332 PECULIARITIES OF THE CARDIAC MUSCLE TISSUE 



different parts of the body which influence the activity of the heart 

 by way of these connecting channels. These correlating impulses, 

 however, have nothing to do with the actual cause of the contractions. 

 In the second place, it must be evident that even if it has been demon- 

 strated that the beat originates in the heart, it still remains to be de- 

 termined whether the impetus to contract is given by the muscle 

 substance or by the nervous elements contained therein. The views 

 held pertaining to this question have been embodied in the so-called 

 neurogenic and myogenic theories of the heart beat. 



Closely related to this problem is another which pertains more 

 directly to the cause of the orderly sequence of the contractions of the 

 different segments of the heart. Thus, it may be asserted that the 

 rhythm of the heart is associated either with the nervous elements or 

 with the muscle tissue. With reference to the automaticity of this 

 organ, the question may then be raised whether its power of remaining 

 active by a self -inducing cause is contained in the first or in the second 

 component. At the present time it is quite impossible to give a 

 definite answer to these questions. We are, however, in possession of 

 certain fundamental facts relating to this topic which may best be pre- 

 sented separately under the headings of the theories just mentioned. 



The Neurogenic Theory of the Heart Beat. This theory which has 

 been proposed by Volkmann, was strengthened considerably by the 

 discovery of Remak that the heart of the frog gives lodgment to nerve 

 fibers as well as to ganglion cells (1849). Upon entering the sinus 

 venosus, the two vagi nerves unite to form a plexus which is situated 

 below the pericardium and embraces numerous ganglion cells. Re- 

 mak's ganglion is connected by means of two septal nerves with another 

 network of nerve tissue which is situated in the vicinity of the auriculo- 

 ventricular groove and is known as Bidder's ganglion. Both ganglia 

 send non-medullated fibers to the neighboring regions of the auricles 

 and ventricle, a few isolated nerve cells being interposed here and there. 

 It was also noted that the apical portion of the heart, embracing the 

 lower one-half to two-thirds of the ventricle, is free from cellular 

 elements. Even more favorable conditions for experimentation 

 prevail in the turtle, because the heart of these animals is larger and its 

 nervous elements are more easily accessible. 



In accordance with this theory, it is assumed that the successive 

 cardiac contractions result in consequence of excitations which are sent 

 out at regular intervals by the cells composing the aforesaid ganglia. 

 Moreover, as each contraction begins near the venous entrance to the 

 right auricle, and progresses from here toward the apex, Remak's 

 ganglion is generally regarded as the motor center of the entire organ. 

 It is held, therefore, that the cause of the automaticity lies within these 

 cells, while the peripheral fibers and cellular elements serve merely as 

 adjuncts which are made use of in the conduction of the wave of 

 excitation to other parts of this organ. It is granted, however, that the 

 separation of these outlying elements from the "pace-maker, " enables 



