cells. The local nervous mechanism in the frog consists of 



FIG. 175. Course of the Nerves in the Auricular Partition, Heart of a Frog, d, Wall of the 

 dorsal branch; v, ventral branch. (Ecker.) 



three chief groups of cells or ganglia. The first group is situated in the 

 wall of the sinus venosus at the junction of the sinus with the right auricle, 

 Remak's ganglia; the second group is placed near the junction between 

 the auricles and ventricles, Bidder's ganglia; and the third in the septum 

 between the auricles, von Bezold's ganglia. Small ganglia have been de- 

 scribed for the base of the ventricle, but no ganglia are present in the apical 

 part of the ventricles, though isolated cells have been found. The nerve- 

 cells of which these ganglia are composed are generally unipolar, seldom 

 bipolar. Sometimes two cells are said to exist in the same envelope, con- 

 stituting the twin cells of Dogiel. The cells are large, and have very large 

 round nuclei and nucleoli, figure 176. As regards the automatic move- 

 ments of the heart when removed from the body, our knowledge has been 

 derived from the study of the hearts of the frog, tortoise, dog, cat, and rabbit. 



