THEORIES OF THE HEART-BEAT 195 



it would automatically occur, then the succeeding pause is longer; i.e., there is 

 a compensatory pause. Also the contraction induced is smaller and the one 

 following the compensatory pause is proportionately larger. This observa- 

 tion can easily be demonstrated on the heart strip, see figure 174, or on the 

 whole ventricle of the frog, which was originally used by Bowditch. 



Nerve influence, nutrition, temperature, etc., will of course affect the 

 extent of the contractions, but under a given set of conditions it is held that 

 the contractions which occur are maximal for the particular state. This 

 is more readily understood when taken in connection with the fact that when 

 a contraction originates in a cardiac cell it is conducted throughout the extent 

 of all the cells of the muscular mass. 



Theories of the Heart-beat. The cause of the rhythmic power of 

 the heart as a whole has been the subject of much discussion and experimen- 

 tal observation. Two leading hypotheses have given inspiration to investi- 

 gators, and now one, now the other theory has attracted followers as new 

 facts have been discovered. The hypotheses that have been advanced 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, has 

 been made to contract with good rhythm when kept at the proper tempera- 

 ture 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 

 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, 

 RemaWs 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 175. 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. 



If removed from the body entire, the frog's or terrapin's heart will con- 

 tinue to contract for many hours and even days, and the contraction has no 

 apparent difference from the contraction of the heart before removal; it 



