518 CIRCULATION OF BLOOD AND LYMPH. 



theory has been developed chiefly by Gaskell and by Engelmann. 

 It assumes that the heart muscle itself possesses the property of 

 automatic rhythmicity and that this property is most highly de- 

 veloped at the venous end. This portion of the heart, therefore, 

 contracts first and the wave of contraction spreads directly to the 

 musculature of the auricle and thence to that of the ventricle. The 

 quickly beating venous end sets the pace, as it were, for the entire 

 heart. The nerve cells and nerve fibers that are present in the heart 

 are upon this theory supposed to be connected with the extrinsic 

 nerves through which the rate and force of the heart beat are regu- 

 lated, but they are not concerned in the production of the beat. 

 Many experimental facts have been accumulated which give 

 probability to this view, and it has been adopted by many, perhaps 

 most, of the recent workers in this field. Some of the facts that 

 favor this theory are as follows: 



1. The anatomical arrangement of the musculature of the 

 heart is not opposed to such a theory. It was formerly stated quite 

 positively that there is no muscular connection between the auricles 

 and ventricles in the mammalian heart, but a number of observers 

 have now demonstrated the existence of a muscular bridge, the 

 auriculo- ventricular bundle, between the two chambers (see p. 496). 



2. The fact that a contraction started at one part of the heart 

 may travel to other portions through the intervening musculature 

 may be said to be demonstrated. Thus, Engelmann has shown 

 that if the ventricle in the frog's heart is cut in a zigzag fashion, 

 so that strips are obtained which are connected only by narrow 

 bridges, a stimulation applied at one end starts a wave of con- 

 traction which propagates itself over all of the pieces. This and 

 similar experiments scarcely permit of explanation on the supposi- 

 tion that conduction from piece to piece is effected by a definite 

 nervous mechanism. So too it has been shown that under certain 

 conditions the normal auriculo-ventricular rhythm can be changed 

 at will to a ventriculo-auricular rhythm. If, for instance, a ligature 

 be tied around the frog's heart between the sinus venosus and the 

 auricle (first ligature of Stannius) the auricle and ventricle cease 

 to beat. In this quiescent condition a slight mechanical stimulus 

 to the ventricle causes it to beat and its contraction is immediately 

 followed by that of the auricle. A similar reversed rhythm may be 

 obtained from the mammalian heart under suitable conditions. 

 Such an experiment makes it most probable that the contraction 

 is propagated from one chamber to the other directly through 

 the muscular connections. It is not possible at present to conceive 

 that a definite mechanism of neurons should work thus in either 

 direction. 



3. There is much probable proof that the heart muscle tissue 

 possesses the property of automatic rhythmical contractions. Ex- 



