192 THE CIRCULATION OF THE BLOOD 



rately determined from the galvanometric record. The exact distance be- 

 tween the contact and the sinoauricular node is then measured and from 

 the data the average transmission time is estimated. From his results 

 Lewis 3 concludes that the transmission rates are uniform from the node 

 to all parts of the auricle, with the exception of the superior vena cava, 

 in which the rate is considerably lower. One thousand millimeters per 

 second represents very fairly the average rate at which the excitation 

 wave travels. On the other hand, Eyster and Meek 8 state that the wave 

 is propagated throughout the sinus node, and that it spreads to the 

 contiguous venae cavse and to the auriculoventricular node with con- 

 siderable rapidity, reaching the mouth of the superior vena cava in 0.01 

 second, whereas its passage to the auricle itself takes 0.02 second. There is 

 therefore a delay in the passage of the wave to the auricle, which indi- 

 cates that the excitation must spread to the auriculoventricular node be- 

 fore involving the right atrium. These authors conclude that "this leads 

 to the inevitable conclusion that the cardiac impulse spreads to the ven- 

 tricle and to the right auricle by different paths, and does not pass to 

 the ventricle through the auricle, as ordinarily stated." 



In the second, or indirect, method, the onset of the negative wave from 

 different leads in the auricle is compared against a standard. For the 

 standard Eyster and Meek have used the record of the mechanical sys- 

 tole of the auricle, but the interpretation of the result is extremely dif- 

 ficult on account of the rate at Avhich the changes are occurring. Lewis, 

 on the other hand, has used the standard electrocardiogram for purposes 

 of comparison. 



Mode of Propagation of the Beat to the Ventricles 



After reaching the auriculoventricular node, the beat is transmitted to 

 the ventricles along the auriculoventricular bundle a fact which has been 

 most clearly demonstrated by the experiments on heart-block. We have al- 

 ready seen (page 174) that although each chamber of the heart of a 

 turtle or frog has a rhythm of its own, this is much more pronounced at 

 the venous end of the heart, and when the transmission of the beat to the 

 ventricles from the auricles is obstructed or blocked, as by compression 

 or partial cutting at the auriculoventricular junction, the ventricles, 

 after coming to a standstill for a time, subsequently contract with a 

 rhythm which is entirely independent of that of the auricles. 



In the mammalian heart the same results may be obtained by arrang- 

 ing a clamp so that it compresses practically nothing but the auriculo- 

 ventricular bundle (Erlanger.) If the compression is extreme, the 

 rhythm of the ventricles is quite independent of that of the auricles, but 

 if it is only partial, the ventricular systoles follow regularly every sec- 



