186 CIRCULATION OF THE BLOOD 



It is most probable that we have to do herd, with a chemical stimulus of 

 some kind, which is to be sought among the products of decomposition formed 

 in the activity of these parts. If this is so, it follows that the inorganic con- 

 stituents which, as mentioned above, must be present in an artificial fluid in 

 order to maintain the heart's activity, and which occur in the blood, are not 

 to be considered as the real excitant of the heart beat, but merely as a 

 condition. 



Since the structure of cardiac muscle agrees essentially with that of skel- 

 etal muscle, and the latter is set in action normally only under the influence 

 of a stimulus communicated to it from the central nervous system, it was 

 for a long time supposed that the rhythmical contractions of the heart were 

 not caused by any specific property of the muscle fibers, but were discharged 

 by intracardial ganglion cells. This view found weighty support in the fact 

 that these cells were demonstrated in just those parts of the heart where the 

 systole begins. In more recent times various authors, notabty Gaskell and 

 Engelmann, have advocated the view that the spontaneous contractions of 

 the heart are of muscular origin, and are due to a special property of cardiac 

 muscle. 



The following facts among others have been adduced as arguments for this 

 conception. The venous sinus in the frog contains a large collection of gan- 

 glion cells, known as Kemak's ganglion, which on the ganglion hypothesis has 

 often been referred to as the originator of the heart beat. Now it has been 

 found that normal pulsations can be started from every other place in the 

 sinus region ; the sinus ganglion is not, therefore, absolutely necessary. In the 

 frog in the normal course of events, the contraction waves probably proceed 

 not from the sinus but from the great veins. These pulsjBspontaneously 

 if they are isolated entirely from the rest of the heart, everr^f the isolated 

 portion contains no ganglion cells. The same is true of the bulbus arteriosus 

 of the frog's heart, in which no ganglion cells are present. Moreover in the 

 heart of the higher invertebrates, and in the spontaneously contractile veins 

 of the bat's wing, notwithstanding diligent search, no ganglion cells have 

 been found. Again the embryonic heart of mammals beats in a perfectly 

 characteristic manner at a time when no nerve or muscle cells have yet been 

 differentiated. 



All these and still other circumstances go to show that a rhythmical, auto- 

 matic activity of contractile tissue can be brought about without the partici- 

 pation of ganglion cells ; and it is, therefore, possible that the automatism of 

 the fully developed vertebrate heart is of muscular origin. The great tenacity 

 of life of the heart speaks strongly for this view also; for from all that we 

 know of ganglion cells elsewhere, they perish in much shorter time than is 

 required for the exsected heart to lose its power of rhythmical contraction. 



The fact that this power is developed to different degrees in different parts 

 of the heart, and that individual parts, like the clamped-off apex of the frog's 

 ventricle, will never pulsate spontaneously under the influence of the normal 

 stimulus, is explained according to the muscular theory by supposing that 

 automatism, originally common to all the cardiac muscle cells, has disappeared 

 in the course of development from some places, notably the apex, but remained 

 in others, notably the region of the venous sinus. 



