THE ACTION OF THE HEART 349 



as the frog and turtle that have a pulsating sinus, there is likewise 

 a pause between the contraction of the sinus and of the auricles. 



If in a beating heart a cut be made between the sinus and the 

 auricles so that they are completely separated, the sinus con- 

 tinues to beat exactly as before; the other chambers of the heart 

 may not beat for a moment, but after a short interval usually 

 resume activity. The rate of beat of these chambers under such 

 circumstances is slower than that of the sinus. Similarly the ven- 

 tricles may be separated from the auricles without affecting the 

 auricular beat, but with the result that the ventricles either fail to 

 beat at all, or beat at a much slower rate than the auricles. Such 

 experiments as these show that the rhythmic power increases the 

 nearer we go toward the venous end of the heart, and also that in 

 the normal heart the most rhythmic portion imposes its rate on 

 the rest of the organ. In order for the heart-rate to be determined 

 as a whole by the beat of the venous end it is evident that there 

 must be a conduction of the impulse to activity from one chamber 

 to the next throughout the heart. This conduction moves over 

 the heart in the form of a wave. 



There are in the frog's heart two places and in that of the mam- 

 mal one place where there is a delay in the passage of the con- 

 traction wave. These are, as already noted, at the junction of the 

 sinus with the auricles and of the auricles with the ventricles. 

 Anatomical study shows that at these junctions most of the 

 cardiac tissue proper is replaced by connective tissue, so that 

 physiological communication between one chamber and another 

 is restricted to small bundles of conducting heart tissue. The de- 

 lay at the junctions is usually explained as resulting from the 

 small size of these conducting paths, which offer on that account 

 considerable resistance to the passage of the contraction wave. 



Neurogenic and Myogenic Theories of the Heart Beat. There 

 are two questions of fundamental importance to an understanding 

 of the mechanism of the heart's action. These are: (1) Does the 

 rhythmic property of the heart reside in its muscular elements 

 or in its nervous elements? and (2) Is the contraction wave con- 

 ducted over the heart by muscle or by nerve-tissue? By the 

 early students of the heart both these properties were attributed 

 to its nervous elements as being more like nerve activities in gen- 

 eral than like those of muscle; and also because the venous end of 



